October 1, 2012

Abstract: 

One-third of the energy you buy probably leaks through holes in your house. Reprinted with permission from Fine Homebuilding, October/November 2012, pages 45-49.

Stopping air is the second-most-important job of a building enclosure. Next to rain, air leaks through walls, roofs, and floors can have the most damaging effect on the durability 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 outdoor 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—as seen below.

Leaky rim joists matter. Floor and wall connections offer many airleakage opportunities. The wall sheathing on this house in Minnesota experienced serious rot because ductwork in the floor framing pulled moist air into poorly sealed rim joists.

The only way you can know for sure that the air coming into a house is clean is to know where it’s coming from. People who say “I want my house to breathe” are really saying “I want to rely on the mistakes that were made by the plumber and the electrician to provide me with fresh air.” That’s exceptionally dangerous. Any air that enters a house through leaks in the building envelope may be loaded with pollutants. The dead squirrel in your attic and the SUV idling in your garage are not going to provide you and your family with fresh indoor air.

Many indoor-air-quality problems are related to poor control of air flowing through an enclosure that has been damaged by exposure to moisture, heat, or UV-rays. Good indoor-air quality comes from having a good air barrier. Only with a good air barrier can we know where the air is coming from and have a chance that air quality (and quantity) can be controlled.

The importance of an air barrier is recognized in canada, where the national building code has required one for 25 years. In the United States, it’s absent from state energy codes and has just recently been added to the 2009 version of ASHRAE’s energy efficiency Standard (ASHRAE 90.1). In 2006, the International Residential Code tightened up the language to require walls to be sealed, and as of 2009, the IECC requires airtightness testing.

Wind can push drafts through a house; air barriers push back many houses built to code are leaky. Air leaks can be responsible for a third or more of the energy loss in typical houses.

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.

The stack effect: When buildings act like chimneys

Like wind, the stack effect can push large volumes of air through an envelope. In winter, warm air in a heated building is lighter (less dense) than cold air outside; that warm bubble of air wants to rise up and out. The flow of air leaving the top of the building draws cold air in through cracks at the bottom. The reverse happens in summer, when hot air outside an air-conditioned house can push cooler indoor air down from the ceiling and out cracks in the basement. This can cause moisture problems on the top floor as humid exterior air is drawn through leaks in the upper floor’s walls—especially for houses with leaky rim joists.

The differences in temperature and pressure are less during the summer than during the winter. When it’s cold outside, the pressure created by the stack effect is 4 Pa per story; when it’s hot, the pressure is about 1.5 Pa per story. However, unlike most other pressures, the stack effect acts every hour of every cold day, so the flows generated by the stack effect are significant.

HVAC equipment can overpower wind and the stack effect

Air pressure created by fans—particularly range hoods, dryer exhausts, and large vent fans—can overwhelm wind and the stack effect. If we blow air into a building, we pressurize it. A 10-Pa pressurization spreads through a building, and pressure increases everywhere. Negative pressurization drops pressure everywhere around the building.

High-end houses often have large, powerful downdraft range hoods with exhaust fans that are rated at 1000 cfm or more. If you don’t provide makeup air for such a large volume of exhaust, the whole house will become strongly negatively pressurized. You’ll start sucking on the garage, and you’ll breathe air sucked backward through . . .

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