Water vapour diffusion describes the passage of moisture through a component in the form of water vapour. The critical models in building physics assume relatively cold, dry air in winter, where vapour diffusion usually takes place from the inside to the outside. In addition to the moisture release by the users (bathing, cooking, washing, sweating, plants), this also results from the remaining building moisture, e.g. from damp construction site screeds, plaster, mortar and concrete components.
With the same humidity inside and outside, there is already a vapour pressure gradient because the cold outside air can absorb less steam than the warmer air inside. This vapour diffusion can lead to condensation in components and consequently to damage to the structure. All exterior components such as wall structures or roof structures must therefore be proven with regard to the condensation water precipitation, which is usually done in the thermal insulation certificate.
Source: bauwion
