Höyrysulun

Höyrysulku, often translated as vapor barrier or vapor retarder, is a material or membrane used in building construction to prevent or control the diffusion of water vapor through building assemblies. Its primary function is to manage moisture within walls, roofs, and floors, preventing condensation that can lead to mold, rot, and structural damage. The effectiveness of a höyrysulku is determined by its permeance, which measures how easily water vapor can pass through it. Materials are classified by their permeance ratings, ranging from impermeable (less than 0.1 perms) to highly permeable (10 perms or more).

In colder climates, the höyrysulku is typically installed on the warm side of the insulation, meaning the interior side of the exterior wall assembly. This placement prevents warm, moist indoor air from reaching the cold exterior sheathing, where it could condense. In contrast, in hot and humid climates, the höyrysulku might be placed on the exterior side of the insulation to prevent humid outdoor air from entering the wall assembly and condensing on cooler interior surfaces. The correct placement is crucial and depends on the climate and the specific building design. Common materials used as höyrysulku include polyethylene sheeting, foil-faced insulation, and certain paints or specialized membranes. Proper installation, including sealing all seams and penetrations, is essential for the höyrysulku to perform effectively.