**Abstract:** The purpose of this paper is to explore the challenges of an old low-standard urban district with a strong historical and cultural heritage and propose more sustainable renovation solutions, acceptable for the residents and municipality. The challenges of physical renovation or refurbishment are complex due to poor condition of the buildings, municipal ownership and governance, mixed management with community and low rents, which are insufficient to cover the costs. The paper discusses the proposed solutions of living standards, supported by the research in two directions: (i) available resources and reuse of materials, (ii) developing a renovation guidance for inhabitants from the building physics perspective, including indoor environment quality. Challenges related to energy efficiency are addressed from the decision-making perspective to overcome the barrier of lack of motivation to invest in energy-efficient measures at the individual and community level. The interdisciplinary approach complements engineering-focused studies with a focus on the comfort conditions and the influence of occupant habits in sustainable buildings. The methods used were literature review, case studies with observations and survey, looking to cover all technical, social, and historical aspects of sustainable renovation of cultural heritage buildings with the same level of importance. Results show that to keep a sustainable, low-cost urban living model, instructions for self-renovation are a valuable guidance for non-professional actors to make more sustainable choices. In conclusion, we can emphasize that inhabitants are accustomed to lower living standards, so the project is aimed to present the proper solutions for improvement as a balance between new sustainable technical solutions, personal self-renovation skills, habits, and health.

**Keywords:** self-renovation; habits and comfort; sustainable building material; cultural heritage buildings

#### **1. Introduction**

An increasing body of scientific literature has put the focus on energy efficiency measures for cultural heritage buildings, but this approach tends to undermine cultural values which are often described only as constrains [1]. In the extensive systematic literature review on sustainable refurbishment of historical buildings, Loli and Bertolin (2018) [2] pointed out the "Scandinavian paradox", which often shows the Scandinavian countries as frontrunners when it comes to implementing and operationalizing sustainability goals [3], but on the other hand, the scientific production on methods for sustainable maintenance and renovation of cultural heritage buildings in Scandinavia is relatively scarce [2]. Norway is quite advanced in terms of energy-efficiency renovation with a yearly rate of 2.5% of the existing building stock compared to the 13 other EU countries where data are available, which have rates between 0.5% and 2.0% [2]. However when it comes to "deep renovation" defined as interventions that fundamentally affect the buildings performance, the European rate is stagnating at 0.2% [4,5].

**Citation:** Senior, C.; Salaj, A.T.; Vukmirovic, M.; Jowkar, M.; Kristl, Ž. The Spirit of Time—The Art of Self-Renovation to Improve Indoor Environment in Cultural Heritage Buildings. *Energies* **2021**, *14*, 4056. https://doi.org/10.3390/en14134056

Academic Editors: Boris Igor Palella and Roberto Alonso González Lezcano

Received: 28 May 2021 Accepted: 28 June 2021 Published: 5 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The renovation of cultural heritage buildings can produce positive effects for the socio-economic regeneration of the cities [6] and boost the application of contemporary living models, sustainable management strategies, and maintenance procedures, which can balance the up-to-date requirements of energy efficiency, human comfort, and operating cost reduction [7] ensuring also energy and economic benefits [6,8].

In this regard, different studies indicated that the most critical factors for deep renovation of existing housing concern the non-technical barrier related to social, logistic, legislative, and financial constraints [9], while behavior and local cultural factors, in particular, can reduce the efficiency of the renovation initiatives, impacting on energy use, conservation state, management, and maintenance operations [6,9,10]. The importance of cultural heritage for the community and the specific character it has built over time are two sensitive elements that need to be combined with the contemporary expectations and responsible and sustainable lifestyle.

For policy makers, supporting self-renovation projects of residential buildings can contribute to lessen social exclusion and spatial segregation and encourage tenants to feel a sense of responsibility for their dwellings [11]. However, the lack of technical and legal knowledge among residents combined with the high costs are still major barriers in the implementation of energy efficient renovation measures [12,13]. In addition, some of the researchers emphasize the lack of motivation as a barrier for more sustainable renovation solutions [14–16], and proposing the focus on social sustainability aspects [17], more concretely: 'factors affecting participation' [18,19], 'relationship between participants' [20–22], 'engagement strategy' [23–25], and 'influence of participation' [26–28]. As stated by Esmaeilpoorarabi et al. (2020) [29], it is important to mobilize resources, improve relationships, promote cooperation, and ultimately achieve community engagement and trust [30,31]

The art of self-renovation in low-income urban communities is quite unique in the sense that it transcends a purely technical approach and often constitutes an opportunity for them to feel more ownership of their homes, promote social inclusion, and strengthen community bonds [32]. Financing and education of workforce are also central challenges that need to be addressed [12]. The social aspects are widely underrepresented in energy research, and although there is no question that technological engineering studies are of utmost importance to improve technical performances of buildings, due to the fact that energy-efficiency relies on human actions and informed choices, more interdisciplinary research is needed to include a softer approach and address social barriers and drivers to a successful energy transition [33–35]

#### *1.1. National Regulations for Conservation of Cultural Heritage Buildings*

The purpose of building protection is to preserve our sources of cultural heritage and history. Building protection became a formal matter in Norway in 1913 when the first national antiquarian was hired. Since then, it has been the Directorate for cultural heritage national antiquarian that has been responsible for cultural heritage policy [36]. In Norway, there is a broad political will to preserve cultural heritage buildings [37]. It points out further that the preservation of cultural heritage can contribute with knowledge and input to sustainable resource management by providing insight into how environmental problems have affected our patrimony and give a better understanding of how they can be solved. About listed buildings, it says: "*From a long-term socio-economic perspective, preserving valuable parts of the building stock rather than demolishing and rebuilding can be significantly more profitable*" [37].

The Norwegian Planning- and Building Act § 31-1 states that when doing renovation or rehabilitation, the municipality shall ensure that historical, architectural, or other cultural value associated with a building is preserved as far as possible. Nevertheless, there is a challenge associated with the maintenance and preservation of the protected building stock due to the ever-increasing demands for lower greenhouse gas emissions [38].

The existing building stock in Norway accounts for around 40% of the total energy consumption in the country, 22% of which is for residential and 18% for non-residential buildings [39]. In addition, most buildings built before 1950 were without insulation, which makes both protection and improvement of this building stock challenging. Fulfilling the requirements related to energy savings could mean major structural interventions and may lead to changes that affect cultural historical values of the building [36].

#### *1.2. Improving Energy Efficiency in Existing Buildings*

According to Ugarte et al. (2016) two of the main problems that low-standard households need to face as a consequence of low energy efficiency are extreme indoor temperatures and high humidity levels. Rehabilitation could address these problems by taking measures with three main outcomes: improvements in indoor air quality, humidity levels, and indoor temperature [40]. For this matter, retrofit programs may focus on a weatherization based on changes in the shell of the building, such as insulation of facade wall, ceiling and basement, facade painting, or replacement of windows and doors [41,42].

Given the environmentally friendly nature of wood, its low maintenance cost, and broad accessibility, wood is one of the most renewable materials in construction [43]. However, when it is used in building facades, it needs careful treatment and maintenance to minimize thermal and humidity discomfort sources in buildings. Energy wastage in the facade tends to be derived from humidity, since it provokes expansions and contractions of the material and mold growth, causing degradation of timber. From an environmental point of view, one of the best ways to protect wood is with linseed oil [44]. It saturates the external layer of the wood, so that it is impossible for water to penetrate the structure. Moreover, it allows water kept inside to evaporate [45]. It is important to plan the functioning of the building envelope for the future degraded climate conditions; in northern Europe, climate adaptation has to take into account moisture resistance (due to increased precipitation) and slight increase of temperatures [46,47].

When refurbishing the wooden structure, two main aspects need to be considered. First, if replacement of a timber member needs to be made, the new item must belong to the same species of wood and have the same quality. Secondly, it is also recommended to use the same (or similar) techniques that the original craftsmen used to convert the timber and assemble it [45].

This way, the identity of the building can be preserved. Moreover, some materials should be avoided or, at least, reduced to a minimum. Such as epoxy resins, steel reinforcements and plastic paint for coating [45].

Existing retrofit programs for low-standard buildings are based on weatherization of households; this way comfort improves and costs are reduced [48]. Weatherization is usually focused on changes in the structure, as insulation of ceilings and walls, air sealing, and duct sealing, and the electricity savings can reach up to almost half the original price [41]. Some of the actions that can be taken in order to achieve a good weatherization are [42,49]:

Regarding the refurbishment of doors and windows, they play an undoubtable role in the ventilation and weatherization of a household, but sometimes their high prices might keep the owners from fixing them. Anyway, it is a key element to address, as old doors can be leaky and have extremely poor insulation; similarly, single glaze windows are inefficient and should be switched to double-glazing or interior storm windows which provide moderate insulation and avoid air infiltration [41].

Insulation is a key element in order to achieve a good energy efficiency level. Some natural and renewable materials have been proved to have a great performance as insulating materials, presenting important advantages regarding costs, ecology, and energy savings, if compared to traditional materials.

Natural fibers, such as technical hemp, jute, and flax have very good mechanical, acoustic, and thermal insulation properties. Moreover, they can be combined in different proportions and be useful in façades, roofs, floors, partition walls, and external walls [50]. Natural fibers, such as technical hemp, jute, and flax, have very good mechanical, acoustic, and thermal insulation properties. Moreover, they can be combined in different proportions and be useful in façades, roofs, floors, partition walls, and external walls [50].

Some additional costs can derive form the insulation of the façade, since activities as scaffolding and painting cannot be overlooked and homeowners rehabilitating a building exterior need to pay for them no matter what [42].

Other non-structure related improvements that could be applied are [42]:

This said, it is clear that some good weatherization methods can be decisive for inhabitants of the building, not only in economic terms but also healthwise, as inadequate heating, cooling, or ventilation of the spaces can be a life-or-death issue [51,52].

#### *1.3. The Scope of This Study*

The scope of the study, considering the transformation of historical and cultural background of Svartlamon, poor housing condition, reuse of materials, insufficient funding, and existing self-renovation model, aligns with the New Bauhaus European initiative which vouches for a softer approach to the challenges in reaching sustainability goals. The European Commission's initiative aims to bridge the world of science and technology with the world of art and culture by developing affordable and accessible living spaces, striving to improve quality of life and highlight simplicity, functionality, and circularity of materials while accounting for the need for comfort and attractiveness in citizens' daily life [53]. Building upon this initiative, this study focuses on three principles:

The paper aims to address some of the challenges linked to unsupervised self-renovation work that can result in alteration of the cultural heritage buildings, negative consequences of the maintenance backlog, and unsustainable technical solutions. At the same time, this study looks into approaches that will increase the sustainable renovation rate that is currently lagging behind due to socio-cultural barriers [54]. The case study proposed in this paper focuses on "soft approach" to energy renovation. This paper deals with energy efficiency from the decision-making perspective addressing the challenge of lacking motivation to invest in energy-efficient measures. The interdisciplinary approach complements engineering-focused studies with a focus on the comfort conditions and the influence of occupant habits in sustainable buildings. It proposes a practical self-renovation guidance for residents with a focus on sustainable technical and technological solutions, focusing on indoor environment, health, and wellbeing. This is motivated by a gap identified in previous studies that revealed communication challenges between public authorities and residents in the field of energy retrofitting of cultural heritage buildings that resulted in

decaying buildings rather than improving their sustainability [13]. This can be also seen as an urban experimentation (Newton and Frantzeskaki 2021) case which can be replicable for similar circumstances.

The main research question is *how to raise the self-renovation standards of cultural protected though poorly maintained area*. The aim is to explore how it is possible to refurbish the houses, keeping all the existing aspects of the community, the history, the culture for re-use of materials, and the low budget and adding new ones like thermal insulation and general improvements of the indoor environment. The analysis of the physical properties of buildings such as ventilation, lighting, thermal weaknesses, and thermal insulation will be explained, in order to propose appropriate measures to meet the growing demand for a pleasant indoor climate while safeguarding the cultural heritage values. In addition, a survey conducted among residents of a low-standard urban community is presented to understand the opportunities and challenges of sustainable self-renovation of cultural heritage buildings.

This paper is structured as such: Section 2 presents the overview of the material and methods used for the study; Section 3 shows the study results; Section 4 presents a guidance for sustainable self-renovation in accordance with the theoretical and empirical background, and Section 5 concludes with the main findings.

#### **2. Materials and Methods**

To address the research question, we investigate in three directions: (i) improving the living standards and culture of a low-income intentional community, (ii) widening the reuse of materials as a part of circular economy solution, and (iii) finding high quality LCC solutions for specific problems. A number of buildings with a focal point on observation in the intentional community of Svartlamon in Trondheim, Norway, were chosen as the case study. This was combined with multiple on-site visits and a questionnaire survey. The case of Svartlamon was selected for its uniqueness and self-organized maintenance and operation system. The economic, socio-cultural, and environmental challenges posed by its ownership structure and eventful history as well as the subversive nature of Svartlamon will be presented in the following section. The present study is a result of several projects running at the Norwegian University of Science and Technology with master's students since 2018.

The data collection and observation have been conducted in collaboration between master's students and the authors of this paper during the spring and fall semesters, from 2018 to 2020 [55,56]. Initial data about Svartlamon's historical and cultural background were collected through informal meetings with Svartlamon housing foundation (Svartlamon boligstiftetse) and desk research at Trondheim Municipal Archive Center (Trondheim Byarkiv). Observation of indoor has been organized by Svartlamon housing foundation several times per year for the students and the research team. The observations of the outdoor environment were taken and documented whenever was needed.

A questionnaire survey was designed to investigate the living conditions in Svartlamon, the self-renovation work organized and done by residents, and their perception of sustainable technologies. The questionnaires included 4 sections; the first section collected demographic information such as age, employment status, composition of the household, living period in Svartlamon, the affiliation to a sub-community, and types of facilities respondents shared with others (bathrooms, toilets, kitchen, living room, laundry room, etc.) The second section on the living conditions consisted of questions about social aspects of the community life, motivation factors for living in Svartlamon, opinions on possible changes or improvements that could be made, and their perception of the core-values of the community. The third section collected data on self-renovation habits and culture, the respondents' involvement in such work, and their will to participate more in the future. The last section of the questionnaire addressed the available technologies within the households as well as their perception of innovative sustainable technologies.

The questionnaire was distributed in digital and hard copy versions to all inhabitants in Svartlamon (ca. 200 people), and answers from 24 people were collected. A qualitative analysis of the results was then conducted by the authors in order to serve as a basis for the recommended renovation measures that respect and reflect the spirit of the Svartlamon community.

#### *The Overview of the Study Area*

Svartlamon is a neighborhood in the city of Trondheim, Norway, with a strong presence of listed buildings (Figure 1). It is inhabited by an alternative low-income community and regulated as the first urban ecological research area in Norway, which makes it a particularly relevant experimental case to explore the challenges of sustainable renovation of cultural heritage buildings.

**Figure 1.** Trondheim Historic Centre, Svartlamon, and National Railroad (©Google 2021, edited by authors).

The origin of Svartlamon dates back to 1860, established as a settlement in the outskirts of Trondheim for dock workers, sailors, and workers from nearby factories. Due to railway construction in 1889, the area was exempted from the bigger district Lademoen and became the dirtiest and poorest part of it; thus, it got nicknamed Svartlamon or "black" Lamon [57]. There was no water or sewage system, which made it a sullied residential area with precarious living conditions. During World War II, some houses were demolished to allow the construction of the Dora II Bunker. Even though Svartlamon was later incorporated into the city, the basic infrastructure was severely lacking (Figure 2) [58]; therefore, by 1980, the municipality changed plans for this district to be demolished and become an industrial area. As the demolition operations started, a small area comprising of wooden houses remained untouched and was squatted by outcasts and criminals.

**Figure 2.** Svartlamon in 1964 (Trondheim Municipal Archive Center).

However, in the 1980s the cultural heritage character of the houses was brought to the attention of a community of artists and activists who occupied the houses and settled in, starting the first wave of basic renovation to repair damages caused by negligent tenants [59]. They made the area livable again and started their own alternative-living

community. Svartlamon is also defined as an intentional community [60], a community '*which choose to live together with a common purpose, working cooperatively to create a lifestyle that reflects their shared core values*' [61].

In 1990, the municipality attempted once more to tear down the Svartlamon area arguing that the houses were unfit for living and had not met the regulations in place for residential buildings. This marked the beginning of a fight between the municipality and the residents who organized themselves into the Svartlamon Residents' Association, '*Svartlamon Beboerforening'*.

After long years of discussions and conflicts, the municipality decided in 2006 (Figure 3) to consider Svartlamon as an experimental urban ecological area, the first of its kind in Norway [62,63]. Since 2001 the Housing Foundation '*Svartlamon Boligstiftelse'* has been acting as a steering organ, collecting the rents, and managing the everyday operations of the community. It also bridges communication between the community and their municipal landlord.

**Figure 3.** Timeline of Svartlamon's history (by the authors).

The challenges it faces today are caused by the long period of neglecting the area, mixed governing and managing structure, unclear distribution of responsibilities, and insufficient maintenance funding as the income is based on low rents. All these lead to poor infrastructure in indoor and outdoor housing condition. Regardless of the very active and social oriented community, their self-renovation attitude and habits and their demographic background with lower income and short-term tenants' contracts lacked enough security to search for high quality solutions and resulted in the multiplication of quick fixes rather than long-term solutions. Today, the contract period is extended to twenty years, which gives better planning foundation for more sustainable renovation solutions and future development.

In line with the aforementioned and having in mind the presented history of its origin and basic function, the settlement can be characterized as industrial heritage with significant social value, as important evidence about the life of ordinary people and their identity [64–66]. In addition, it has a technological and scientific value with regard to the history of manufacturing, engineering and construction, as well as a significant esthetic value in terms of architecture, design, and planning. These values exclusively refer to industrial heritage, its materials, components, equipment, and method of installation in industrial environments, as well as written documentation and intangible records related to the memory of the people and customs [67].

Svartlamon is covered by zoning plan R0219b, which came into effect on the 27th of June 2006. Based on the zoning plan, urban ecological efforts at Svartlamon include both physical and process related efforts. The physical experiments involve testing of new and affordable solutions in housing types, technology, and architecture. With the main focus on utilizing physical resources in the area [68].

Process-related trials, on the other hand, involve testing new planning, management, rehabilitation, and collaboration processes, with the main focus on utilizing the human resources in the area. According to the zoning plan, the concept of urban ecology therefore implies a holistic view where development in the area is based on an interaction between the physical and human resources, where one cannot be seen separate from the other [68].

Further on, the zoning plan states regulations in regard to refurbishment at the area. According to § 3–5, in the zoning plan, it is specified that: "*Existing buildings shall be preserved except for the buildings marked as demolition objects in the planning map of the area. For existing buildings, no major reconstruction, extensions or facade changes can be made without this being presented to the Cultural Heritage Management Office in advance.*" [68]

In this study, we highlight both the technical, social, and cultural aspects with equal importance. This combined approach of sustainable renovation of cultural heritage buildings is seen as an added value to tackle the challenges in this domain.

#### **3. Results**

#### *3.1. Case Study Description—Observation*

At Svartlamon, there are a total of 25 residential buildings with 130 dwellings/tenancies. The total gross area of the residential buildings is approximately 7000 m2. About 200 people live in the dwellings rented by svartlamoen boligstiftelse by the municipality. There are also four commercial buildings, and the gross area that the culture and industry foundation rents from the municipality constitutes ca 2500 m2 (Table 1).

#### **Table 1.** Characteristics of Svartlamon.

About 80 people work in varying positions in Svartlamon. In addition, the kindergarten is managed by the municipality. The area is regulated as special conservation area and has about 35 antiquarian classified buildings (residential buildings and outbuildings) [69].

Svartlamon is a district located to the east of the Trondheim city center, which hosts in addition to the residential buildings, a reusable shop, free shop, cultural festival, concert venue, kindergarten and book café. Most residents share some facilities like bathrooms (toilets and showers) and kitchens. The old houses are wooden and brick buildings. Inhabitants tried to renovate some parts of the buildings since their construction in the 19th century, but the buildings are in a general bad condition now. Lack of money and complex organization between the municipality and inhabitants lead to insufficient investments in maintenance of the buildings and regeneration of the area. The buildings are owned by the municipality, and since 2001, the housing foundation has been responsible for collecting rents and ensuring the maintenance. This was formalized in the contract that binds the foundation and the municipality, by which the municipal landlord transferred all its responsibilities for maintenance and preservation to the foundation. Today, the foundation manages 151 leases divided into 35 houses with about 200 people living there. The foundation has three people working full-time, a manager, a carpenter, and an electrician. The money collected by the foundation finances the salaries of the three employees, renovations, and maintenance operations. In order to keep the rent as low as possible, residents are expected to do as much work as possible by themselves. The Housing Foundation has a permanent office and meeting place on site. In addition, in 1990, Svartlamon residents' association was founded with the purpose to preserve the houses and to defend low-income people's right to live downtown. Everyone who has a lease with the Housing Foundation is automatically a member of the Residents' Association. They

have monthly meetings and govern with a flat structure and consensus. The residents of Svartlamon have a leading role in the renovation. They manage to do almost everything by themselves with low budget, reusing materials from the old houses from other parts of Trondheim and trying to keep the buildings in their original state. The focus of this study is mostly on the technical and technological aspects, but reflecting also the economic, organizational, social, cultural, and historical ones.

Svartlamon is located close to the city center of Trondheim, with a climate type dominated by the winter season, cold period with short daylight, relatively little precipitation which is mostly in the form of snow, and low humidity. It is located north of the humid continental climate. There is an average of 272.0 days of precipitation. The average temperature for the year in Trondheim is 4.8 ◦C. The warmest month is July with an average temperature of 13 ◦C. The coolest month is January, with an average temperature of −3 ◦C.

Wooden buildings (Figure 4) are houses with bedrooms and kitchen. Each house does not necessarily have a bathroom. The community shares few bathrooms. Most of the houses were built at the end of 19 century; they are now deteriorated even though people tried to keep them in a satisfactory condition. Most of them are two-story houses. In the area, we can notice a sloping terrain, some gardens, pebbles on the ground, and brick floor in some paths. There are no paved roads or sidewalks between houses. The site is located near the Strandveien road with cars and bus. There is some lighting next to the main road but not in the smaller paths between houses. There is also the railway right next to houses. Most of the houses do not have any basements except the storage room for the ancient milk shop.

**Figure 4.** Svartlamon wooden buildings (NTNU).

Foundations are in bad condition. Houses have many windows, but they are damaged, showing problems with condensation and humidity. All the facades are painted; however, the painting is very old and is deteriorating (Figure 5). Roofs are not insulated, and all houses have a steep roof. Many houses have tile roof. Clay tiles are a traditional kind of roofing material, it is a material that is both widely available and easy to shape into forming a channel to direct the flow of water. Some of the houses have a corrugated metal roof which is inexpensive and quick to install. Mainly, the rainwater gutters and downpipes are in good condition close to the roof, but the lower parts are worse (leakages), causing constant humidity on the walls, allowing moisture to seep into the houses, so molds, fungus, and mosses are spotted. They affect the health and well-being of people. The façades are complicated to insulate due cultural protection, so the design of the facades and windows should be original. Walls, doors, windows, partitions, and finishes are wooden and old.

**Figure 5.** Deteriorating façade (NTNU).

Many walls are not straight, and they lean (Figure 6), likely because the houses were built on poorly compacted embankments or on a heterogeneous soil. No drainage systems could be seen around them. Some of the load bearing elements are rotten. Even some entry doors are not straight. Most of the houses do not have any bathroom, and the only sink in their house is the kitchen one. The houses with bathrooms share them. The plumbing system is old and partly maintained but not to the standards that we can find in newer houses. The electrical system is at its full capacity, old but safe.

**Figure 6.** Leaning house (NTNU).

A typical example of housing block is presented: two buildings Strandveien 19 and 21 (Figure 7), from 1893, with four flats each, shared facilities in the common basement, and a shop in the ground floor. The foundation material is stone; basement walls are made of brick, and upper floor levels have a wooden structure. During the time only some small interventions were done, and foundations were stabilized in 2001; a wind barrier on the north façade was installed in 2016, and the damaged wooden panels were replaced (Figure 8). Both houses need major refurbishments as some foundations and walls are falling apart, and the structure is in a very bad condition, showing many cracks.

A comparison of the floor plans shows some smaller changes in the usability of the space in the floors and basement. An apartment in the basement is changed to a bathroom and storage place for inhabitants, and some internal walls were removed to create a larger common space. In one of the flats in the second floor, an extra bedroom and own bathroom was built.

An analysis of technical condition is presented, based on the observation (Figure 9). The foundations of the two buildings are connected and have the same structure, consisting of stones. They support brick walls with a thickness around 0.40 m and a height equal to the basement height, which is up to 3.00 m. There is no drainage system on the outer side of the walls. The inner walls structure from the basement on is carried by wooden columns. The wooden façade is made from stacked timber with a thickness of 7 cm.

**Figure 7.** Strandveien 19 and 21 (Trondheim Municipality, 2016).

**Figure 8.** Renovation of the façade (Trondheim Municipality, 2016).

**Figure 9.** Illustrated section of the building in Strandveien 19 (Sekkal, 2019).

The structure of the slabs is made of primary beams resting on the columns and the brick wall. The wooden slab is made in classic form: secondary layer of beams with space of 50 cm between each, another layer for wooden panel floors, and clay or gravel in between as insulation. The frame of the buildings is classic with a wooden truss, which is supported by the columns in the walls. Above the trusses, there are purlins that support perpendicular wooden boards. The distance between the trusses is more than 6 m.

Windows are not the original ones, and they are in simple or double glazing. However, we saw that some frames are damaged mainly due to humidity.

 Some of the residents put an internal insulation.

The walls have many cracks both inside and outside the buildings. A large crack in the cladding of the north façade between the two houses extends along the entire height of the ground floor, and the thickness is from 1 to 2 cm. From documented material, it is there from 2012 and is still the same size, without any changes (Figure 10). There are many other smaller cracks in the brick walls, missing cement binders and in the wooden walls as well. On the north façade several humidity traces could be seen (Figure 11).

**Figure 10.** Crack between the buildings Strandveien 19 and 21 (NTNU).

**Figure 11.** Humidity damage (NTNU).

The situation extended for a longer period could cause the structural and health problems due to humidity indoor (Figure 12), which could freeze during the winter, or formation of molds and fungi. In addition, the cladding contact between the brick and wooden wall is not tightened enough (Figure 11, upper left corner). There are some damages on the south wall: some pieces of bricks are falling; there are cracks between the bricks with missing binders and traces of humidity coming from the outside, i.e., from the soil (the bedrock in Svartlamon is very shallow). The analysis shows various causes of humidity in the wall: cracks outdoor, poor cladding condition, no drainage, capillary rise, bad condition of the gutters causing too much water streaming on the façade, and the wooden board above the wall of the ground floor does not play its part to 'drop' off water. The humidity traces are located at the top of the wall or on the ground, behind the cracks, in the contact area of two different cladding materials, and behind the wooden façade, panels are not tightened enough and have no vapor barrier.

The ventilation is not efficient in the buildings. The air exchange is only through the doors and windows, somewhere through leaks in the walls, and small extractors in bathrooms (Figure 13). There is no ventilation system in the kitchen.

**Figure 12.** Indoor wall in one of the rooms (NTNU).

**Figure 13.** Integrated humidity control, occupancy sensor, and downtime (NTNU).

The acoustic is problematic, and there is a lot of sound transmission. During the visit to one of the apartments, the discussion from another one was heard as well as a washing machine from the common stairs. Inhabitants made an agreement between the residents to respect a certain schedule of "making noise". The noise could be heard through the windows of the north façade, which is facing to the railway.

The electrical system is old, safe, but insufficient, so they should consider new equipment. They use mostly electrical heaters. The lighting conditions are quite poor.

The municipality conducted an evaluation of the maintenance backlog of the building stock of Svartlamon in 2016. Their findings, which align with our on-site observation (Table 2), support the evidence that the available funds for maintenance were very low compared to the required renovation work (examples given in Table 3 for Strandveien 19 and 21).

**Table 2.** Problems of the building stock based on site-observation.

**Table 3.** Estimated renovation costs compared to available funds (Trondheim Kommune, 2016).

In the case study of Svartlamon, different local resources were found. The observations revealed several workshops available at no cost to the residents; photos of the workshop are shown in Figures 14 and 15. Here, the renters have both materials and tools that can make it easier to maintain their home and houses. There is also a dedicated group that manages the workshops.

**Figure 14.** Workshop session on window restauration (Svartlamon residents association, 2016).

**Figure 15.** Workshop at Svartlamon (NTNU).

The workshop voluntary group manages the workspaces (Figure 15) at Svartlamon with associated tools, materials, and a recycling stock that is available both to the residents and to the residents association.

They receive a lot of recycled materials from both outside and inside Svartlamon. From time to time, Svartlamon also receives old material and equipment from houses that the municipality is tearing down. The idea is that access to tools and materials should stimulate reuse and local refurbishment. It is stated as a cautious estimate that the voluntary group organizing the workshop produces about 250–300 h of work per year [69].

Based on their own observations and this information, the students decided to investigate solutions that could improve the living standards in the buildings in a more sustainable way with respect for the low-budget and self-renovation culture deeply anchored in the Svartlamon community.

#### *3.2. Results of the Survey*

Based on the results of the questionnaire, relevant information regarding the living conditions in Svartlamon are presented in this section. 54.2% of the respondents share bathrooms and toilets with other households. The respondents rated their relationship to their neighbors above average (on a scale from 0–6, 100% rated it above 3; with 58.3% rating it as a 6/6). The main factors that influenced their choice to move Svartlamon were Social aspect/sense of belonging (83.3%), Freedom (79.2%), Budget/economic reasons (66.7%), Having friends/family that were already living in Svartlamon (66.7%). To the open question on what they liked the most about living in Svartlamon, the social aspects came out the most, including community feeling, local democracy, shared facilities and green areas, economic freedom (non-binding leases, possibility to settle in a mobile home, no mortgage, cheap rent that allow to focus personal investments elsewhere, e.g., "pursuing their dreams"), general feeling of freedom (to set up art installations, start new activities and events, DIY projects). To the open question on whether they would like to change anything in Svartlamon, most respondents would like to have an even bigger community engagement, i.e., that more people would actively participate in the collective activities and decisions.

The interesting thing about the self-renovation culture in Svartlamon is that a majority of the respondents have experience in renovation work (Figure 16).

**Figure 16.** Participants' involvement in renovation/refurbishment projects.

Furthermore, results point towards a strong community aspect of these projects with respondents being involved not only in their own house renovation but also those of other residents (Figure 17), and a further engagement of the community at large beyond the household (Figure 18).

**Figure 17.** The type of buildings and the participants involvement in renovation plans.

**Figure 18.** Involved groups in renovation plans.

However, most of them did not discuss the project with the community beforehand (Figure 19), and a vast majority expressed their will to be involved in future projects (Figure 20).

**Figure 19.** Participants interest in involvement in future renovation plans.

**Figure 20.** Respondents' engagement with community before renovations.

Regarding the technologies available to their household, results show that the most basic needs are met, while more sustainable technologies are not (Figure 21). However, a majority of respondents expressed no need for new technologies (Figure 22).

From the open question about the use for new technologies in their daily life, some respondents expressed a will to have more sustainable energy sources such as solar panels combined with thermoelectric generator, heat pumps, and bio toilets.

"*We are saving up to buy a thermoelectric generator which will help us utilize our solar energy system better during the dark months of winter, via the wood stove. We are also saving up for a bio/compost toilet*". (Survey respondent)

Others expressed a need for keeping up the basic comfort with examples of kitchen fans and floor heating systems. For the majority who expressed no need for new technologies, respondents expressed a will to keep the simple living standards as they are for cultural reasons or lack of space in their dwellings.

"*I have what I need", "My house doesn't need anything else, nor is our house big enough", "The rooms are original 1880s, and I like the quiet atmosphere in the rooms*". (Survey respondents)

An analysis is designed to address challenges and find solutions from different perspectives that could be useful for inhabitants with low budget and a 'do it yourself' mindset.

The students covered different areas, such as indoor temperature, air quality, noise disturbance, energy consumptions, cracked walls, fire safety, and lack of private bathrooms.

#### **4. Guidance for Sustainable Refurbishment for Improving Indoor Environment**

As a result of the observations and the survey, the students developed under supervision of academic and professional experts, a practical guide in form of a small magazine for residents in which they addressed each part of the sustainable renovation work (Figure 23, Trouillon et al. (2019)).

**Figure 23.** Practical guide developed by NTNU Students for Svartlamon residents.

The first part of this magazine recalled information about the cultural heritage background and communicated the results of the survey. Further on, each element of the sustainable renovation guide was addressed in the most practical way yet easily understandable for a non-expert audience. This practical guide provided background information on the buildings' symptoms, the importance of addressing them, the potential for reusing materials, and the possible renovation measures to be taken.

The following section goes into the details of each element covered more succinctly in the magazine.

#### *4.1. Reuse of Materials*

While global concerns are saving energy, protecting the environment, as well as the scarcity of resources, re-use and recycling take place in the very first steps towards a promising future. The inhabitants and the entire community of Svartlamon have a good understanding of these issues, and many of them explain that they are living in this neighborhood not only for the low rent price, but rather to live a simple life and have a low environmental impact. Furthermore, this practice is the preferred one in the community, both for economic and environmental reasons. Svartlamon could also benefit from a collaboration with Loopfront, a digital platform created in Trondheim that enables the reuse of materials in the construction industry, promoting a circular economy [70].

More specific information about the reuse of construction materials is presented below in order to provide a basis for technical guidance for non-professional residents who engage in self-renovation work.

#### **Bricks**

Brick is a robust material with a long service life. Thus, recycled bricks have great potential for use in new constructions. Recycled bricks can be used in different contexts depending on the technical characteristics of bricks. Reclaimed bricks that are frost-proof can be used for cladding in facades while those that are not frost-proof are used in a plastered façade. Bricks used over several floors must have sufficient compressive strength. The reuse potential of bricks varies for different periods of time. Bricks covered with paint or treated with surface treatment that may contain PCBs, chlorine paraffins, heavy metals and other hazardous substances should be avoided [71,72].

A project at Lilleborg in Norway shows the following figures on the cost of unit price for reuse of bricks:

Results of the Lilleborg project indicate that due to the costs associated with sample extraction and quality control, the reuse volume per demolition object should be at least 50,000 bricks [72].

#### **Wood**

Results from the project "Reuse House Trondheim" showed that wood is one of the most popular materials for reuse [73]. Re-used material groups include all types of treated and untreated wood, glued-laminated timber, and wood fiber products, where spruce and pine are the most commonly used types of wood in Norway. Wood is most often used in constructive elements. It makes up about 30–40% of the total waste during a demolition. In the "Reuse House Trondheim", approximately 85% of the timber-frame and cladding were made of reclaimed wood, and smaller wood elements like doors, window frames, and kitchen interiors were also reused [74]. It is important to keep a record of quality assurance process when re-using or repurposing wood so that the material keeps its quality and usability. Wood that should be avoided is CCA- and creosote impregnated wood which belongs to the group impregnated wood and is considered hazardous waste.

#### **Metal**

Metals such as steel, zinc, copper, and aluminum components are sorted for reuse. One should be aware that materials that belong to the hazardous waste category should not be reused. Surface treatment must be assessed against limits for hazardous substances and whether there may be a risk of leakage [74]. One must be careful about the metal components with surface treatments that contain sensing substances (asbestos, heavy metals, Pcb, chlorine paraffins).

#### *4.2. Indoor Environment*

In the case of low-standard buildings, retrofit recommendations are based on weatherization of households, to improve indoor comfort and reduce energy-consumption costs. Weatherization is usually focused on changes in the structure, such as insulation of ceilings and walls, air sealing, and duct sealing.

#### **Ventilation**

To have a healthy indoor environment, fresh air is required in buildings to lighten and minimize odors, to improve the oxygen level for respiration, and to increase thermal comfort [75]. Natural ventilation could be a good environmentally friendly solution in Svartlamon buildings to improve air quality as the budget is very tight and the cultural heritage must be preserved [76]. This ventilation system can not only bring in fresh air using the natural force of wind and provide a high ventilation rate, which can be cost and energy efficient compared to mechanical systems, but also (if applied properly) can provide the opportunity of accessing daylight in buildings [77].

The proposed solution implies having straight pipes (re-used when applicable) through the roof and the ceilings of the apartments in order to exhaust the polluted air from the secondary rooms (kitchens in this case). Each apartment would have its own pipe as shown in Figure 24 which refers to a possible solution for the house of Strandveien 21. Putting vents through the walls is also a good solution for air change. However, the location and orientation of these pipes are crucial in terms of controlling the airflow, pollution release rate, and the amount of fresh air coming in [78], not having vents close to the exhaust pipe, but at the opposite side to improve airflow and preferably placed facing the wind, but also not close to the windows which is a cold area.

**Figure 24.** Simplified drawing of the proposed ventilation system for Strandveien 21 (Practical guide by Gendarme, 2019).

Ventilation outlets would be a vertical duct (at the opposite side of the air inlets) from the ceiling of the apartment to the roof of the house but also air extractors. The vertical straight pipe will evacuate hot air through the roof due to the low density of heated air.

#### **Sound insulation**

Materials which will be used for sound insulation need to be selected carefully. The goal is to prevent, or at least reduce, the sound coming from other apartments since it is the most annoying sound source. For keeping a low budget, only the walls (separating the common stairs and the apartments) and the floors (separating the first floor and the second floor) are addressed. Sound absorbent materials act as springs in a mass–spring–mass system. The sound absorbent is a soft, porous material that dampens sound and which is placed between two rigid walls considered as the masses [79]. These rigid walls stop a part of the sound according to the mass law: the heavier and denser a material, the higher the acoustic insulation is. The growing environmental awareness triggered a shift towards more environmentally friendly materials from renewable resources, such as waste wool and recycled polyester fibers [80]. Wooden fiber is good for acoustic since it is quite heavy but also mineral wool that has a good absorption factor at all frequencies [81,82].

Frames can be created in front of the existing walls separating the apartments from the common stairs/area, in which the insulation will be placed. For optimal insulation, the frame must be detached from the wall. That way, if one wall vibrates, it does not transmit its vibration to the second wall.

#### **Lighting conditions**

Ensuring good natural lighting conditions in a building is important to improve the health and wellbeing of residents [83]. A solution to enhance natural lighting in buildings would have been putting larger windows, but the cultural heritage regulations make this impossible. The alternative solution is to choose colors that reflect natural light. The reflection value of light can vary from very high percentage for white glossy finish to almost null for mat black. This can affect daylight illuminance in the room and impact the visual comfort of residents [84]. Finally, improving the artificial light equipment in the rooms can contribute to better conditions.

#### **Windows**

One solution will be to use windows with the lower U-value than windows already installed in order to reduce heat loss through window, especially in the north façade where there is the least solar gain [85–87]. However, windows with low U-Value are expensive and have a lower transmittance which deteriorates the properties of natural lighting [88]. A better performing frame like an insulated frame allows to reduce heat loss through the frame, and if they are well installed, they will highly reduce air leakage [89]. However, some of them could be expensive, and the design of the building does not allow to put any windows, as the external visual aspect must be kept. In the cheapest way, it is recommended to stop air leakage around the windows just with a seal and the use of reclaimed material to replace damaged parts.

#### **Wind barrier**

Airtight building envelope in lightweight constructions in cold and moderate climates is normally realized by a continuous interior air- and vapor-tight barrier. However, a proper interior air barrier that fulfills the stricter environmental requirements is usually labor intensive due to many internal joints in buildings (e.g., interior walls, perforations necessary for electrical and plumbing devices) [90,91].

Given the high possibility of unwanted infiltration of cold air in buildings through forced convection, using "wind barrier" can help to protect the outside insulation layer from such cold air infiltration into the constructions [92]. The wind barriers can also serve as a drainage plane to prevent water infiltration into the structure [92].

#### **Air leakage**

Uncontrolled infiltration of air through the building results from holes in the envelope (chimneys and ducts), gaps between building components especially at the roof, joints around movable elements such as doors and windows, and penetration of air through building components under pressure from wind. These infiltration means a high heat loss (up to 30%). Reducing infiltration will therefore significantly reduce heat loss and reduce overconsumption [93]. The following actions can be taken to reduce infiltration:

An old building breaths due to its air infiltration weaknesses, ventilation is mainly due to low airtightness. Therefore, in reducing air leaks, the building will be more airtight and this type of natural ventilation will be less efficient. It is important to consider ventilation in the renovation planning, in order to avoid reduction of inside air quality causing moisture, material degradation and health problems for the residents thereafter [94].

#### **Insulation of exterior components**

Given the considerable influence of building envelope properties on the construction energy performance, one of the most common methods to improve energy efficiency in constructions is to insulate the internal and external walls [95]. To minimize energy use in buildings, providing thermal and moisture insulation in the layers tend to be very efficient. Thermal insulation can be installed on the external or internal side of the building envelope. However, in case of historical buildings, applying insulation in the external walls should be done in a more careful way due to the necessity of preserving the ancient and distinctive appearance in such buildings [96].

However, the influence of moisture transfer and condensation inside the walls (which would affect the thermal performance of the materials as well) is usually overlooked. For instance, it is shown by Barbosa and Mendes (2008) [97] that ignoring the influence of moisture in thermal insulation layers may lead to underestimation of yearly heat flux in buildings, which will result in huge waste of energy. Hence, it is crucial to view both thermal and humidity aspects in installing insulation in both internal and external wall layers.

The given circumstances in Svartlamon offer both advantages and disadvantages in terms of wall insulation of external walls. The usual measures used in the performance of insulation techniques are usually characterized by a high-cost factor. However, Svartlamon has to deal with limited budgets. Due to this fact as well as the historical and heritage characteristics of buildings in this area, it is not advisable to install insulation in the outer shell of the building, as a composite thermal insulation system. On the contrary, techniques can be used which may differ slightly from the state of the art or which do not fully comply with the regulations. The loosened requirements for building standards in Svartlamon offer a great advantage in this respect. Thus, apart from fire protection, the energy standards for new buildings do not necessarily have to be fully met. In the following section, different possibilities will be explained which can lead to an enhancement of the living situation and comfort.

Internal insulation is characterized by numerous disadvantages, as poor execution can result in extensive consequential damage [85]. Internal insulation in historical buildings can also lead to reduction of floor area, change of spatial room pro-portions, loss of historic fabric, and influence on the hygrothermal behavior of the insulated walls [98]. However, with careful execution it also offers several advantages with regard to Svartlamon. For instance, single rooms such as the living room can be insulated as desired to reduce heating times and energy loss. Although this means that valuable living space is lost, it also saves costs as no scaffolding is necessary, and the wooden cladding of the façade does not have to be removed and replaced.

Due, among other things, to the widespread use of timber-/timber-framed constructions in Norway, blow-in insulation should also be taken into consideration. With the help of this technique, all cavities are filled with loose bulk material and the insulating effect of the wall is significantly increased without a change in the external appearance of the building being visible after the measure. However, the installation should be carried out or at least supervised by a specialized company, since a deficient execution leads to large consequential damages. Nevertheless, it is a very cost-effective and above all very fast method with few working hours. There are also numerous sustainable and natural materials available for the choice of insulation material. For example, old newspapers can be collected and recycled to provide a much cheaper alternative to manufactured raw materials.

The structural and physical properties of the exterior walls are influenced by the right choice of materials and therefore also by the choice of insulating materials. Nowadays, many different insulating materials are used in the building industry. The Tables 4 and 5 were adapted from Gabriel and Ladener (2018) [99] and Kolb (2014) [100] and developed by one of the student [101] to be incorporated in the practical guidance and each give an overview of the individual properties of the selected insulating materials and techniques.

**Table 4.** Insulation materials and their properties (Beck, 2019).

**Table 5.** Insulation techniques and their potential for self-renovation (Beck, 2019).

Provided information consists of approximate costs per square meter for the materials and their technical parameters as well as indications regarding insulation techniques that could be more or less easily applicable for self-renovation work. A point system is used to describe the insulating efficiency, summer heat protection, and moisture control (Table 4), whereby five filled out points stand for very high (very good) and five empty points for low (very bad). The same point system is used to assess the insulation techniques (Table 5).

#### **5. Discussion and Conclusions**

The purpose of this study was to explore the challenges related to sustainable refurbishment of an experimental urban ecological area with a strong historical and cultural heritage. We proposed possible renovation solutions and instructions with respect to the self-renovation culture that dominates in the community while also accounting for the complex ownership structure, governance, and insufficient funds for the maintenance backlog. Findings from this paper provided insight in two aspects of self-renovation:

The research prepared together with master's students from NTNU is based on on-site observation and a close collaboration with the housing foundation of Svartlamon, thus providing solid basis for recommending real-life practical solutions that reflect the needs and the will of the community while trying to improve the overall sustainability of the building stock.

Based on the research, this study presents answers to the research question *How to raise the self-renovation standards of a culturally protected although poorly maintained area*?

Results show that in order to keep a sustainable, low-cost urban living model, instructions for self-renovation including information on their impact on sustainability are valuable as a guidance for non-professionals to make informed choices. We can emphasize that inhabitants are used to lower living standards, so the project is aimed to present the proper solutions for improvement as a balance between new technical solutions, personal self-renovation skills, habits, and health. This study addresses the challenges related to lack of motivation, knowledge, and communication that constitute major barriers to the implementation of sustainable renovation measures by proposing concrete actions based

on state-of-the-art energy-retrofit interventions that account for the pre-existing cultural and economic factors influencing community decisions and capabilities.

The community has had a long history of fighting for the preservation of the area as a low-standard independent neighborhood. Despite of the maintenance backlog, the will of the community to work together to renovate the buildings is an asset. Having only a low budget and little knowledge, they constantly found creative ways to address their problems, for example, the infrastructure and network systems for re-using materials are already well implemented. However, the local storage of the materials is a limit to their ambitions, and the community as well as the municipality could benefit from being part of a wider network as proposed by Loopfront. This pre-existing culture should be combined with more sustainable solutions such as improving the thermal insulation, upgrading the windows with better glazing and frames, etc. With an improved maintenance strategy and access to the proper tools and resources, the community could develop their self-renovation culture towards more efficient and environmentally friendly practices.

Recommendations for future self-renovation work should provide some key-knowledge to the inhabitants about insulation, structure, indoor climate, building physics, and energy efficiency. This would allow them to make better-informed choices when engaging in self-renovation projects. All aspects of a renovation plan are closely linked and should be considered simultaneously. For instance, repairment, sealing of cracks and sealing of air infiltration weaknesses, adding new layers like wind barrier and insulation lead to buildings more airtight which means that ventilation system must be considered and improved. Regarding the indoor comfort in the building, it is relevant to point out that they were not initially designed for the modern lifestyle and hygiene habits; inhabitants' activities and use of the kitchens and bathrooms have caused humidity damages that should be fixed in order to insure an acceptable indoor air quality.

Furthermore, when developing a practical maintenance guide, the effects and the actual cost-benefit factor for the residents of Svartlamon must also be considered, and a balance must be found between technical solutions and user behavior.

To improve the living standards in Svartlamon, thermal comfort and building physics, the proposed solutions and recommendations aforementioned might be a promising start to sustainably improve the houses in the long-term.

**Author Contributions:** Conceptualization, C.S., A.T.S. and M.V.; formal analysis, C.S., A.T.S. and M.V.; investigation, C.S. and Ž.K.; methodology, C.S. and A.T.S.; resources, Ž.K.; supervision, A.T.S.; visualization, M.J.; writing—original draft, C.S., A.T.S. and M.V.; writing—review and editing, C.S., A.T.S., M.V., M.J. and Ž.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank all the master students who took part in the course "Refurbishment technology Specialized course" at NTNU Faculty of Civil ad Environmental Engineering since 2018 for their work and their innovative thinking which are the foundations of this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Airflow Analysis of the Haida Plank House, a Breathing Envelope**

**Roberto Alonso González Lezcano <sup>1</sup> and María Jesús Montero Burgos 2,\***

**Abstract:** The Haida plank house is one of the most important models built by the native American Indians. Built on the southwest coast of Canada, it adapts the tradition of the ancient pit houses to the requirements of the humid and cold climate characteristic of the Haida Gwaii Islands. This construction is composed by two main pieces: the central pit covered by a wooden envelope. Both protect its dwellers and their hearths. The ventilation system is based on two solutions: the gaps between the wall planks and a smoke hole that can be opened or closed in the roof at will. The aim of the present research is to analyze the way these two elements arrange the indoor airflow in order to ensure the comfortability of the house. Four cases have been proposed, according to four different dimensions for the gaps: 1, 2, 3 and 4 cm. Each case has been doubled in order to determine how the state of the smoke hole affected the corresponding results. This way, it has been concluded that if the gaps' width becomes higher than 4 cm, the airflow velocity comfort level would be exceeded. It is been possible to observe how the state of the smoke hole influences the way the air moves around the dwelling.

**Keywords:** ventilation; CFD analysis; archaeology; architecture; native American Indians; traditional architecture; vernacular architecture

#### **1. Introduction**

The dwellings built by the Native American Indians are one of the most interesting examples of primitive architecture that can be found. When analyzing them, it can be seen that they were designed in the search for one main goal: to achieve a construction that takes advantage of its environment and of the available resources in the most efficient way [1]. In order to succeed, it was necessary to have a deep knowledge about the way that the environment worked and the advantages and disadvantages offered by those resources. Such was the case of these communities.

When the European explorers arrived in North America by the end of the 15th century, they found a bunch of cultures that represented a period of history that was already lost in their own continent [2–4]. Among the members of those expeditions, there were not only sailors but also artists, historians, engineers, botanists, or anthropologists, whose work is extremely valuable nowadays [5]. Specifically in the architecture field, they gathered a great amount of information about the dwellings they had the opportunity to visit. Dimensions, building materials, building processes, or indoor distribution are some of the features described in the documentation they created.

The present research is focused on one of these dwellings, the plank house built by the communities settled in the coast of southwest of Canada and northwest of the United States. The goal of this work is to analyze the passive ventilation strategies that are contained in its design.

**Citation:** Lezcano, R.A.G.; Burgos, M.J.M. Airflow Analysis of the Haida Plank House, a Breathing Envelope. *Energies* **2021**, *14*, 4871. https:// doi.org//10.3390/en14164871

Academic Editor: Alessandro Cannavale

Received: 28 May 2021 Accepted: 3 August 2021 Published: 10 August 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The study case was built by the Haida, one of the most important groups of the zone. Two types of houses were built by this community, according to two different structural solutions. The use of six or four beams was the main decision to make (Figure 1).

**Figure 1.** (**a**) Four beams model based on [1] (p. 270); (**b**) Six beams model based on [1] (p. 270).

The first one is probably the eldest [1] (p. 271). The house that has been chosen for the present research, dwelling number 3 from the village of Ninstints, corresponds to this structural solution.

The Haida name for the village was "Red Cod Island Town", which clearly indicates the way of life of its inhabitants. Its first European visitors changed its name from the one of the chief of the tribe, Nañ stîns, "He who is two". It contained twenty dwellings, although at present, just seventeen of them can be located, as can be seen in Figure 2 [6] (p. 103).

**Figure 2.** Plan of Ninstints (dwelling number 3 in red) based on [6] (p. 10).

As usually happens in this type of villages, the line of dwellings was arranged along the coast line in order to ease the entry to the ocean and to take advantage of the airflow that arose between the mountains and the ocean. This mechanism was a very useful feature of these locations, since this airflow kept the wooden structures of the dwellings from being rotten. The red cedar wood (*Thuja plicata* Donn ex D. Don) was chosen by the Haida, since it is naturally covered by a special oil that also protects it from moisture [7] (p. 63).

Walls were composed by a line of wooden planks (Figure 3). It was necessary to leave a little gap between them, so the airflow could enter into the house. As told by Underhill [8] (p. 84), one of these dwellings was rebuilt some time ago by modern Indians. When it was finished, they lit a hearth indoors, and smoke filled the house completely. They could not understand how their ancestors could have lived in such a smoky place. An Indian elder

man who had seen all the process explained them how important those gaps between the planks were: "Why, it was the cracks. With enough cracks, you have fine circulation of air".

**Figure 3.** Exploded axonometric of a Haida dwelling construction system based on [9] (p. 171).

The door consisted of a hole carved in a totem pole, and it usually was covered with another plank, hung from a cedar bark rope, and blocked with some poles fit in the structure [10] (p. 21).

This way, the structure of the dwelling was solved by means of wooden beams and poles, and its envelope was composed of wooden planks. Roofs were built in two different ways: wooden planks or bark sheets. The first option was used by the richest families, while the second possibility was chosen by the rest of the community [10] (p. 21). Whatever solution was chosen, these pieces were overlapped, such as the tiles used in modern roofs, in order to keep rainwater away from the house; finally, they were weighted down with stones [11] (p. 72).

The indoor space was arranged around a central terraced pit [12] (p. 146B). It was the place where the hearth was lit. There could be several hearths, if there was more than one family living in the house. The roof planks could be removed in order to let the smoke out.

As explained before, the target of the present research work is to analyze the airflow system that triggers indoor ventilation. There are three elements to focus on: the dimensions of the gaps between the planks, the influence of the smoke hole in the indoor airflow, and the way the airflow behaves in the pit.

#### **2. Materials and Methods**

#### *2.1. Materials*

The dwelling analyzed was built by the Haida in the village of Ninstints, Haida Gwaii Islands, British Columbia. Specifically, it is dwelling number three. It was built according to the oldest type of structure, as explained before, and it had a central pit, which was the usual piece of this type of dwelling [10] (p. 107). Its floor plan was 14.1 per 14.85 m. The pit (9.45 per 9.9 m) was 1.8 m deep; it was delimited by two steps that were each 75 cm high. The height of the house has been obtained by taking the Haida dwelling drawn by Niblack in 1890 (Figure 4); [13] (p. 42) as reference. This way, its height has been estimated as 3.5 m for the side walls and as 5.03 m for the ridge of the two main façades.

**Figure 4.** Cutaway section of Haida dwelling based on [13] (detail of plate XXXV).

#### *2.2. Methods*

First of all, the ventilation rate of the dwelling has been calculated in order to determine if it was high enough. This way, the 2nd Florida Solar Energy Center method has been used [14] (pp. 101–103). This method is used to dimension the inlet and outlet areas in cross-ventilated spaces. It is based on the pressure increase between the inlet and the outlet openings. The results must be read taking into account that they are not influenced by the ambient temperature and that this method assumes that every inlet opening has the same positive pressure value, and every outlet has the same negative pressure value.

According to this method, the ventilation rate is calculated according to the following formula:

$$\mathbf{Ach} = \left[ \mathbf{A} \cdot \mathbf{W} \cdot \sqrt{\left( \mathbf{f}\_3 \cdot \mathbf{f}\_4 \cdot \mathbf{PD} \right)} \right] / \left[ \mathbf{4.3} \times 10^{-4} \cdot \mathbf{V} \right]. \tag{1}$$

"A" is equal to the total effective area of the openings (m2) and is determined following the next expression:

$$\mathbf{A} = (\mathbf{A}\_{\rm o} \cdot \mathbf{A}\_{\rm i}) / (\mathbf{A}\_{\rm o} \,^2 + \mathbf{A}\_{\rm i} \,^2)^{0.5}. \tag{2}$$

The outlet area is represented by "Ao" (m2), and the inlet area is represented by "Ai" (m2). "W" is equal to the wind velocity (m/s). There are two correction factors represented by letter "f", f3 and f4. On the one hand, "f3" introduces the effect of the nearby constructions and is equal to "g/h", being "h" the height of the nearby obstacles and "g" the distance between them. On the other hand, "f4" is determined by the floor number where the case study in question is located. "PD" is the subtraction between the windward pressure coefficient (WPC) and the leeward pressure coefficient (LPC) [14] (p. 102).

The dwelling has been modeled in Autodesk Revit 2020®. This model has been exported as a SAT file in order to carry out the corresponding calculations in Autodesk CFD 2021® [15]. These calculations have been developed taking into account the average conditions of the zone where these dwellings were built. This way, in order to obtain representative results, ten locations have been taken, and their wind velocity average has been determined (Figure 5). Just summer wind velocity has been taken into account, since that is the season when ventilation is more important.

As can be seen in Figure 6, the model has been placed in a box. Air material has been assigned to this element, as it has to the gaps between the planks, the smoke hole, and the air mass located inside the house. Wood material has been assigned to the planks, floor, and roof.

As boundary conditions, wind force (8.24 m/s) (Figure 5) has been assigned to one of the sides of the box, normal to the longitudinal axe of the house. Zero pressure has been assigned to the opposite face. This way, the latter absorbed the windflow emitted by the former. The top and longitudinal sides of the box have been characterized as "slim/slippery". The free sides of the gaps have also been assigned zero pressure and the smoke hole has as well (Figure 6).

The dimensions of the air box were established by the software Autodesk CFD®. Its width is five times the width of the dwelling, its length is five times the length of the dwelling, and its height is five times the height of the dwelling.

The solution mode was established as "steady state". The resolution of the mesh was defined as 1, the edge growth rate was defined as 1.1, two points were the minimum on the edges, and 10 was the number of points for the longest edge. These meshes were designed automatically by the CFD software. Calculations have been carried out accepting the Autodesk CFD® criteria, since it limits automatically the optimum amount of iterations to achieve convergence. This way, "Intelligent solution control" and "Automatic convergence assessment" have been selected and set as "tight". The calculation system has been developed from the algorithm SIMPLER, which belongs to one of the best-known CFD tools family. The turbulence was simulated by means of the model k-epsilon, which has been widely developed and studied. The validity of the results that have been obtained are backed by the use of Autodesk CFD®, which is a software that has been tested since 2013. Its calculations have been compared in numerous research studies, such as the one developed in the Victoria University of Wellington by Dr. Jing Li [17]. Autodesk® provides an ample compilation of verification studies; some of them compare test models against experimental results, and others compare them against empirical hand calculations [18].

**Figure 6.** Screenshot in which boundary conditions are indicated (Autodesk CFD®).

Four alternatives have been calculated according to the gaps dimensions: 1, 2, 3, and 4 cm wide. Each case is calculated twice, since the smoke hole can be either open or closed. Five locations have been chosen inside each model, as shown in Figure 7. Each location has been divided into several levels, from level 0 to level 450, in such a way that one point was placed each 50 cm (Figure 7).

**Figure 7.** (**a**) Axonometry, screenshot showing the location of the five points analyzed (Autodesk CFD®); (**b**) Floorplan of the same model (Autodesk CFD®); (**c**) Section of the same model (Autodesk CFD®).

#### **3. Results**

#### *3.1. Ventilation Rate*

As explained before, the ventilation rate of this dwelling has been calculated according to the Second Florida Solar Energy Center method in order to determine if the openings were wide enough to accomplish this minimum requirement. The Spanish Building Technical Code establishes that this value has to be higher or equal to 0.63 ach in abodes.

Calculations have been carried out and, as can be seen, the plank house whose gaps are 1 cm wide accomplished this requirement (Table 1).

**Table 1.** Ventilation rate for 1 cm wide gaps.

Once it was determined that the ventilation rate was high enough if the gaps between wall planks were equal to or higher than 1 cm wide, the research could be focused on analyzing the way the airflow moved inside the dwelling, varying the dimension of these gaps.

#### *3.2. Airflow*

#### 3.2.1. Airflow above the Central Pit

The velocity magnitude has been calculated in five columns, each one composed by nine points from 50 to 450 cm high. Each column has been calculated for two cases, since the smoke hole could be either opened or closed.

As can be seen in Figures 8 and 9, the velocity magnitude increases as the gaps' width does. The thinner the gaps are, the more regular the velocity is along the corresponding column.

**Figure 8.** Values of velocity for the five columns of points, which is analyzed when the smoke hole of the plank house is opened (m/s) (Microsoft Excel®; Autodesk CFD®).

**Figure 9.** Values of velocity for the five columns of points, which is analyzed when the smoke hole of the plank house is closed (m/s) (Microsoft Excel®; Autodesk CFD®).

"A" is the column that gathers the higher velocity values (Figure 7). The highest values are not obtained by the thinnest gaps, in such a way that wind passes through the gaps that are 1 cm wide more slowly than through the 4 cm gaps. This difference is higher if compared with the closed-smoke hole model. Even though the velocity through the 1 cm gap is lower when the smoke hole is closed, it can be seen that it is almost invariable for the rest of the widths.

The greatest oscillations occur in columns "B" and "D", which are both located near the envelope. The velocity varies along them in an irregular way from the floor to the top of the house. However, their lowest values are reached close to the work plane (about 70 cm high), and the highest ones are found near the ceiling.

The most irregular case is the one of column "C". Its levels are similar to "B" at 50 cm below the floor, and they reach velocities as high as the "A" column when approaching the ceiling. So, the airflow behavior at this point is a combination of both "A" in the highest part of the dwelling and "B" in the lowest one. Whether the smoke hole is opened or closed, the result is the same.

Column "D" is near the outlet gaps. Column "C" is close as well, but the velocity is higher inside the gaps located besides "D". The difference between the results obtained for "C" and "D" is based on the proximity of the former to the smoke hole: that is, the element that triggers the velocity increase in "C" at 300 cm high. Actually, the velocity magnitude is almost null near the ceiling in column "D", which means that the smoke of the hearths could be trapped in this zone. The same velocity level takes place in column "C" below 300 cm high.

The column that is clearly influenced by the state of smoke hole is "E". As can be seen, the airflow is much more regular if this element is closed rather than if it is opened. The most remarkable fact is that the pit stays almost the same. The airflow velocity does not vary in this zone whatever the state of the smoke hole. These graphs also reflect that as expected, the velocity near the smoke hole is higher when it is opened.

In the following diagrams, the way the wind flows along the longitudinal axis of the house, the same as the wind vector, can be observed (Figures 10 and 11). They show how the opened smoke hole creates a quiet zone in the leeward elevation. This zone becomes smaller when the smoke hole is closed, and it turns softer and bigger as long as the gap between the planks becomes bigger. This fact reflects the way Haida transformed these dwellings along the year by moving these boards in order to let the air in. This way, there was no limit between indoors and outdoors, as happened in tipis.

**Figure 10.** Airflow velocity and direction inside and nearby the plank house, whose smoke hole is opened (**a**) 1 cm gap; (**b**) 2 cm gap; (**c**) 3 cm gap; (**d**) 4 cm gap (Autodesk CFD®).

The airflow velocity inside the dwelling is lower when the smoke hole is closed, but it is higher on the part of the roof surface touched by the airflow from indoors. This way, the roof erosion was more intense when the smoke hole was closed.

**Figure 11.**  Airflow in the Central Pit

By means of the following diagrams (Figures 12 and 13), it has been possible to observe how the airflow behaves inside the central pit, where the hearths were lit. As they show, the wider the gaps are between the planks, the bigger the turbulence is inside the pit. They are even more complex if the smoke hole is closed.

As explained before, the airflow velocity was very low in this zone. This turbulence helped to keep the its air clean, but at the same time, the smoke would spread over the place more easily.

When the smoke hole is closed, the airflow in the pit stays in it and cannot exit the house directly. The only case where it becomes possible is the one of the 4 cm gaps (Figure 12f). If the gaps are thinner, the air in the pit has to be dragged by other flows that are able to exit.

**Figure 12.** Airflow traces in the pit when the smoke hole is opened. (**a**) 1 cm gap; (**b**) 2 cm gap; (**c**) 3 cm gap; (**d**) 4 cm gap; (**e**) Axonometry corresponding to (**a**); (**f**) Axonometry corresponding to (**d**) (Autodesk CFD®).

**Figure 13.** Airflow traces in the pit when the smoke hole is closed. (**a**) 1 cm gap; (**b**) 2 cm gap; (**c**) 3 cm gap; (**d**) 4 cm gap; (**e**) Axonometry corresponding to (**a**); (**f**) Axonometry corresponding to (**d**) (Autodesk CFD®).

Instead, when the smoke hole is opened, the air in the pit usually exits by itself. In addition to the turbulence, it is important to take into account the velocity of the air particles. Figures 8 and 9 show that the airflow is almost static in the pit if the gaps are 1 cm wide.

#### **4. Discussion**

According to the results, it can be asserted that the plank house was a pit covered by a breathable envelope. The pit was the main area of the house, where the hearths were lit and where daily life took place. The origin of this space is lost in time, but it can be found in several Indian dwellings. In different ways, it is the main piece of the Navajo hogans [1] (pp. 325–336), of the pit-houses built by the Thompsons groups [1] (pp. 176–179) and, even, of the sod-houses built by the Yup'ik communities [19] (pp. 119–156). The thermal stability was the target to accomplish in most of these cases, but as shown in previous studies [20], there are some examples in which this function is not so clear. The plank houses are one of them. The depth of the pit is not enough to influence the indoor temperature of the house, so it has to have another aim. As can be seen in the previous graphs, it is a peaceful zone inside the house, where the hearths could be lit up and where the cold air from outdoors decelerated.

The idea of versatility is as important in the plank house as it was in the tipi, which is the most important nomad dwelling. The planks of Haida houses could be removed or displaced as the hide envelope of the tipis could be opened or closed. This way, both models search for the continuity between indoors and outdoors, as nomad tribes do in their daily life [21]. However, the most important point is that these communities analyze conscientiously their environment and take advantage of the opportunities it offers to them [22]. As Atta [23] asserts, an affordable and sustainable housing design requires integrating economic and environmental solutions from a social point of view. Traditional architecture offers a unique opportunity to investigate how these three factors can coexist successfully.

As the research developed by Knowles and Easton established [1,24], Native American architecture is much more complex than expected at first sight. Their research was focused on Acoma dwellings, which are adobe pueblos from New Mexico. Knowles determined that the orientation of Acoma dwellings let the houses take advantage of the solar radiation as much as possible; meanwhile, Easton analyzed the ascending currents of air that occur along the main elevation of the Pueblo houses.

As another example, the tipis could be taken. Their conical design was based on the shape of a Cottonwood tree leaf [25] (p. 7). This specie of tree is well known for being able to resist strong winds, as the tipis do. Its leaves move, adapting to the wind direction in order to survive. This is the way the ventilation system works in the tipis, taking advantage of the main wind in such a way that it takes out the hearth smoke from indoors.

The present research is rooted in the field of virtual archaeology, which is a discipline that offers an enriching amount of opportunities for other areas. The virtual reconstruction of disappeared buildings can show forgotten solutions that are particularly useful nowadays. In this case, the reconstruction of the Haida plank house has been possible thanks to the archaeological records about Ninstints as well as due to the journals written by the first European and Russian explorers. Research studies such as those developed by Trebeleva and Glazov [26] are undoubtedly essential to understand how the ancient buildings were designed and the technical solutions they contained.

The plank house that has been analyzed is the one built by the Haida community in the Haida Gwaii Islands. However, the "plank house" model can be found all along the Pacific coast of North America in different ways. From California, where the Hoopa built their own models, to the coastal Salish in the Vancouver Island and Puget Sound, who dwelled their shed houses, there are enough models to carry out a deeper comparison study.

#### **5. Conclusions**

By analyzing the results shown previously, it can be concluded that if the gaps between the planks of the Haida dwelling are wider than 4 cm, the summer airflow comfort limits (1.1 m/s) would be exceeded inside the building [27] (p. 305). However, it should be taken into account that the present research has assigned the same width to every gap in each case, which is a variable that could be studied in further research.

This way, it can be seen that the envelope of the Haida plank house let the indoor area breath. This dwelling is composed of two elements: this envelope and a central pit. The Native American dwellings were usually designed around a central hearth that is protected by an envelope [1]. This envelope can establish a solid limit between indoor and outdoor, as, for instance, happens in the New Mexican adobe pueblos, but there is another group of dwellings that uses this envelope to let the indoor area breath. Among them, the Wichita grass houses can be found. As occurs in the Haida plank houses, their grass envelope is designed to let the smoke out; that is to say, the whole envelope breathes. This type of solution can also be found in the thatched British cottages, confirming the idea that this type of solution is common in high humidity locations. What turns the Haida plank house into a special case is the combination of this breathing envelope and a central pit. As can be seen in Figures 10 and 11, if the smoke hole is closed, this zone is protected from the air currents that take the smoke out and keep the cedar planks dry. Thereby, the smoke hole works as a ventilation trigger and can be used to renovate the air of the dwelling at will.

By means of modern technology, this type of envelope is being developed at the moment by many architecture research centers and offices. For instance, the design developed by the engineer Tobias Becker from Stuttgart can be cited [28]. His envelope is composed of multiple cells controlled by changes in air pressure. This way, they regulate the incident light and the airflow as skin pores and pigments regulate a living being. In addition, the idea behind the Al Bahar towers should be highlighted [29]. Built in Abu Dhabi by Abdulmajid Karanouh and Aedas, their envelopes are integrated by multiple cells inspired in the Arabic lattice. These cells are closed or opened according to the meteorology, since they can let the air and the solar radiation in or they can shield the buildings from them. The prototype developed in Los Angeles by professor Doris Kim Sung also offers an interesting option [30]. Called Bloom, this experiment proposes the use of metal sheets that curl when heated in such a way that the whole structure casts shadows and lets ventilation under specific areas when temperature rises. Obviously, all these projects are technically much more complex than the plank house built by the Haida several centuries ago, but

the concept hidden behind all of them was the same. The point was not just to design a breathing envelope but to get the best of every element that composes a building by means of a deep knowledge of the possibilities offered by the environment in order to save the available resources.

**Author Contributions:** Conceptualization, R.A.G.L. and M.J.M.B.; methodology, R.A.G.L. and M.J.M.B.; software, R.A.G.L. and M.J.M.B.; investigation, M.J.M.B.; resources, R.A.G.L.; writing original draft preparation, review and editing, R.A.G.L. and M.J.M.B.; supervision, R.A.G.L.; project administration, R.A.G.L.; funding acquisition, R.A.G.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors wish to thank CEU San Pablo University Foundation for the funds dedicated to the Project Ref. USP CEU-CP20V12 provided by CEU San Pablo University.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Implementation of a Ventilation Protocol for SARS-CoV-2 in a Higher Educational Centre**

**Alberto Meiss \*, Irene Poza-Casado \*, Alfredo Llorente-Álvarez, Héctor Jimeno-Merino and Miguel Ángel Padilla-Marcos**

> RG Architecture & Energy, Universidad de Valladolid, Av. Salamanca, 18-47014 Valladolid, Spain<EMAIL_ADDRESS>(A.L.-Á<EMAIL_ADDRESS>(H.J.-M<EMAIL_ADDRESS>(M.Á.P.-M.) **\*** Correspondence<EMAIL_ADDRESS>(A.M<EMAIL_ADDRESS>(I.P.-C.)

**Abstract:** The most recent research confirms that airborne transmission may be the dominant mode of SARS-CoV-2 virus spread in the interior spaces of buildings. Consequently, based on some prescriptions that implemented natural ventilation during face-to-face lessons in a university centre, an experimental characterization of several complementary options aimed at reinforcing the prevention and safety of the occupants was carried out. The action protocol adopted was based on the combination of mandatory natural ventilation, a maximum contribution of outdoor air supply in the air conditioning system, and the use of filtering devices located inside the classroom. All the strategies were incorporated concomitantly with necessary compliance with the basic conditions of social distance, occupation, use of masks and guidelines for use and cleaning within educational buildings. The suitability of this protocol was further evaluated throughout the teaching day with students and teachers by measuring the CO2 concentration. The results showed that the measures implemented successfully removed the possible pollutants generated inside.

**Keywords:** indoor air quality; COVID-19; educational buildings; air purifier; airborne transmission

#### **1. Introduction**

Throughout the current global pandemic caused by the SARS-CoV-2 virus, there has been a significant evolution of the knowledge of contagion. In the early stages of preventive action, the recognized contagion mechanisms were [1]:

At the beginning of February 2020, it was still not considered necessary for the population to wear face masks. In this way, mandating the population to assume protection mechanisms that might not make sense was avoided [2]. It was in May 2020 when the mandatory use of masks was decreed in Spain [3], following the pattern of the rest of the developed countries, with the predominant aim of blocking the emission of "large droplets" infected with the virus.

Only recently, in April 2021, has the World Health Organization (WHO) begun to as-sess airborne transmission, although emphasis continues to be placed on the fomite path and large droplets [4]. However, there is a current endorsement by a growing number of scientists who do not agree with this limited view [5].

In the field of interior spaces in buildings, the term bioaerosol includes the droplets exhaled by people which can be between 0.4 and 100 μm and remain suspended in the air

**Citation:** Meiss, A.; Poza-Casado, I.; Llorente-Álvarez, A.; Jimeno-Merino, H.; Padilla-Marcos, M.Á. Implementation of a Ventilation Protocol for SARS-CoV-2 in a Higher Educational Centre. *Energies* **2021**, *14*, 6172. https://doi.org/10.3390/ en14196172

Academic Editor: Christopher Micallef

Received: 1 September 2021 Accepted: 23 September 2021 Published: 27 September 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

for a long time [6]. In other contexts, the emission sources can be any living beings in any terrestrial or marine ecosystem.

At an early date (July 2020), the Directorate of the Architecture Technical School in Valladolid developed a supplementary strategy to face a possible airborne transmission method in addition to the strict measures adopted regarding social distance, occupancy, the use of masks and guidelines for the use and cleaning of the building.

It is important to inform society that bioaerosols may be the dominant mode of transmission of the SARS-CoV-2 virus, with some contribution from the fomite pathway and an even smaller contribution of large droplets.

#### **2. State of the Art**

*2.1. Bioaerosols and SARS-CoV-2*

Bioaerosols can contain in their structure:

The size of an individual SARS-CoV-2 virus is very small (around 0.10 μm), which enables its lodgement in the bioaerosols generated by an infected person (Figure 1).

The virus uses the bioaerosol as a means of transport and, depending on the bioaerosol's size, can affect different organs of the recipient body, which in turn influences the viral load that will determine the contagion.

All bioaerosols smaller than 100 μm can be inhaled. There are three main mechanisms for the deposition of bioaerosols in the respiratory tract (Figure 2):

#### *2.2. Classification of the Droplets Emitted by People*

In the case of sedentary or light activity (1.0 to 1.2 met), human emissions through the respiratory tract are limited to events such as breathing, speaking, coughing or sneezing. In each of these events, a mixture of air and droplets of different sizes and at a variable speed is exhaled (Table 1).

**Figure 2.** Mathematical model of particle deposition of nasal breathing [7].

**Table 1.** Average emission per person and event [8].

Their distribution based on sizes also varies according to the type of event, which influences the way in which contagion from the infected person could occur (Figure 3, [9–12]). A usual classification establishes three ranges of size:

**Figure 3.** Droplets emitted according to the type of event.

#### *2.3. Evaporation*

Evaporation processes affect bioaerosols emitted by people. Thus, the distribution according to droplet size is not only dependent on the event but also a transformation through evaporation (Figure 4). This significantly affects the transition droplets (between 100 and 300 μm).

Thus, the rate of evaporation of the droplets depends on the humidity conditions of the environment (the lower the humidity, the higher the evaporation) and the emission speed (depending on the type of event). Based on these parameters, part of the droplets precipitate and others undergo an evaporation process, which allows them to diffuse as bioaerosols (Figure 5). Large droplets are those that could cause infection if they hit the eyes, nose, or mouth of a healthy person. In these cases, social distance greater than 1 m serves as effective protection, as well as the mandatory use of masks.

**Figure 5.** Behaviour of emissions according to their diameter [14].

It is the particles smaller than 100 μm that normally acquire the properties of an air-diffused bioaerosol, which, when inhaled, transmit the disease through the respiratory tract.

A fundamental question is to know how long the SARS-CoV-2 virus remains infectious in bioaerosols emitted by a patient. The life expectancy of the virus is conditioned by the loss of viral infectivity.

However, there is still no consensus on this matter since the time varies between 1 and 3 h under typical temperature and humidity conditions (20 ◦C and 50% RH) [15–17]. This range of time is important in order to confirm that any virus present in the air of a room has lost its contagiousness after a few hours. On the other hand, it has been demonstrated that the virus remains infectious over much longer periods of time on surfaces (Table 2).

**Table 2.** SARS-CoV-2 viability in different materials [15].

*2.4. Dynamic Behaviour of Bioaerosols*

Once the droplet is emitted by the infected person, its dynamic behaviour is determined by the simultaneous and combined action of a system of forces. The acting forces are the gravity force, the buoyancy force and the drag force (Figure 6). The preponderance of one over the others basically depends on the size of the droplet, which determines its diffusion and permanence in the air of the room.

**Figure 6.** System of forces affecting the bioaerosol.

In the case of the larger droplets, the drag force (from the impulse speed) and the gravity force are dominant, so their impact or their precipitation is favoured after a few seconds. In the case of bioaerosols, buoyancy is the main force, favouring their diffusion and increasing their permanence in the air.

#### *2.5. Standard Protocol*

Therefore, it is verified that bioaerosols serve as a vehicle for transporting the SARS-CoV-2 virus when they are emitted from an infected person. Its diffusion capacity is very wide, being able to follow a highly variable pathway from its emission until the bioaerosol leaves the room or the virus loses its infectivity. For this reason, it is necessary to adequately ventilate interior spaces.

The instructions received from the government authorities [18] motivated the development of a first strategy in the classrooms throughout the 2020/2021 school year:

Although the measures were correct as a basic prevention mechanism, a series of drawbacks that could affect the desired level of prevention were identified:

Accordingly, in order to supplement the above measures and increase the prevention of possible contagion, an experimental study was carried out [19]. The indoor air quality (IAQ) was assessed assuming strict compliance with the aforementioned provisions, and alter-native proposals based on the use of air purifiers were evaluated.

#### **3. Methods**

#### *3.1. Experimental Study in a Typical Classroom*

Successive tests were carried out in a representative classroom of the building of the Architecture Technical School of the University of Valladolid (Figure 7) under different ventilation conditions.

**Figure 7.** Typical classroom of the Architecture Technical School in Valladolid.

The classrooms have an air conditioner that vertically blows heat-treated air into the room through three inlets in the ceiling and recirculates the indoor air, which is mixed with fresh outside air (50% of the airflow, as a preventive measure against SARS-CoV-2). The windows were modified to allow total opening of their openable surface.

Tests were carried out according to the concentration drop method. In this way, aerosols were released in a controlled way up to a certain concentration, achieving homoge-

neous distribution by means of fans located inside the classroom. Once the concentration was stabilized, measurements were taken with six sensors (S1 to S6) distributed through-out the room.

To simulate the behaviour of bioaerosols, the emission of smoke was used, since they behave in an analogous way: the smoke contains aerosols with a similar range of sizes (the visible part of the smoke) and gases (recognizable by their smell, which do not constitute a vehicle for virus transport).

The behaviour of smoke emission is analogous to that of bioaerosols released in an environment. When a person smokes, the smoke does not fall to the ground quickly, but rather concentrates in front of the smoker and then mixes with the airflows. If this occurs in a poorly ventilated room, the smoke accumulates. In these situations, only a small fraction, less than 10%, is deposited on the interior surfaces, and the rest remains floating in the environment until the interior air is renewed.

The concept of smoke serves also to illustrate a situation between two people [18] where one person is exhaling smoke and the other wishes to inhale as little of it as possible: this is how the chances of contagion are reduced. This experience carries over to the use of masks: the aerosols are filtered with the masks, but the gases are not. The fact that the odour penetrates the mask does not mean that the mask is not retaining the aerosols.

A smoke machine (1.2 kW model Fz-1200) was used to carry out the tests. Its operation consisted of the combustion of oils that emit fumes with bioaerosols analogous to the composition of the air, allowing its traceability in its airflow path, as well as measuring its concentration by means of multipurpose sensors.

#### *3.2. Filtering Device*

As a significant improvement to the initial protocol, it was proposed to incorporate an air filtration device into the classroom. The mechanical capture of particles and aero-sols was done by means of a HEPA H14 filtering surface with an efficiency greater than 99.995%. The efficiency represents the percentage of particles that each type of filter can retain with respect to the most penetrating particle size (MPPS), which is normally be-tween 0.15 and 0.25 μm. Thus, a HEPA H14 filter with an efficiency of 99.995% for a particle size of greater penetration of 0.15 μm implies that for every 100,000 particles of that size, the filter only allows 5 of them to pass through. For particles of other sizes, the filter has an even higher retention capacity.

The flow rates treated depend on the adjustable flow rate (SETPOINT), according to the data supplied by the manufacturer (Figure 8):

**Figure 8.** Adjustable flow rate of air filtration device.

A parameter that requires further study is the flow pattern of treated and contaminated air in order to determine the recommended position and height of the air purifiers within the classroom [20]. For the tests conducted for this study, the device was placed in a central position in the space. Since a mixture flow pattern was achieved inside the studied classroom, it was possible to determine that adequate purification of the polluted air was guaranteed [21–25].

#### **4. Results**

The experimental study consisted of 12 cases with different configurations regarding the opening of windows and doors, the operation of the air conditioning system with the supply of outdoor air, and the use of a filtration device:

Throughout all the tests, the outdoor temperature was in the range of 8 to 10 ◦C, which represents a typical winter situation in Valladolid (Spain).

The results obtained in the tests are summarised in Table 3 and Figure 9, which reflect the average values of the sensors distributed within the room.

**Table 3.** Aerosol concentration decay time to 10μm/mm3.

**Figure 9.** Evolution of the concentration drop of aerosols emitted in the classroom.

#### **5. Discussion**

*5.1. Conclusions Derived from the Results*

#### *5.2. Verification of the Ventilation Conditions*

Finally, after the previous comparative analysis, the classroom under study was subjected to a test under typical conditions, that is, with 29 students and a teacher during the usual course of a class. During the test, the outside temperature was 3 ◦C on average, so the openings were partially closed (50% of their surface) in a similar way to Case 11.

In this case, the tracer gas was the CO2 concentration. Although CO2 is not a pollutant since its concentration was far from dangerous levels for health, it is nevertheless a good indicator of the emission of human bioaerosols, because it is strictly related to them.

The classroom was intermittently occupied (from 9:00 to 11:00 on Tuesday and Wednesday, and from 9:00 to 12:00 on Thursday). However, the total CO2 concentration remained very low (maximum values of 515 ppm with an outdoor CO2 concentration of 400 ppm), as shown in Figure 10.

**Figure 10.** Verification of the ventilation conditions through CO2 concentration measurement.

Under these circumstances, the combination of these three ventilation mechanisms can be considered as a means to strengthen the prevention measures adopted in the studied classroom.

#### **6. Conclusions**

In conclusion, it can be affirmed that it is necessary to strengthen prevention mechanisms against the risk of contagion of SARS-CoV-2 in buildings lacking centralized controlled mechanical ventilation systems, where the main strategy consists of window opening to promote natural ventilation.

The most appropriate methodology needs to address the characterization of each of the possible strategies in the building. In the case studied, the protocol adopted consisted of the combination of natural ventilation, the maximization of the outdoor air supply through the air conditioning system, and the use of filtering devices. Simultaneously, the basic conditions of social distance, occupation, use of masks and guidelines for the use and cleaning of the building must be met.

As a final note, it is necessary to mention that throughout the 2020/2021 school year with 100% face-to-face teaching, there was no case of contagion among students and teachers inside the classrooms of the centre.

**Author Contributions:** Conceptualization, A.M. and I.P.-C.; methodology, A.M. and I.P.-C.; formal analysis, A.M., A.L.-Á. and M.Á.P.-M.; investigation, A.M.; resources, A.L.-Á.; data curation, A.M. and H.J.-M.; writing—original draft preparation, A.M.; writing—review and editing, A.M. and I.P.-C.; visualization, A.M.; supervision, I.P.-C.; project administration, A.L.-Á.; funding acquisition, M.Á.P.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research "Metodologías de estudio y estrategias de mejora de eficiencia energética, confort y salubridad de centros educativos en Castilla y León. Investigación básica" was funded by Junta de Castilla y León, Consejería de Educación, grant number VA026G19 (2019–2021).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Review* **The Influence of Outdoor Particulate Matter PM2.5 on Indoor Air Quality: The Implementation of a New Assessment Method**

**Dominik Bekierski \* and Krystyna Barbara Kostyrko**

Thermal Physics, Acoustics and Environment Department, Building Research Institute (ITB), 1 Filtrowa Str., 00-611 Warsaw, Poland<EMAIL_ADDRESS>
**\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** Epidemiological research has shown that there is a positive correlation between the incidence of disease and mortality in humans and the mass concentration of particulate matter. An average 1 g of suspended dust emitted in a room results in the same exposure as 1 kg emitted to the outside air. In this study, the authors described the state of knowledge on dust pollution inside and outside buildings (I/O ratios), and methods of testing the PM infiltration process parameters. According to the law of indoor–outdoor particle mass balance and the physical basis of aerosol penetration theory, a relatively simple but new method for estimating the penetration factor *P* was tested. On the basis of the curve of dynamic changes of internal dust concentration in the process of particle concentration decay and next of the followed curve of dynamic rebound of particle concentration, authors measured penetration factor of ambient PM2.5 through building envelope. Authors modification of the method is to be used for determining the value of the particle deposition rate *k* not from the course of the characteristics in the transient state (the so-called particle concentration decay curves) but from the concentration rebound course, stimulated by natural particle infiltration process. Recognition measurements of the mass concentration of suspended PM2.5 and PM10 particles inside the rooms were carried out. In this study, the choice of the prediction particle penetration factor *P* calculation method was supported by the exemplary calculation of the *p* value for a room polluted by PM2.5. The preliminary results of the penetration factors determined by this method *P* = 0.61 are consistent with the *P* factor values from the literature obtained so far for this dimensional group of dusts.

**Keywords:** particulate matter; dust pollution; IAQ; indoor–outdoor concentration ratio; penetration factor; air quality control

#### **1. Introduction**

The aim of the article is to present the authors' proposal for a new method of assessment of the influence of outdoor particulate matter PM2.5 on indoor air quality indirectly by determining the value of the parameter that determines the intensity of dust penetration into the interior of the building. This parameter is the penetration factor, i.e., the ratio of the amount of dust entering from the outside, ending the infiltration processes, to the amount of dust retained outside.

In this study, the authors described the state of knowledge concerning dust pollution inside and outside buildings (I/O ratios), and methods of testing the PM infiltration process parameters. Methods known in the literature are based on the observation of the process of dynamic air pumping from a dusty external environment into a room with the use of an exhaust blower or an air cleaner. However, the process is not only dependent on the difference in PM concentration levels but more on the O/I air pressure difference produced. The authors' proposal includes the observation of the PM concentration rebound curve of the dust indoor concentration level increasing caused by the natural process of dust infiltration from the outside, to achieve dust balance in the exterior and interior of the building protected by a dust-permeable envelope.

**Citation:** Bekierski, D.; Kostyrko, K.B. The Influence of Outdoor Particulate Matter PM2.5 on Indoor Air Quality: The Implementation of a New Assessment Method. *Energies* **2021**, *14*, 6230. https://doi.org/ 10.3390/en14196230

Academic Editors: Roberto Alonso González Lezcano and Christian Inard

Received: 5 August 2021 Accepted: 22 September 2021 Published: 30 September 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The authors' field study aimed to estimate indoor air quality, especially particulate matter concentration in relation to the infiltration process through the buildings' envelope. Several analyses based on air quality measurements and calculation procedures were undertaken. On the basis of the curves of dynamic changes of internal dust concentration in the processes of particle concentration decay and next followed curves of dynamic change particle concentration rebound the authors proposed the method of estimating the value of the penetration factor of ambient PM2.5 through a building envelope. Information on the value of the P factor can be a reliable parameter that classifies buildings in terms of resistance to dust infiltration into their interior, which partly characterizes the condition of the envelope structure and its resistance to dust penetration into the building. According to research in residential buildings, a large proportion of both groups of particles and their mass concentration are attributed to human activities such as cooking, smoking, vacuuming, gas stoves, the burning of solid fuels and candles, electrical appliances, cleaning, washing and walking. Both sources influence the characteristics of indoor particulate matter, and their concentrations are increased by the resuspension of particles deposited on internal surfaces in the air.

The term PM10 is used for particles with an aerodynamic diameter <10 μm. The term PM2.5 defines aerodynamic particles with a diameter <2.5 μm. The coarse fraction contains particles with an aerodynamic diameter between 2.5 μm and 10 μm. The "ultrafine particles" fraction has an aerodynamic diameter <0.1 μm. A commonly used indicator describing particulate pollution is the mass concentration of PM10.

Thatcher and Layton's [1] experiments concluded that PM > 5 μm may be resuspended, particles < 5 μm are not easily resuspended and particles < 1 μm show almost no tendency to re-suspend at high human activity.

In the project INDEX PM [2], it was demonstrated that indoor air PM dust pollution mainly consists of salt (ammonium sulphate, ammonium nitrate, sodium and potassium chloride), soot (elemental carbon, EC), minerals (silicon oxides, aluminium, calcium, iron, manganese, titanium, zinc, etc.), organic substances (shredded organic matter, also called organic carbon, OC) and materials of biological origin (bacteria and fungi, dandruff, pollen, fragments of plants and insects). All of these components have different particle size distributions and mechanisms, which are strongly dependent on particle size and are involved in removing particles from the indoor air. Sedimentation is the most important mechanism for the removal of coarse particles (>10 μm), diffusion and the agglomeration/coagulation of ultrafine particles (<0.1 μm).

Particles (0.1–1.0 μm) are most stable in air. A typical input fraction (fraction of the total PM mass emitted from the source that will be inhaled by the whole population) for an indoor PM source is 10−<sup>2</sup> to 10−3, compared to 10−<sup>5</sup> to 10−<sup>6</sup> for outdoor PM sources: An average 1g of suspended dust emitted in a room results in the same exposure as 1kg emitted to the outside air. Consequently, while internal sources are less efficient and their total emission loads are smaller than the PM of external sources, each of these sources, if present, may dominate both the sum of individual exposures and the inhalation risks of PM.

Between 1999 and 2016, the Canadian Census Health and Environment Cohort (Can-CHEC) conducted panel studies involving approximately 2.5 million Canadians [3]. The relationship between non-accidental mortality and the concentration of fine particulate matter (≤2.5 μm; PM2.5), ozone (O3) and nitrogen dioxide (NO2) in the environment was investigated. Models with combinations of these contaminants have also been tested and PM2.5 was found to be associated with an increased risk of non-accidental mortality, i.e., lung cancer, diabetes and ischemic heart disease. The results obtained provide evidence that long-term exposure to these three key components of ambient air pollution is associated with an increased risk of non-accidental mortality. Cumulative risk models suggest that exposure to PM2.5 alone does not fully characterise the toxicity of a mixed atmosphere and cannot fully explain the mortality risks associated with exposure to environmental contamination. The strongest cumulative risk estimate is for mortality from diabetes (HR = 1.180; 95% CI: 1.125, 1.236). Assuming the additive exposure of individual pollutants, HR (Health Risk) in 95% CI (Confidence Interval), as estimated using three pollutant models showed that a change in exposure by an average of 5% for all three pollutants together resulted in impact synergies of 1.075. The effect of PM2.5 and O3 was weaker than the effect of NO2 alone, but HR increased with the combination of all three pollutants, i.e., adding NO2.

It has also been proved that the strongest correlation between PM2.5 concentration and mortality increased by 1.5% when PM2.5 concentration increased by 10 μg/m3 per day [3], because particulate matter <PM2.5 enters into the bloodstream.

The health effects of PM2.5 were also studied by Hvelplund [4]. The Brownian motion of particles in the respiratory system is only visible for particles below 0.05 μm. The same tendency was found by Wang et al. [5], who investigated the deposition efficiency of particles with aerodynamic diameters between 0.001 and 300 μm moving through the nasal cavity with an air mass flow of 0.6 m3/h. Studies in this area have shown that the lowest deposition efficiency is in the range of 0.015 to 10.0 μm due to the low influence of diffusion and the effects of impact and gravity settling.

The number of exhaled particles is debatable, e.g., most particles below 1.0 μm were predicted to escape through the bronchiolar orifices. It is not the case, however, that all particles escape because some of them are absorbed directly into the bloodstream through the bronchioles, which in human airways make up most of the levels of the tracheobronchial tree. Particles larger than 5–10 μm are usually removed from the upper respiratory tract, indicating that the mucosa and nasal hair contribute greatly to the filtration of particles.

According to [4], inhalation exposure to particulate pollutants is one of the main threats to public health. Most existing airway morphometry models are theoretical or semi-empirical in nature; they were designed to predict the deposition fraction for an averaged subgroup of the general population. It is difficult to tailor a quick and accurate prediction to suit individual needs.

Hvelplund et al. [4] aimed to analyse the local particle deposition along an anatomically reconstructed model of the airways, which was developed from computed tomography images of healthy subjects. Computational simulations of airway fluid dynamics show that most particles are deposited in the bronchi. The accumulation of particles (0.1–2.0 μm) is the smallest fraction, about 11%, of sediment in the lower respiratory tract. Increasing the aerodynamic diameter >2.0 μm of the particles increased the fraction of deposited particles. This study determined the size related to the site of particle deposition in the airways. It turned out that most of the particles in the studied bands are deposited in a narrow region of the airway model, such as the BC (bronchi) and larynx.

Already in the 1980s, many epidemiological studies had shown that PM2.5 has obvious side effects on human health Lin et al. [6]. Research in China on the relationship between PM2.5 and human health has fully demonstrated that PM2.5 can increase the incidence of heart and lung disease, respiratory disease, cardiovascular disease, cancer and other diseases, and even the risk of death. Long-term exposure to environmental PM2.5 may be an important risk factor for hypertension and is responsible for the significant burden of hypertension in adults in China, as it leads to a reduction in lung function. PM2.5 is a risk factor for childhood asthma, attributed to decreased immunoregulation and deterioration in ventilation function. Exposure to PM2.5 can also affect reproductive health.

The I/O indicator describing the temporary steady state of dust pollution in a building is related to the process of infiltration of suspended dust from the outside into the building's interior and can be described with the assumption that there are no internal sources generating dust particles:

$$\frac{\text{I}}{\text{O}} ratio = \frac{\text{C}\_{in}}{\text{C}\_{out}} \tag{1}$$

Lee et al. [7] found that room temperature and floor level (expressed by the story where the room is located in the building) are powerful I/O predictors and gave the I/O equation for PM2.5, which is:

$$\frac{1}{\text{O}} = 0.629 + 0.0102 \cdot T - 0.00654 \cdot FL \tag{2}$$

where *T* and *FL* are temperature and floor level, respectively.

The indoor/outdoor (I/O) index is a measure for assessing the difference between the levels of indoor solid concentrations and the current outdoor concentrations, but it may be also recognised as an indicator of the persistence of particulate matter sources inside buildings.PM concentrations are affected by dust infiltration from the outside into buildings and internal sources. I/O ratios can vary considerably due to the building's design, location and the different activities of the occupants. The I/O ratio is also calculated to compare the dynamics of flows between indoor and outdoor PM in different apartments and buildings. As far as health protection is concerned, it is clearly the best when the I/O ratio for each dimensional fraction of dust is less than 1, asthisshows that the structural properties of a residential building reduce the penetration of PM from outside and, accordingly, reduce the residents' exposure to PM.

In epidemiological studies of external contamination, the concentration of external contaminants was previously used as an indicator of the stressor. Thus, the appropriateness of using suspended dust concentrations from the building surroundings as a proxy in personal exposure studies was questioned, and both outdoor and indoor concentrations began to be measured to investigate their correlation with external conditions, as well as the correlations among other pollutants. Positive correlations were found for PM, O3 and NO2.

While exposure to indoor air pollution changes under the influence of many factors, such as the type of microenvironment and the source of internal pollution, building characteristics and its location, ventilation parameters and assumed comfort conditions (Branco et al. [8]), as well as individual building occupant activity, studies have shown that only some of these factors are important. Therefore, several parameters of I/O interaction (Kalimeri et al. [9])were selected in order to minimise the error resulting from the use of concentrations in the external environment as a surrogate for estimating the exposure of the PM environment to humans indoors. These parameters are the infiltration coefficient (*Finf*) (i.e., the equilibrium fraction of particles from the environment that have penetrated into the interior and remain suspended), the *P* coefficient of penetration efficiency of particles through the leaks of the building envelope (i.e., the fraction of particles that entered the interior from the infiltration path through the external walls of the building, *P<Finf*) and the I/O ratio (the ratio of internal to external concentration of similar PM particles). Lv et al. [10] argued that regression analyses of the ratio of inside and outside particle concentration values confirm that the degree of correlation of changes in these concentrations expresses the infiltration coefficient.