Primer

How "Smart" Vapor Retarders Work

Vapor retarders are supposed to keep our buildings dry, but they sometimes keep them wet instead. Enter the "smart" vapor retarder.

August 27, 2012

Vapor retarders are meant to protect our building assemblies from getting wet, but they can also slow drying. When humidity levels differ drastically between indoors and outdoors, vapor diffusion can result, sending water vapor into wall cavities and other building assemblies. A dedicated vapor retarder manages this “vapor drive.”

But moisture seldom enters a wall or roof cavity just because of vapor diffusion—leaky roofs and poorly detailed walls that leak vapor-laden air are much more common culprits—and once things get wet, vapor retarders can trap moisture, contributing to mold and rot.