wasserdampfdurchlässigen

Wasserdampfdurchlässigkeit, in English water vapor permeability, is the property of a material that determines how readily water vapor can diffuse through its structure. It is governed by the material’s porosity, pore connectivity, thickness, and by external conditions such as temperature and humidity.

The measure of diffusion is expressed by several related quantities, including the diffusion coefficient, the water vapor permeability, the permeance, and the diffusion resistance. In building physics, diffusion resistance is often represented by the dimensionless diffusion resistance factor μ (relative to air), while permeance and permeability quantify the rate at which vapor passes through a given thickness. Units vary by method, with common SI units such as kg/(m·s·Pa) for permeability and, in other systems, terms like permeance or “perms.” The diffusion process is driven by gradients in water vapor partial pressure, which depend on temperature and humidity.

Testing and standards: established methods include the cup method (desiccant or humidified) per standards such as EN 1931 and ISO 12572, and in North America ASTM E96 for water vapor transmission rate. Results are influenced by sample thickness, boundary conditions, and, in some cases, hygroscopic sorption properties of the material.

Application and design: in building envelopes, higher Wasserdampfdurchlässigkeit supports outward drying and moisture management when climate, ventilation, and indoor humidity are properly controlled. Materials with very low diffusion resistance can act as vapor barriers, reducing drying potential if not paired with appropriate design. Conversely, highly permeable materials can mitigate condensation but may require careful detailing to avoid unintended moisture ingress.

Limitations: permeability alone does not determine moisture performance. Surface properties, sorption behavior, condensation risk, and overall climate strategy must be considered in an integrated design approach.