A vapour-permeable membrane, also called a vapour-open membrane, is a thin sheet that blocks liquid water but allows water vapour to pass through. It is typically fitted on the cold (outer) side of an insulation layer so that any moisture trapped inside the construction can diffuse out, while rain or melted snow cannot enter.
How it works
A vapour-permeable membrane has a micro-porous structure or consists of a layered film. The openings are large enough for vapour molecules but too small for liquid water droplets. Vapour permeability is expressed as the Sd-value (equivalent air layer thickness for diffusion):
- Sd ≤ 0.3 m — strongly vapour-open (typical for underlay roofing membranes)
- Sd 0.3 – 1.5 m — vapour-retarding
- Sd ≥ 1.5 m — vapour-tight
The lower the Sd-value, the more easily vapour can pass through.
Applications
Vapour-permeable membranes are commonly used as:
- Roofing underlay — beneath tiles or slates, on top of the sarking or directly on the insulation. Catches any water that penetrates the outer skin
- Wind barrier in a facade — on the outer face of the insulation behind cladding such as timber or stone
- Timber-frame construction — on the outside of the wall assembly, allowing construction moisture to diffuse outwards
- Insulation retrofits — in existing roofs to maintain a vapour-open structure
Combining with a vapour control layer
A vapour control layer (VCL) or vapour barrier is normally fitted on the warm (inner) side of the insulation to limit moisture entering the structure. The rule of thumb is *tighter inside than outside*: this prevents warm, moist indoor air from condensing inside the construction.
Installation tips
- Tape all joints with the manufacturer’s recommended tape
- Keep the membrane free of tears and punctures
- Do not confuse with vapour-tight PE film
- On pitched roofs, lay with sufficient overlap in the direction of drainage
Related terms
- Vapour control layer
- Vapour barrier
- Vapour diffusion
- Insulation
- Timber-frame construction
- Roof sarking
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