Vapour retarder
A material that slows the diffusion of water vapour through an assembly, used to keep moisture from condensing inside walls, floors or ceilings.
What is Vapour retarder?
Vapour retarders are climate-specific and direction-specific, which is why they are so often installed wrong. In a heating-dominated climate the retarder generally belongs on the warm-in-winter side; in a cooling-dominated climate that logic can reverse, and a poorly placed interior polyethylene sheet can trap moisture and cause rot. Common findings include a vapour retarder installed on both sides of an assembly, kraft-faced batts installed backwards in a crawl space, and polyethylene under a finished basement wall. It is also worth distinguishing vapour diffusion from air leakage — air leakage moves far more moisture, and air sealing usually matters more.