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.

WHAT IS AN AIR BARRIER, AND WHERE CAN I BUY ONE?

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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