A vapor barrier (or vapour retarder) is a material—typically polyethylene plastic sheeting—installed in building assemblies to control moisture movement and prevent condensation problems. In Canadian climates, vapor barriers are typically installed on the warm side of insulation in walls and ceilings to prevent indoor moisture from reaching cold surfaces where it could condense. Proper vapor barrier installation prevents moisture damage, mould growth, and insulation degradation. Building codes specify vapor barrier requirements based on climate zones. Vapor barrier effectiveness depends on continuity—gaps, holes, and improper sealing reduce performance. The term 'vapor retarder' acknowledges that materials slow rather than completely stop moisture movement. Vapor control is one component of overall building moisture management.