Vapor Retarders and Air Barriers: Managing Moisture in Building Envelopes

When one side of a wall, roof, or foundation assembly is colder than the other, moisture in the air can condense on a cold surface inside the assembly, potentially causing mold problems and structural decay. Moisture from air can get into a wall cavity through air leaks or, in smaller quantities, by diffusing through a permeable material such as drywall. Many people think in terms of vapor barriers addressing both of these problems, but there are two distinct functions: preventing air leakage, accomplished with an air barrier; and controlling moisture diffusion, which calls for a vapor retarder.

The air barrier can be installed toward the inside, toward the outside, or on both sides of an assembly (see EBN Vol. 17, No. 6). It has to be continuous because even a small hole can leak lots of moist air. With vapor retarders, on the other hand, complete coverage is less important than placement.