Höyrynvastusluku

Höyrynvastusluku, often translated as vapor barrier rating or vapor permeance, is a measure of how easily water vapor can pass through a material. It is commonly used in construction and building science to assess the performance of materials used in walls, roofs, and other building envelopes.

The unit for vapor permeance is typically perm (US) or ng/(Pa s m²) (SI). A higher permeance value indicates that more water vapor can pass through the material, while a lower value signifies that the material is more resistant to vapor transmission. Materials with very low permeance are often referred to as vapor barriers, while those with higher permeance are called vapor retarders.

Understanding höyrynvastusluku is crucial for controlling moisture within buildings. In colder climates, vapor barriers are typically placed on the warm side of the insulation to prevent interior moisture from migrating into the wall cavity and condensing. In warmer, humid climates, the strategy may be reversed or a more vapor-permeable assembly might be used to allow moisture to dry out.

The selection of appropriate materials based on their höyrynvastusluku helps to prevent condensation, mold growth, and structural damage, thereby improving indoor air quality and the longevity of the building. Factors such as temperature and humidity gradients across the material also influence vapor transmission, so design considerations must take these into account.