housewraps and vapor barriers

By now, almost all builders know the importance of installing a water-resistive barrier (WRB) behind siding. Most types of siding leak, so it’s a good idea (and a code requirement) to install a WRB to protect your wall sheathing from any wind-driven rain that gets past the siding.

Nowhere in building design has there been more confusion or more dramatic change in recommended practice than with vapor retarders. Thirty years ago, we were told to always install a polyethylene (poly) vapor barrier on the warm side of the wall. Then we were told to forget the poly and go with an airtight layer of drywall (airtight drywall approach). Insulation contractors, meanwhile, often said to skip the vapor barrier; we need to let the wall or ceiling cavity dry out.