UPDATED on May 15, 2015
Every couple of weeks, someone sends me an e-mail with a description of a proposed wall assembly and an urgent question: “Do I need a vapor retarder?” Energy experts have been answering the same question, repeatedly, for at least thirty years. Of course, even though I sometimes sigh when I read this recurring question, it’s still a perfectly good question.
The short answer is: if your wall doesn’t have a vapor retarder, there is no need to worry. Builders worry way too much about vapor diffusion and vapor retarders. It’s actually very rare for a building to have a problem caused by vapor diffusion.
A while back, I collected seven questions about vapor diffusion, and published them (along with my answers) in a blog called “Vapor Retarders and Vapor Barriers.” Since new questions keep showing up in my In box, I decided it was time for another Q&A roundup on vapor diffusion. Here are nine more questions on the topic.
A. Water vapor is water in a gaseous state — that is, water that has evaporated. It is invisible. It is present in the air we inhale, and (in even greater concentrations) in the air we exhale.
When this invisible water vapor moves through building materials, the phenomenon is called vapor diffusion.
A. In the 1970s and early ’80s, builders were taught that it was important to install a vapor barrier (usually, polyethylene sheeting) on the warm-in-winter side of wall insulation and ceiling insulation. Most textbooks and magazines explained that a vapor barrier was needed to keep the walls dry during the winter, and that walls without vapor barriers would get wet.
This was bad advice, for several reasons. First of all, outward vapor diffusion through walls during the winter almost never leads…