A breathable membrane is a sheet that allows water vapour to pass through but blocks liquid water. It is placed on the exterior (cold side) of an insulated wall or roof. The membrane lets moisture trapped within the structure escape outward, while keeping rain and wind out.
Why is a breathable membrane needed?
Every insulated structure contains moisture — from inside (warm, humid air) or from condensation. This moisture must be able to escape outward, otherwise problems arise:
- Mould in the insulation or on the timber
- Timber rot from prolonged dampness
- Reduced insulation performance — wet insulation insulates poorly
The breathable membrane on the outside allows this moisture to escape while protecting the structure from rain and wind.
Breathable vs. vapour barrier — the difference
| Breathable membrane | Vapour barrier | |
|---|---|---|
| Position | Exterior (cold side) | Interior (warm side) |
| Water vapour | Allows through (outward) | Blocks |
| Wind/rain | Blocks | Not the primary function |
| Purpose | Let moisture escape | Keep moisture out |
They work together: the vapour barrier on the inside prevents moisture from entering the structure, the breathable membrane on the outside lets any moisture escape.
Where is a breathable membrane used?
- Timber frame construction — Over the OSB board on the exterior
- Roof structure — Over the rafters, under the counter battens (roofing underlay)
- Facade cladding — Behind lap siding, in front of the ventilation cavity
Installation
1. Apply horizontally — Start at the bottom, work upwards
2. Overlap — At least 10-15 cm overlap per strip
3. Seal — Tape overlaps and connections with special tape
4. Do not perforate — Every hole is a leak. Use special nails or staples.
5. Ventilation cavity — Between the breathable membrane and the exterior cladding, always leave an air gap of 20-30 mm
Common mistakes
- Breathable and vapour barrier swapped — Breathable goes outside, vapour barrier inside. Reversing them is the most common cause of moisture problems in timber frame construction.
- No tape on the joints — Unsealed joints let wind and rain through.
- No ventilation cavity — The breathable membrane must be able to “breathe” on the outside. Without an air gap it does not work.
Sd value
The vapour permeability is expressed as the Sd value (equivalent air layer thickness):
- Sd < 0.5 m = breathable (what you want on the outside)
- Sd > 100 m = vapour-tight (what you want on the inside)
- In between = vapour retarder (limited permeability)
Related terms
- Vapour barrier
- Timber frame construction
- OSB board
- Battens
- Insulation value (R-value)