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Air Leaks Waste Energy and Rot Houses

One third of the energy you pay for probably leaks through holes in your house. Air leaks can also cause moisture and indoor air quality problems.
Drawing of how air affects a house -- wind, stack effect, and mechanical equip.gif

The three most important forces affecting air movement in homes are equipment fans, wind, and the stack effect.

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 biggest effect on the durability of a house. Uncontrolled air flow through the shell can not only carry moisture into framing cavities, causing mold and rot, it can account for a huge portion of a home’s energy use and cause indoor air quality problems to boot.

So tight houses are good houses, right?

Tight houses are better than leaky houses — with a caveat: tight houses without a ventilation system are just as bad as leaky houses 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 leaks but from a known source; for this to happen, the house needs an adequate air barrier [see “What is an air barrier and where can I buy one?,” below] and a controlled ventilation path.

In leaky homes, large volumes of air — driven by exhaust fans, the furnace fan, the stack effect, and wind — can blow through the home’s floor, walls, and ceiling. Because air usually contains water vapor, these uncontrolled air leaks can cause condensation and mold.

According to Dr. John Straube of the Building Science Corporation, the only way you can know for sure that a home’s incoming air is clean is to know where it’s coming from. “People who say, ‘I want the walls of my house to breathe’ are really saying, ‘I want to rely on mistakes made by the plumber and the electrician to provide me fresh air. I don’t want to seal up my house because I want to rely on mistakes to breathe.’ And that’s exceptionally dangerous,” Straube says.

It’s no wonder that ‘_Build Tight, Ventilate Right_’ is the battle cry of building scientists.

Dead squirrels make bad air filters

Any air that enters a house through leaks in the envelope may be loaded with pollutants. As Straube points out, you can’t rely on breathing through the dead squirrel in your attic or the SUV in your attached garage to provide you and your family with fresh air.

Many indoor air quality issues are related to poor control of air flowing through an enclosure that has been damaged by exposure to moisture, heat, or UV rays. According to Straube, 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.”

Air barriers in the building code — Canada: 1; USA: 0

It’s important to control air flow — not only air flow through the building enclosure, but also air flow within the building enclosure: from room to room and between basement, living space, and attic.

According to Straube, the importance of an air barrier is recognized in Canada — where the national building code has required one for almost 20 years now. In the US, it’s absent from state energy codes, ASHRAE’s Energy Efficiency Standard (ASHRAE 90.1), and the International Residential Code, “even though we’ve known from a research point of view for probably 30 years that this is a really big deal,” Straube says.


An air barrier consists of materials assembled and joined together to prevent air leakage between the conditioned space and unconditioned space — that is, indoors and outdoors.

Housewrap is not (necessarily) an air barrier

Some products — for example, drywall, plastic, or housewrap — are considered air barriers. But when building scientists talk about a home’s air barrier, they’re not talking about a single material; they’re talking about a collection of materials that reaches from the basement slab, around the entire exterior of the building’s thermal envelope, and across the finished ceiling.

A typical air barrier incorporates more than a dozen materials

Common components of an air barrier system are poured concrete; sill seal; wall sheathing; housewrap; contractors’ tape; caulk; spray foam; gaskets; window glass; drywall; polyethylene; and weatherstripping. If any of these materials are leaky, the home’s air barrier may be compromised.

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  1. scott | | #1

    Love the "dead squirrel"
    Love the "dead squirrel" analogy.

  2. Jason | | #2

    old home air leaks how should a person go about properly (or at least to the best the building will offer) air sealing a 1945 house? I have a 2-story cape cod with full basement, hydronic heat, no insulation in walls, brick on lower level, wood siding on upper. I do have new windows. Natural gas bills well over $200 in winter. My original exterior sheathing is actually 18" tall x 8' wide sheetrock with what looks like (hopefully!) a light building paper (black) backing. No other vapor/moisture barrer. The sheetrock strips were was also used as lath on the interior, with plaster over it. This has me a little concerned about blown-in insulating the walls, with respect to condensation. thanks for any comments.

  3. GBA Editor
    Martin Holladay | | #3

    Response to Jason
    The usual approach is called blower-door-directed air sealing. It is performed by a weatherization contractor or a home performance contractor. To learn more, see Blower Door Basics.

  4. 7u8zfUR5vB | | #4

    Dr. Doesn't mean smarter. Proof, is Mr. Sraube.
    You think "controlled" ventilation is better? Try building hundreds of homes a year, and realizing that the un-knowing occupant is truely now in control of the "controlled" fresh air. As a Dr. you dare compare a SINGLE EASILY DISTURBED source of fresh air as "safer" or "cleaner" air than natural air change? What happens when your controlled ventilator that is a moving part bound to fail....fails? What is worse, dead squirrel in your attic, or a dead rodent in your ONLY pipe bringing in fresh air???? Hmmm? Grandma sprays a wasp nest not knowing it is on their fresh air inlet? Will the homeowner automatically know how to check/maintain the fresh air system? btw Dr., news flash....most air leaks are not penetrations "mistakes" but they are doors, windows, and EXPANSION GAPS REQUIRED BY SHEATHING MANUFACTURERS. Why are these gaps actually the best source of fresh air possible today? Because they are 1/8" all the way around the house, bulletproof, 100% foolish occupant proof fresh air. Should you have to filter outdoor air to make it good quality air? Oh crap the Dr. didn't mention outdoor air is the STANDARD of air quality. Dr. Needs to find a different subject to talk about, tell me something, if I asked you "If every home in the country had an ERV and supertight house, how many houses in this country would have a 100% properly designed/maintained/operating fresh air system?" Unless you can whole-heartadly say ALL HOMES WOULD RECEIVE .35ACH OR BETTER, 100%, than your WHOLE argument on this page is you wasting people's time with YOUR ignorance. HEALTH always trumps some Dr.s dream energy savings. Try something that will help you Dr, try building homes for a while before you mislead people on how it should be done! Give back your doctorite! I lived in a home over 130 years old, no air barriers, no thermal barriers, no was in EXCELLENT condition. Guess we should have had more Dr.s input 130 years ago, so that houses can last longer? Controlled fresh air can work, as long as a Dr. in home energy lives there and takes care of the system. For the rest of housing, please take this page based on home building ignorance off of the web.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Greg Hecker
    Because your comments are laced with so many insults directed at John Straube, it's hard to determine your point. But, if I'm guessing correctly, it sounds like you prefer to live in a leaky house without a mechanical ventilation system instead of a tight house with a mechanical ventilation system.

    Although plenty of people live in houses like that, there's a big problem with such houses: they use a lot of energy to keep warm in the winter and cool in the summer. And wasting energy (only to have the energy escape through leaks in the home's envelope) is a luxury that most people -- and, as it turns out, the entire planet -- can no longer afford.

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