Vaporpermeable

Vapor-permeable, sometimes written as vapor permeable, refers to materials or membranes that allow water vapor to diffuse through them more readily than liquid water. These materials are designed to resist liquid water while permitting moisture in the form of vapor to pass, enabling moisture to dry from a building assembly or product surface.

In construction and housing, vapor-permeable materials are used to manage moisture within walls, roofs, and other assemblies. They aim to reduce the risk of condensation and mold by letting trapped moisture escape rather than accumulate, while still providing barriers to rain, wind, or other liquid water. This approach is part of a broader moisture management strategy that also considers air leakage, thermal performance, and drying potential.

Moisture movement through vapor-permeable materials occurs primarily by diffusion of water vapor. The rate of transmission is quantified as water vapor transmission rate (WVTR) or permeance, with values varying by material composition, thickness, temperature, and humidity. In practice, products marketed as breathable or vapor-open often feature non-woven or micro-porous structures and are used as exterior weather barriers or interior vapor retarders, depending on climate and building design.

Key considerations include compatibility with other layers in a system, the local climate, and the risk of moisture being trapped if air barriers or vapor barriers are not properly integrated. While vapor-permeable materials aid drying, they do not provide full liquid water protection and must be used with appropriate weather protection and drainage details.