The term vapour barrier describes a diffusion-tight component layer that is intended to prevent moisture in the form of water vapour from penetrating into another component and causing damage there. According to DIN 4108-3, vapour barriers are defined as adiffusion-tight layer with a sd value (water vapour diffusion equivalent air layer thickness) of at least 1500 m. In contrast, the less dense vapour barriers (DIN 4108-3: diffusion-inhibiting layer) have a sd value of 0.5 to 1500 m.
Most vapour barriers are made of plastic films or cardboard or paper laminated with aluminium. The incorporation of mesh meshes increases the tear strength of the sheets. Due to the aluminium coating, most vapour barriers also have an insulating effect and can thus contain immissions such as electrosmog.
Typical areas of application for vapour barriers or barriers are on the inner (warm) side of roof insulation in order to prevent its moisture penetration by water vapour diffusion from the inside to the outside. This is especially the case for non-rear-ventilated roofs with vapour-proof materials such as sheet metal coverings, flat roof foils and green roofs. However, vapour barriers are hardly used for pitched roofs anymore due to the possible massive moisture damage caused by damage to these membranes and the development of newer products (see moisture-variable vapour barrier).
Source: bauwion
