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 sd, 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 sd = µ x d [m].
A layer in a building component with a vapour diffusion equivalent air layer thickness sd ≥ 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.
