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.

Glossary

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.

Related terms

Building envelopeThe physical separation between the conditioned interior and the outdoors, comprising the roof, walls, windows, doors, floors and foundation.Crawl space encapsulationSealing a crawl space with a heavy vapour barrier over the ground and up the walls, usually combined with insulation, air sealing and humidity control.Drip edgeMetal flashing installed along roof eaves and rakes that directs runoff into the gutter and away from the fascia and sheathing.EfflorescenceThe white, powdery mineral deposit left on masonry when water passes through it and evaporates at the surface.

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