Vapour pressure distribution is the way water vapour pressure is divided across the individual layers of a wall, roof or floor build-up. By calculating the pressure drop in each layer, builders can determine where interstitial condensation is most likely to occur. It is a key concept in building physics and the basis of the Glaser method.
How does vapour pressure distribution work?
The warm indoor side of a building element has a higher water vapour pressure than the cold outside. That difference drives vapour diffusion through the materials. Each layer offers resistance described by its Sd-value (vapour-equivalent air-layer thickness in metres). The higher the Sd-value, the larger the pressure drop across that layer.
By calculating each layer in sequence, a graph of the actual vapour pressure is produced. This is then compared to the saturation vapour pressure (which depends on temperature). Where the actual line touches or crosses the saturation line, condensation forms.
Application
Vapour pressure distribution is used for:
- Glaser calculations — assessing interstitial condensation risk in walls and roofs
- Renovation detailing — checking whether internal insulation causes moisture problems
- Timber-frame design — placing vapour control layers and breather membranes correctly
- Thermal-bridge analysis — combined with thermal calculations
The rule of thumb “vapour-tight inside, vapour-open outside” follows directly from this logic: resistance decreases in the direction of vapour flow so moisture can escape.
Limitations
The classic Glaser method is steady-state and ignores moisture storage in materials as well as drying-back during summer. For complex assemblies a dynamic calculation according to EN 15026 or with software such as WUFI is preferred.
Related terms
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