A house that leaks air

In modern home construction, we attempt to build a tight envelope. While most do an acceptable job of sealing up some of the homes components, more often than not, air sealing is overlooked. This can have some pretty serious consequences.

Approximately 30% of the energy your home produces leaks through the various gaps, cracks, and holes in your house.

Controlling air leaks is the second most important job of the buildings envelope. Next to rain, air leaks through walls, roofs, and floors can have the most damaging effect on the longevity of a house. Uncontrolled airflow through the shell not only carries moisture into framing cavities, causing mold and rot, but it also can account for a huge portion of a home’s energy use and can cause indoor-air-quality problems.

A tight house is better than a leaky house, with a caveat: A tight house without a ventilation system is just as bad as a leaky house with no ventilation system—maybe worse. energy efficiency requires a tight shell; good indoor-air quality requires fresh outside air. Ideally, the fresh air should come not from random accidental leaks of unknown size and quantity, but from a known source at a known rate. For this to happen, the house needs an adequate air barrier and a controlled ventilation path.

In a leaky house, large volumes of air—driven by exhaust fans, the stack effect, and wind—can blow through the floor, walls, and ceiling. because air usually contains water vapor, these uncontrolled air leaks can cause condensation, mold, and rot.

What pushes and pulls air through a house? Three things: wind, fans, and the stack effect. Wind is somewhat predictable, or at least its average speed and direction are. Fans include kitchen and bath exhaust fans, HVAC equipment, fans, and clothes dryers. The stack effect generates pressure because warm air rises, pushing up and out on the ceiling in cold weather (see “How It Works” in FHB #213 and at FineHomebuilding.com). Peak wind loads that are listed in the model building codes are fairly high (usually more than 20 lb. per sq. ft. or 1000 pascals). On average, however, local wind pressure is quite a bit lower. For a house, 5 Pa is likely, and in a high-rise building, maybe 40 Pa, 50 Pa, or even 60 Pa. The pressure exertedby a blower door—50 Pa—is roughly equal to the pressure of a 16-mph wind or around 1 lb. per sq. ft.

A low-slope roof (less than 3-in-12 pitch) is usually under negative pressure when the wind blows over it. Air is sucked up through the roof because the aerodynamics of the wind passing over the roof’s leading edge cause negative pressure. On a house with a steep roof (more than 3-in-12), the pressure is positive on the windward side and negative on the leeward side.

Wind is highly complicated, however. When wind tries to flow around buildings, the highest pressure is on the “sweet spot” in the middle. As the wind goes around corners, it creates large swirls and negative pressures (visible on snowy roofs after a high wind). Wind-related structural damage often occurs at these high-pressure spots or at areas of low-pressure swirling.

Wind exerts positive pressure on the windward walls of a building, causing air leaks on the side of the building facing the wind.

On the leeward side, negative pressure sucks indoor air through walls and windows.

In short, air that leaks through your walls, windows, roof, or basement can result in nothing but trouble. Building tight and ventilating right remain the best advice.




This article was originally produced by John Straube, Ph.D., P.Eng who is a professor of building science at the University of Waterloo in Ontario.