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Vapour barrier

A vapour barrier is a membrane on the warm side of the insulation that limits how much moisture from indoor air can migrate into the wall or roof.

Also known as vapor barrier, vapour control layer, VCL, vapour retarder, air barrier

Indoor air carries moisture from cooking, washing and breathing. If it diffuses or leaks into a cold wall, it can condense inside the construction and cause mould and rot. The vapour barrier, usually a polyethylene sheet or a vapour-control board, stops that.

It only works if it is continuous. Laps must be sealed and every penetration for cables and pipes taped, which is why many designs add a service cavity inside the barrier so installations do not need to pierce it.

Related terms
Building envelopeThe building envelope is the physical barrier between inside and outside: the external walls, roof, lowest floor, windows and doors. Wall panelA wall panel is a prefabricated section of wall: a timber or steel frame, often already fitted with sheathing, insulation, windows and services. It is built in a factory and assembled on site. Moisture content (timber)Moisture content is the weight of water in a piece of wood expressed as a percentage of its dry weight.
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