Vapour diffusion (construction)

Vapour diffusion means: (water) vapour molecules move through a structural element. In case of different vapour pressures, the vapour molecules always move into the direction of the lower value. The vapour pressure depends on the temperature and the air humidity. Example exterior wall of a building: In winter the vapour molecules move from the inside to the outside and in summer from the outside to the inside. Vapour barriers and vapour retarders, as e.g. special foils, injections, etc., are used to stop resp. reduce vapour diffusion.