Chapter 3: The Building Enclosure

Chapter 3: The Building Enclosure2020-02-19T22:53:10-07:00
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Fig. 3-1 Northfield High School at Paul Sandoval Campus in Denver, CO
(General Contractor: GE Johnson Construction Co., Architect: LOA Architects) Photo by Brad Nicol

The building enclosure (i.e., building envelope) is a system of materials, components, and assemblies physically separating interior environment(s) from the exterior environment. As an environmental separator, the enclosure must control the flow of heat, air, water (liquid water and water vapor) and much more. The building enclosure must also provide support against physical loads on the building (e.g., air pressures, gravity loads, impact, etc.). The building enclosure must also provide an acceptable interior and exterior finish and help distribute utilities through the building. An appropriately designed building enclosure benefits a building’s serviceability, comfort, durability, and heating and cooling energy use. It also contributes to a healthy indoor environment.

The elements of the building enclosure include roofs, above- and below-grade walls, windows, doors, skylights, exposed floors, the basement/slab-on-grade floor, and all interfaces and details in between. Many of these elements are visible in Fig. 3-1. The focus of this guide is above-grade masonry systems; where appropriate, roof, floor line, foundation, fenestration transition details, and typical wall penetration details are also discussed as they relate to the masonry wall systems.

Within building enclosure design, there is a relationship between the loads that act on the building enclosure and the various layers and materials that control these loads. This chapter describes each aspect of this relationship with respect to masonry wall systems.

Building Enclosure Loads

Over the life of a building, the building enclosure is subjected to a wide range of interior and exterior environmental loads.

Interior Environmental Loads

Interior environmental loads include temperature, relative humidity, and vapor condensation as well as liquid water and water vapor associated with human activities, operating heating/cooling and ventilation systems, and potential defects in appliances, sprinklers, and interior plumbing.

In general, these loads are relatively predictable and can be controlled through various heating, cooling, and ventilation strategies and regular building maintenance.

Fig. 3-2 Contiguous United States climate zone map, as referenced from Figure C301.1 of the 2018 International Energy Conservation Code1

Exterior Environmental Loads

Exterior environmental loads typically include solar radiation, rain, snow, ice, hail, vapor condensation, wind, temperature, relative humidity, insects, pests, and organic growth. The climate zone in which the building is constructed, in addition to its site specific features, dictates the magnitude and duration of these environmental loads. Site-specific features, including the local terrain, can further vary the properties of the regional exterior climate by creating a microclimate specific to a building site. For example, a project in a regional high wind area adjacent to other buildings or a heavily wooded area of similar or greater height
will reduce the wind and driving rain exposure of the building.

As depicted in Fig. 3-2 and Fig. 3-3, four climate zones exist in Colorado and southern Wyoming and include Zones 4, 5, 6, and 7. These climate zones impose a wide range of exterior temperatures; however, the masonry design guidance provided within this guide is generally applicable to all zones within Colorado and southern Wyoming. Colder temperatures of Zones 5, 6, and 7 may require special considerations for some masonry installations to minimize freeze-thaw cycle risk and is discussed in Chapter 4.

As shown in Fig. 3-4, Colorado is located within a relatively low rainfall area; however, the importance of providing good enclosure design for moisture management is still critical. The impact of snow melt or rain (i.e., liquid water) flowing over or falling on the exterior surface of the building and the masonry cladding creates the most critical loads acting on the enclosure with respect to long-term enclosure durability. As such, two factors further determine the loads that act on the enclosure: 1) the building’s form and features, and 2) the enclosure’s ability to shed water or provide a continuous water-shedding surface.

Fig. 3-3 Colorado and surrounding climate zones as excerpted from Fig. 3-2
Fig. 3-4 Contiguous United States total annual rainfall levels2
Fig. 3-5 On this façade, sunshades above window penetrations, canopy cover over a storefront window, and a parapet coping that projects beyond the wall face deflect or divert moisture and/or sun away from the enclosure.
Photo by Bryn MaRae
Fig. 3-6 Generous roof overhangs, gutter systems, and sloped sill and water table elements all serve to reduce the water load on the building enclosure.
Photo by Bryn MaRae
Fig. 3-7 Balcony elements on this façade are sloped away from the veneer to ensure runoff from rainfall and snowmelt is directed away from the wall system. Sloped sills are proud of the masonry veneer and form a drip to shed water before it has an opportunity to drain onto the masonry veneer below.

Building Form and Features

Region and site-specific climate determine the expected rain, snow, hail, solar, and wind loads. However, a building’s form and features dictate to what degree these loads act on the building enclosure. On a larger scale, building form and features include the building height as well as geometry, inclusive of canopies, balconies, and roof overhangs. On a smaller scale, form and features include the masonry cladding, fenestrations, cornice elements, counterflashings, and drip edges that all act as part of the water-shedding surface. Examples of building form and features on projects that used masonry wall systems are shown in Fig. 3-5, Fig. 3-6, and Fig. 3-7.

Specific to above-grade wall systems, exterior architectural elements such as balconies, canopies, or roof overhangs can deflect rainwater or snow accumulation away from fenestration systems, cladding elements, and building entrances. Conversely, building form and features can increase the severity of loads such as water by concentrating runoff at specific areas of the building enclosure. For example, a canopy that drains water onto masonry wall cladding could cause staining and efflorescence, cladding damage, or worse; water ingress into the building. In Colorado and southern Wyoming, it is especially critical in snowfall areas that a building’s form and features are designed for snow accumulation and snow melt.

Water-Shedding Surface

The water-shedding surface is the outer surface of the building enclosure; particularly, the anchored veneer or CMU wall face at the field-of-wall area. This surface deflects and drains most of the exterior water from the system; thus, the water-shedding surface reduces the water load on the underlying elements of the system. Due to its importance, the water-shedding surface is depicted on the masonry system figures and details throughout this guide.

Control Layers

In this guide, water, air, thermal, and vapor control are described; the control of sound, fire, light, and contaminants are related to the concepts discussed within this guide but are not covered in detail.

Water, air, thermal, and vapor enclosure loads are controlled by specific layers called control layers. These layers comprise systems of materials or stand-alone materials that are intentionally selected and located within the enclosure as shown in Fig. 3-8.

Fig. 3-8 Building enclosure load and control layer relationships

When control layers are intentionally designed to control a specific load, they are said to have a primary relationship with the building enclosure load. Some control layers also control other loads indirectly and form a secondary relationship. Primary and secondary relationships are depicted in Fig. 3-8 with a solid and dashed line, respectively. As an example, the building enclosure load of air, more specifically air pressure, is controlled by the air control layer (the primary relationship). This layer comprises the air barrier system. The air control layer also has secondary relationships; it assists with controlling water, heat, water vapor, sound, and fire.

Using the control layer concept to evaluate assemblies and details follows industry best practices and can be useful to assess specific assemblies and details being considered for a project. Applying the control layer concept can help all parties better understand the role and importance or functions of the systems and materials associated with each layer and can help identify areas where control layers may be missing, discontinuous (if required to be continuous), or inappropriately redundant.

A general summary of the water, air, thermal, and vapor control layers is provided on page 21. Both Chapter 6 for anchored masonry veneer wall systems and Chapter 7 for single-wythe CMU wall systems discuss the control layers specific to each masonry wall system.

Additionally, throughout this guide, control layers are shown on system assembly figures and on detail figures as shown in Fig. 3-9.

System Rain Control Strategies

There are three categories of rain control strategies available for above-grade masonry walls:3
  • Screened and drained (i.e., rainscreen)
  • Mass (i.e., storage)
  • Perfect barrier
Rainscreen and mass strategies are discussed further below. Perfect barrier walls are not included; however, perfect barrier assemblies or materials—such as a conventional roof membrane assembly or window glass—occur in some details.  
Fig. 3-9 Window head detail with wood-framed backup wall and anchored masonry veneer (top). The accompanying water-shedding surface and control layers of the detail are also described (bottom).
Fig. 3-10 Anchored masonry veneer wall with steel stud-framed backup wall structure. Masonry wall systems, which control rain using a rainscreen strategy, include these characteristics.
  1. Cladding: the anchored masonry veneer to shed water.
  2. Air Cavity: behind the veneer to allow for drainage. This cavity may be vented or ventilated.
  3. Drain Holes or Gaps: through the veneer system so drained water can leave the air cavity. Flashings are typically placed at drain hole or drainage gap locations (e.g., ledger supports, base-ofwalls, doors, windows, etc.) to direct draining water to the exterior environment.
  4. Drainage Plane: the face of a water-resistive barrier membrane (i.e., the water control layer). This membrane acts as a drainage plane within the air cavity behind the veneer. Flashings, whether flexible membranes or sheet metal, are also part of the drainage plane.

Rain Control: Rainscreen Strategy

The anchored masonry veneer wall systems described in this guide control rainwater with a rainscreen strategy. This rain control strategy assumes that some water makes its way through the cladding plane
while the building is in service. In this guide, a rainscreen strategy assumes the characteristics discussed in Fig. 3-10.

Rainscreen wall systems can exhibit good resistance to water penetration and are less sensitive to water ingress than the other two rain control strategies. However, good performance relies on implementing proper details and ensuring acceptable construction practices are followed. Improperly executed details are frequently sources of water ingress, critically affecting the moisture performance of the assembly and contributing to premature weathering or efflorescence of the masonry veneer system. 

A continuous air control layer is not a requirement of the rainscreen strategy but is typically provided to control condensation, improve rain control, and to meet energy code requirements.

Rain Control: Mass Wall

The single-wythe CMU wall systems in this guide controls rain through a mass strategy. 

Mass walls control rainwater by absorbing some water and shedding the rest. Enough hygric mass is present within the wall to store the water so it can be released later during times of drying. Because mass walls store water and do not use drainage, they do not require a drainage plane. The greater the mass within the wall, the greater the storage capacity, which is similar to the concept of thermal mass. 

In modern times, mass walls such as CMU walls are thinner than historic mass wall systems. Thinner systems benefit a building’s usable square footage and reduce material or labor costs; however, these walls have less mass and often have continuous mortar joints that may incur hairline cracks. Unless modified, these walls have less moisture storage capacity and would be more vulnerable to rainwater leaks than historic mass assemblies. 

As a result, designs for new mass wall systems must compensate by either only using them in low rain load applications (dry climate, first story, etc.), reducing their exposure (overhangs, drips, etc.) and/or improving their water-shedding characteristics—either with hydrophobic admixtures, with penetrating surface-applied water repellents, or with surface-applied nonpenetrating vapor-permeable coatings. These changes minimize wetting, reduce absorption, and increase watershed while still allowing the wall to dry the small quantities of rain that may still be absorbed.

Perfect Barrier

Perfect barrier walls are not discussed within this guide; however, perfect barrier assemblies or materials (such as a conventional roof membrane assembly or window glass) are common. While further discussion of the perfect barrier category of rain control is beyond the scope of this guide, the glossary contains a definition of a perfect barrier system.

Chapter References

  1. International Code Council. 2018 IECC International Energy Conservation Code. (Country Club Hills, IL: International Code Council, Inc., 2018).
  2. RDH Building Science Inc. United States Total Annual Rainfall Levels. Map. Seattle, WA: n.p., 2017. Data courtesy of National Oceanic and Atmospheric Administration (accessed August 3, 2017).
  3. John Straube. “BSD-013: Rain Control in Buildings.” Building Science Corporation, published August 23, 2011,
  4. ASTM International. ASTM E2178-13 Standard Test Method for Air Permeance of Building Materials. (West Conshohocken, PA: ASTM International, 2013).
  5. ASTM International. ASTM 2357-11 Standard Test Method for Determining Air Leakage of Air Barrier Assemblies. (West Conshohocken, PA: ASTM International, 2011).
  6. John Straube. “BSD-014: Air Flow Control in Buildings.” Building Science Corporation, published October 15, 2007,
  7. ASTM International. ASTM E283-04. Standard Test for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen. (West Conshohacken, PA: ASTM International, 2012).
  8. International Code Council. 2018 International Building Code. (Country Club Hills, IL: International Code Council, Inc., 2017).

Control Layer Summary

Water Control Layer

The water control layer is a continuous control layer designed and installed to act as the innermost boundary against water intrusion. 

The water control layer of an above-grade wall system will vary based on the wall system’s rain control strategy (see “System Rain Control Strategies” on page 19) as described further in Chapter 6 for anchored masonry veneer wall systems and Chapter 7 for single-wythe CMU wall systems.

Air Control Layer

The air control layer controls the flow of air through the building enclosure, either inward or outward. Air flow is significant because it impacts heat flow (space conditioning), water vapor transport, and rain penetration control. The air control layer includes the air barrier system which is:
  • Impermeable to air flow.
  • Continuous across the building enclosure.
  • Strong enough to transfer the forces that act upon on it (e.g., mechanical pressures, wind pressures, and stack effect) back to the structure.
  • Stiff enough that it does not distort such that the performance properties of the system are changed.
  • Durable over the life expectancy of the building enclosure.6
Specific to the scope of this guide and the International Energy Conservation Code (IECC),1 an air barrier material has an air permeance less than 0.004 cfm/ft2 at 1.57 psf (75 Pa) when tested to ASTM E2178.4 An air barrier assembly has an air permeance of less than 0.04 cfm/ft2 at 1.57 psf (75 Pa) when tested to ASTM E23575 and ASTM E283.7 The air barrier system extends to the masonry system’s details and transitions including fenestration systems, the air barrier membrane of a conventional roof system, and the roof membrane of an inverted roof membrane system as well as spray foam (of a minimum thickness, often determined by manufacturer testing) and sealant joints necessary to transition between assemblies.

Thermal Control Layer

The thermal control layer controls the heat flow across the building enclosure. The placement and continuity of the thermal control layer is an important factor of a thermally efficient building enclosure. While all materials within the building enclosure contribute to the thermal control layer, some materials may increase heat flow and be considered a thermal bridge, while others substantially minimize heat flow.

In the masonry wall system, materials that contribute to minimizing heat flow across the system include thermal insulation, low-conductivity framing elements, and thermally improved glazing systems. Materials that increase heat flow and may be considered thermal bridges include some anchored masonry ties and shelf angle supports. Careful consideration of thermal bridges or thermal discontinuities should be addressed by design. 

Chapter 8 sections regarding thermal performance further discuss types of thermal insulation and wall components that create thermal bridges specific to the masonry wall systems addressed in this guide.

Vapor Control Layer

The vapor control layer retards or greatly reduces the flow of water vapor through the building enclosure. The ease with which water molecules diffuse through a layer is known as vapor permeance. The 2018 International Building Code8 (IBC) defines three classes of vapor permeance:

  • Class I: Materials that have a permeance ≤ 0.1 perm (i.e., vapor barrier)
  • Class II: Materials that have a permeance > 0.1 perm and ≤ 1.0 perm (i.e., vapor retarder)
  • Class III: Materials that have a permeance > 1.0 perm and < 10 perms

Although not defined by the IBC,8 a Class IV vapor permeance designation is often used for materials with a vapor permeance > 10 perms. 

Section 1405.3 of the IBC8 defines code requirements for vapor control in Colorado and southern Wyoming climate zones. 

The location and need for a vapor control layer is dictated by the location and vapor permeance of the materials and systems that form the remaining control layers in the above-grade masonry wall system. A vapor control layer may not be necessary for some masonry wall systems. Specific recommendations for vapor control are provided in Chapters 6 and 7.


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