vesihöyrynläpäisevyyttä

Vesihöyrynläpäisevyys, often translated as water vapor permeability, refers to the ability of a material to allow water vapor to pass through it. This property is crucial in many applications, particularly in construction and textiles. In buildings, a material with good water vapor permeability allows moisture that accumulates within the building envelope to escape to the outside, preventing condensation and potential damage to building materials. This is often described by its "Sd value" or diffusion equivalent air layer thickness, where a lower Sd value indicates higher permeability.

In textiles, water vapor permeability is a key factor in determining breathability. Fabrics that are highly permeable allow perspiration vapor to escape from the body, keeping the wearer dry and comfortable, especially during physical activity. This is a vital characteristic for outdoor and activewear. Conversely, materials with low water vapor permeability act as barriers, preventing moisture passage in both directions. This can be desirable for waterproofing or for creating controlled environments, but it can also lead to discomfort if moisture is trapped. Factors influencing a material's water vapor permeability include its porosity, chemical composition, and thickness.