dampdiffusieweerstand

Dampdiffusieweerstand, often translated as moisture diffusion resistance, is a property of building materials that describes how strongly a material resists the diffusion of water vapor. In practice it is quantified by the diffusion resistance factor, denoted by μ, a dimensionless number. The higher the μ, the less water vapor can diffuse through the material.

μ is defined relative to a reference material with known diffusion characteristics; in many European contexts this reference is a standard for water vapor diffusion through air. For a multilayer construction, the total diffusion resistance is approximately the sum of the resistances of the individual layers. Materials with low μ are highly permeable to water vapor and support drying of assemblies; materials with high μ act as vapor barriers and slow moisture transfer.

Manufacturers provide μ or related diffusion-permeability data for common materials in data sheets, lab tests, or building codes. In practice, μ is determined by tests that measure steady-state vapor diffusion or inferred from vapor permeability data. Standards such as EN 1931 are commonly used in Europe, while ASTM standards are used in other regions. The diffusion resistance is influenced by porosity, pore connectivity, temperature, and moisture content, so values vary with environmental conditions.

Understanding dampdiffusieweerstand is important for hygrothermal design, condensation risk assessment, and ensuring proper drying of buildings. It interacts with factors such as relative humidity, temperature, and air exchange, and is used in moisture transport models to predict indoor humidity distribution and the risk of mold or degradation in building envelopes.