Humidity-variable directional vapour barrier

The present invention relates to a humidity-variable directional vapor barrier, its use for the sealing of buildings as well as a system comprising this vapor barrier. Vapor barriers are usually used in roof structures of buildings in order to control the diffusion of water vapor into the structure. They are designed to prevent the penetration of humidity into insulating layers, but allow at the same time the drying of a building. Known vapor barriers have an issue in that in spaces with high humidity, like the bathroom or kitchen, there is a high water vapor diffusion in winter, which may lead to the condensation of water in the roof insulation and/or in the insulation of outer walls and thus to structural damage. Likewise, a condensation of water may occur in the roof structure. The present invention provides a vapor barrier comprising at least two layers, of which one layer is humidity-variable and the other layer essentially is not humidity-variable. Thus, the present invention relates to a variable-humidity directional vapour barrier which comprises at least two layers, wherein one layer (layer 1) is humidity-variable and the other layer (layer 2) is humidity-independent. For layer 1, the quotient for the water vapour diffusion resistance from the sd value at 25% mean relative air humidity to the sd value at 71.5% mean relative air humidity is greater than 3. For layer 2, the quotient for the water vapour diffusion resistance from the sd value at 25% mean relative air humidity to the sd value at 71.5% mean relative air humidity is less than 1.5. Furthermore, the invention relates to the use of the vapour barrier for sealing buildings and to a system which comprises said vapour barrier. Furthermore, the invention relates to the use of a certain film for sealing a space in buildings which is closed by an exterior skin, wherein the film comprises defined film sides X and Y or layers 1 and 2 and is arranged in a certain way.