Vapour diffusion is the natural transport of water vapour through a material as a result of a difference in water-vapour concentration or vapour pressure between two sides. It is an important building-physics concept because it determines how moisture moves through a wall, roof or floor construction, and therefore where condensation and possible moisture damage can occur.
How does vapour diffusion work?
Water vapour behaves like a gas and tends towards equilibrium: it moves from an environment with high vapour pressure to one with low vapour pressure. In a typical heated home indoor air is warmer and more humid than outdoor air, especially in winter. Vapour therefore diffuses from inside to outside through the building envelope.
With a sound build-up the vapour leaves the construction without forming condensation. With the wrong order of materials the vapour can cool below the dew point somewhere inside the construction, where it then condenses.
Sd value and vapour resistance
The vapour resistance of a material is expressed as the Sd value (in metres):
- Sd < 0.5 m — vapour-open (e.g. mineral wool, some membranes)
- Sd 0.5-20 m — vapour-retarding (plywood, OSB, kraft paper)
- Sd > 100 m — vapour-tight (PE film, bitumen, metal)
The µ value is the ratio of a material’s vapour resistance to that of still air. Sd is calculated as µ × thickness.
Rule of thumb: vapour-tight on the warm side
For a healthy temperate-climate construction the rule is:
Vapour resistance decreases from inside to outside.
So on the warm side there is a vapour-retarding or vapour-tight layer (membrane, OSB); in the middle the insulation; and on the outside a vapour-permeable, weather-resistant membrane. Any vapour that does enter the construction can then escape outward.
Points to consider
- Air tightness — air leaks transport many times more moisture than diffusion ever does
- Detailing — most problems originate at corners, penetrations and window junctions
- Vapour-open vs vapour-active — modern membranes can adjust their Sd value to the surrounding humidity
Application
Vapour diffusion calculations are carried out per EN ISO 13788 (or the Glaser method) to assess whether a build-up risks interstitial condensation.
Related terms
- Vapour barrier
- Vapour control layer
- Vapour-permeable membrane
- Dew point
- Building physics
- Insulation
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