Building scientists have learned a great deal about air barriers since the 1970s. They now recognize that air barriers are key to how long a building will last, how much energy it will require to heat and cool, and how comfortable its occupants are going to be. In high-performance houses, few building components are more essential, and building and energy codes have become much more stringent. As a result, houses built to code now must be relatively tight. Building a house to the Passivhaus standard is impossible without a carefully detailed air barrier.

Getting rid of unwanted air leaks accomplishes three essential goals:

There is no single method, and no single material, that makes an effective air barrier. Instead, air barriers are really a number of different materials that work together. Everything from caulk and spray foam to rubber gaskets, drywall, housewrap and sheathing can be part of the mix. What counts is how these materials are installed. The success or failure of the air barrier depends largely on a number of tradespeople understanding how an air barrier works, and their role in ensuring its integrity.

Although there are lots of ways of getting to the same goal, there are two rules of thumb that apply: the air barrier should be next to the insulation layer, and it should be continuous.

Vapor barriers and air barriers serve two different functions. Vapor barriers stop (or at least slow down) moisture that moves by means of diffusion. Air barriers, on the other hand, stop the movement of air. Air barriers can be vapor-permeable or vapor-impermeable.

When it comes to keeping unwanted moisture out of wall and roof cavities, air barriers are much more important than vapor retarders. Far less moisture is transported by diffusion than by an air leak. A vapor barrier…