Vapor retarders have been one of the most confusing aspects of building design in terms of recommended practice and application over the last thirty years. Initially, it was recommended that vapor retarders be installed on the warm side of walls in the form of a polyethylene (or poly) vapor barrier. After that, it was recommended that the poly be left out in favor of airtight drywall, although there were others recommending that the vapor barrier be skipped altogether so that cavities could dry out. To this day, the confusion continues and the proper direction has still not been established. The latest idea is a smart vapor retarder that can react to humidity conditions.
Read the full article here: Smart Vapor Retarders