Vapour barrier

Vapour barriers are materials made from foil, that are used wherever building materials, usually insulation products, classically mineral wool, are to be protected from moisture penetration. They are used especially for the insulation of pitched roofs, but also for flat roofs or partition walls in dry construction. They form a vapour-impermeable layer between the cladding and the insulation material. On the one hand, this is to prevent the loss of warm air from the interior to the outside, but at the same time it is also to prevent air saturated with water from passing through the building components and then precipitating the water in the insulation layer - which must inevitably lead to mould in the long term.

Since both water vapour and temperature are transported with the air, a vapour barrier fulfils both purposes. However, a complete seal is usually not desired at all, as this would also prevent the drying of building components if water were to get in somehow. For this reason, vapour barriers that completely prevent the passage of air are only a niche product in today's building industry. Vapour barriers come in a variety of designs. One of the most important distinguishing features is the so-called Sd value. This indicates the diffusion resistance for a water vapour molecule. Specified in metres, this means that such a molecule needs the same time to pass through the foil as the same distance through a standing layer of air. This means that the higher the Sd value, the tighter the barrier.