![]() ![]() In warmer climates, we were told to install the poly vapor retarder on the outside of the wall cavity, because the inside of the air-conditioned house was colder than the outside. By installing the vapor retarder on the inside of the wall, we would keep that moisture out of the wall cavity where that water vapor might condense. ![]() As vapor-laden air cools off, it is able to hold less moisture, and if it gets cold enough the moisture in the air will condense (i.e., it reaches the dew point) - causing problems by wetting the insulation or rotting wood framing. The idea was that we wanted to prevent water vapor from migrating from inside the house (where it was warmer) outward through the building envelope. Smart Vapor Retarders for Walls and Roofsįorget Vapor Diffusion - Stop the Air Leaks! The rule was to install it on the “warm side.” In northern climates, that meant that the vapor retarder should be on the inside (installed on the inner face of wall studs and rafters) before installing drywall. We installed one of these new materials on our house.īack when poly was the default choice as a vapor retarder (called vapor barriers back then), the recommended placement of that layer varied depending on where you lived. Some experts are now looking to vapor retarders whose vapor permeance changes based on the humidity conditions. And I’m not sure we’re totally out of the woods yet. Insulation contractors, meanwhile, often said to skip the vapor barrier we need to let the wall or ceiling cavity dry out. Then we were told to forget the poly and go with an airtight layer of drywall ( airtight drywall approach). Thirty years ago, we were told to always install a polyethylene (poly) vapor barrier on the warm side of the wall. Nowhere in building design has there been more confusion or more dramatic change in recommended practice than with vapor retarders. ![]()
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