Diffusion-open building (also called ‘breathable construction’) is a building principle in which the outer envelope is built so that any moisture inside the structure can escape outwards without letting water or wind in. It is an alternative to the strictly vapour-tight approach and is widely used in eco-building, timber-frame construction and sustainable new build.
Principle
The idea behind diffusion-open building is that a house can ‘breathe’: moisture that inevitably enters the construction (through diffusion, condensation or construction moisture) should be able to escape without restriction. The rule of thumb is that vapour resistance decreases steadily from inside to outside.
A typical build-up consists of:
- Internal finish — plaster, clay or wood-fibre board (slightly vapour-retarding)
- Vapour-retarding layer with a low Sd value (1-5 m), such as OSB or an adaptive vapour control layer
- Insulation — natural materials such as wood fibre, flax, hemp or cellulose
- Vapour-permeable weather-resistant membrane or vapour-permeable wood-fibre board
- Cladding with a ventilated cavity
Advantages
- Moisture-tolerant — small air leaks or construction moisture do not lead immediately to mould
- Comfortable indoor climate — natural humidity buffering by the materials
- Healthy — lower risk of interstitial condensation
- Sustainable — often combined with natural, renewable materials
Points to consider
- Air tightness still matters — diffusion-open does not mean air-permeable; leaks carry far more moisture than diffusion
- Material choice is critical — all layers must be co-ordinated for their Sd value
- Detailing — junctions at windows, penetrations and ground floors need extra care
- Calculation recommended — a hygrothermal calculation (Glaser or WUFI) is advisable to rule out condensation risk
Difference between diffusion-open and vapour-tight
- Vapour-tight building — all moisture must be removed by ventilation; the envelope is fully sealed
- Diffusion-open building — moisture may move through the construction; only works with good air-tightness and ventilation
Application
Diffusion-open building is popular in:
- Timber-frame construction
- Straw-bale and clay building
- Restoration of heritage buildings with timber floors
- Bio-based construction with natural-fibre insulation
Related terms
- Vapour-permeable membrane
- Vapour diffusion
- Vapour barrier
- Air tightness
- Eco-building
- Timber-frame construction
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