Vapour barrier is a layer of building material (e.g. vapour barrier foil) whose function is to prevent the passage of water vapour from inside the building into the building structure (walls, ceilings). It protects against condensation inside the building envelope, which can lead to serious problems such as material degradation, corrosion, weakened thermal insulation or the growth of mould and mildew.
The vapour barrier is placed on the 'warm' side of the envelope, on the inside of the building. In this way, water vapour that naturally seeks to penetrate to the outside of the building is prevented from penetrating into the building envelope.
