Moisture-variable vapour barriers (other names: climate membrane, humidity-adaptive or intelligent vapour barrier) usually consist of 1- to 3-layer films made of polyethylene (PE), polypropylene (PP) or polyamide (PA) and have a variable vapour diffusion resistance. At low relative humidity, i.e. in winter, they are highly diffusion-inhibiting, at high relative humidity, i.e. in summer, they are very permeable to diffusion. Due to the warm outside air and the sunlight on the roof in summer, the diffusion flow runs through the roof from the outside to the inside. Any moisture that may have accumulated in the roof flows to the inside and can escape through the vapour-permeable vapour barrier. Conversely, the penetration of warm, humid air from the interior into the roof structure in winter is prevented by the high Sd value.
Source: bauwion
