Vapour barrier

A vapour barrier is the layer in a building component which reduces the diffusion of water vapour in the construction. The layer is defined by the vapour diffusion equivalent air layer thickness s_d, which defines the thickness of an air layer providing the same resistance to vapour diffusion as a layer of a material with the thickness d, and the vapour diffusion resistance µ as given by s_d = µ x d [m].

A layer in a building component with a vapour diffusion equivalent air layer thickness s_d ≥ 1500 m is by definition a vapour seal. Membranes possessing a diffusion equivalent air layer thickness which depend on the relative humidity are called moisture adaptive vapour barriers.