A smart vapour control layer — also called a humidity-variable vapour retarder, and in Dutch klimaatfolie — regulates the moisture balance of a construction: it is both vapour-retarding and vapour-open, because its vapour resistance moves with the humidity inside the build-up.
An ordinary vapour barrier, a polythene sheet for example, has a fixed high resistance. In winter that is exactly what you want — it keeps warm, moist indoor air out of the insulation — but it also means that any moisture that does get in can never leave. A smart membrane solves that: at low humidity (winter, dry indoor air) the resistance is high; at high humidity (summer, damp construction) it drops sharply, so the build-up can dry back towards the inside.
Manufacturers express this as an sd value, the vapour-diffusion-equivalent air layer thickness in metres. For smart membranes that value runs, depending on the product, from a few tenths of a metre at high humidity to many metres in dry conditions. A classic polythene barrier sits far above that at a single fixed value.
| Ordinary vapour barrier (e.g. polythene) | Fixed high vapour resistance. Keeps moist indoor air out in winter, but any moisture that does get in can never leave. |
|---|---|
| Smart vapour control layer, low humidity (winter, dry indoor air) | Vapour resistance is high. |
| Smart vapour control layer, high humidity (summer, damp construction) | Vapour resistance drops sharply, so the build-up can dry back towards the inside. |
Where it applies
- Pitched roofs and dormers — especially where an insulated roof is relatively vapour-tight on the outside (bitumen, boarding), the ability to dry back inwards is essential
- Internal wall insulation of existing buildings — a risky intervention in which a humidity-variable membrane reduces the chance of interstitial condensation
- Timber frame and refurbishment — where the build-up cannot be fully controlled and some tolerance is welcome
- As the airtightness layer — the membrane doubles as the airtight plane of the construction
Practical tips
- Airtight matters more than vapour-tight — an open seam or a hole at a service penetration carries many times more moisture than diffusion through the membrane itself. Use the matching tape, grommets and sealing adhesive from the same manufacturer
- On the warm side — the membrane always belongs on the inside (warm side) of the insulation
- Fill the cavity completely — leave no air gaps between insulation and membrane; air circulating against cold surfaces causes condensation
- Do not perforate it — allow a service void of roughly 30 to 50 mm behind the lining so cables and pipes never pierce the membrane
- Calculate when in doubt — for unusual build-ups a hygrothermal assessment (Glaser method or a dynamic simulation) is more reliable than a rule of thumb
Related terms
- Vapour control layer
- Climate façade
- Airtightness
- Indoor climate
- Green roof
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