The term vapour barrier describes a diffusion-inhibiting component layer that is intended to reduce or slow down the entry of moisture into another component in the form of water vapour. According to DIN 4108-3, vapour retardants are defined as diffusion-inhibiting layers with an sd value (water vapour diffusion equivalent air layer thickness) of 0.5 to 1500 m. In contrast, vapour-tight vapour barriers (according to DIN: diffusion-tight layer) have an sd value of at least 1500 m. 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.
Source: bauwion
