höyrynvastusluvulla

Höyrynvastusluku, often translated as vapor retarder or vapor barrier, is a material or system designed to control the diffusion of water vapor through building assemblies. Its primary function is to prevent moisture from migrating from warmer, more humid interior environments to colder, drier exterior environments, where it can condense within wall or roof structures. This condensation can lead to a range of problems including mold growth, material degradation, and reduced thermal performance of insulation.

The effectiveness of a vapor retarder is measured by its permeance, which is the rate at which water vapor can pass through it. Permeance is typically expressed in perms (US) or ng/(s·m²·Pa) (SI). Materials with low permeance are considered effective vapor retarders. Common vapor retarder materials include polyethylene sheeting, foil-faced insulation, and certain types of paints and coatings. The placement of a vapor retarder within a building envelope is crucial and depends on the climate. In colder climates, the vapor retarder is typically installed on the warm side of the insulation, while in mixed or hot-humid climates, it may be placed on the exterior side or a vapor-permeable material might be used on the interior. Proper installation is key to ensure the vapor retarder is continuous and free of holes or gaps to maintain its effectiveness.