An air barrier or air retarder is any material on a building that prevents the movement of air from the interior to the exterior (infiltration) or from the exterior to the interior (exfiltration). Also called housewraps, these materials are barriers to air and liquid water but not to water vapor. Typical exterior housewraps are not vapor retarders and will allow water vapor to diffuse easily through them.
Infiltration can comprise almost 50 percent of all heat loss from a home during the winter months. The major sources of air infiltration are sill plates (25 percent), wall outlets (20 percent), and duct systems (14 percent), followed by windows (12 percent).1 These sources can be reduced by the use of caulking, weatherstripping, and air infiltration barriers. It is important to note that homes need some fresh air to remove odors, chemicals from interior sources, and even exhaust gases. Tests have shown that a building with a poorly air-sealed exterior suffers accelerated heat loss or gain because of outside air infiltration into
Figure 4.3 Housewrap. (Owens Corning)
the wall cavity. Exterior air retarders protect the wall from the effects of weather and help eliminate air infiltration.