A vapour barrier is a diffusion-tight component layer that prevents water vapour from penetrating other components and thus prevents moisture damage. The classification into the categories vapour barrier and vapour barrier is regulated in DIN 4108-3. Accordingly, vapour barriers are defined as diffusion-tight layers with an sd value (water vapour diffusion equivalent air layer thickness) of at least 1500 m. For the less diffusion-tight vapour retarders (DIN 4108-3:diffusion-inhibiting layer), the sd value is between 0.5 and 1500 m.
Vapour barriers are usually made of plastic films laminated with aluminium, bitumen welding membranes, bitumen roofing membranes or polyethylene films. Mesh meshes significantly increase the tear resistance of vapour barriers.
The requirements for the sd value of a system-compatible membrane can be created or checked with the help of a diffusion calculation according to DIN 4108, Part 3, Section A6.
Vapour barriers must always be installed on the inner (warm) side of a wall structure in order to prevent moisture penetration by water vapour diffusion from the inside to the outside.
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