Do I Need a Vapor Retarder?
Energy Nerd: Most buildings don’t need polyethylene anywhere, except directly under a concrete slab or on a crawlspace floor.
Every couple of weeks, someone emails me a description of a proposed wall assembly and then asks, “Do I need a vapor retarder?” The short answer I give is that if the wall doesn’t have a vapor retarder, then there probably is no need to worry. It’s rare for a building to have a problem that’s caused by water-vapor diffusion. (Vapor retarders slow vapor movement and have a permeance rating of between 0.01 and 1. With a permeance of less than 0.01, vapor barriers essentially halt vapor flow.) Water vapor is water in a gaseous state — that is, water that has evaporated. The passing of water vapor through building materials is known as vapor diffusion.