In construction, breathing refers to the ability of a material or building assembly to absorb moisture (water vapour), temporarily store it, and release it back to the environment. This process is also called moisture buffering and is essential for a healthy indoor climate.
How does breathing work?
Building materials that can breathe have an open porosity. Water vapour can migrate through the pores — from the side with high humidity to the side with lower humidity. This is the principle of vapour diffusion.
A breathing material:
- Absorbs moisture when the relative humidity is high (e.g. during cooking or showering)
- Releases moisture when the humidity drops
- Prevents condensation because moisture is transported gradually rather than settling on cold surfaces
Application
Breathable materials are used in:
- External walls — lime mortar, clay plaster and unglazed brick are naturally vapour-permeable
- Insulation — mineral wool, wood fibre insulation and cellulose allow vapour transport
- Membranes — vapour-permeable membranes (breather membranes) allow water vapour through but keep liquid water out
- Interior finishes — clay plaster and lime plaster buffer moisture better than a sealed latex paint
Why is breathing important?
A construction that cannot breathe risks:
- Moisture accumulation — trapped moisture builds up inside the construction
- Mould growth — prolonged high humidity behind vapour-tight layers causes mould
- Material degradation — timber can rot, metal can corrode
Related terms
- Vapour-permeable construction
- Vapour diffusion
- Moisture regulation
- Indoor climate
- Mineral wool
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