Vapour-control (foil-backed) boards
What does a foil backing on plasterboard actually do?
Water vapour resistance factor (μ): how many times more the board resists water vapour than the same thickness of still air — used with thickness in condensation-risk calculations. Declared to EN 12524 in dry-cup (low humidity) and wet-cup (high humidity) regimes; gypsum passes vapour more easily when humid, so wet-cup values are lower. Duplex boards' metallised film raises μ to ~1400 versus ~10 for plain board.
A foil-backed (Duplex) board does one specific job in a wall’s building physics: it holds back water vapour. That is a different task from resisting liquid water, and confusing the two puts the board — and the condensation risk it is meant to manage — in the wrong place.
Vapour resistance, μ and Sd
Water vapour resistance is quoted as the factor μ (mu): how many times more the material resists vapour than the same thickness of still air. A plain gypsum board has a μ of around 10; the metallised film on a Duplex board raises it to roughly 1400 — a step change, which is the whole point of the backing.
In practice designers often work with the Sd value — the equivalent air-layer thickness in metres, Sd = μ × board thickness — because it can be added up across the layers of a wall in a condensation-risk assessment (the approach set out in BS 5250 for controlling condensation in buildings).
What a vapour control layer is for
Warm indoor air carries moisture. As it moves out through a cold wall it can reach a point where it condenses inside the construction — interstitial condensation — which over time damages the fabric. A vapour control layer on the warm side of the insulation slows that outward diffusion so it stays below the risk threshold.
Because it works by resisting diffusion, its position in the build-up matters: a vapour-control board is a layer in a designed wall, not a general-purpose upgrade you add anywhere.
Vapour control is not moisture resistance
This is the distinction that matters on site. A foil-backed board manages vapour diffusion, but it is generally unsuitable for humid rooms in the way a Type H moisture board is — the two jobs are declared under different standards (EN 14190 for the vapour-control laminate, the EN 520 H classes for moisture resistance), and a board strong in one is not automatically strong in the other.
drywallDB keeps the vapour-control family separate from the moisture family in its equivalence logic, so a foil-backed board and a moisture board are never offered as silent substitutes for one another.
Frequently asked questions
- Is a foil-backed board waterproof or moisture resistant?
- Neither, in the everyday sense. The foil is a vapour control layer — it slows water-vapour diffusion. For liquid-water tolerance you want a Type H moisture board; for wet areas, a tanking system.
- What does the μ value tell me?
- μ is the vapour resistance factor — how much more the board resists vapour than the same thickness of air. Plain board is about 10; a foil-backed board about 1400. Multiplied by thickness it gives the Sd value used in condensation calculations.
- Which side of the wall does a vapour control board go?
- On the warm side of the insulation, so it slows moisture-laden internal air before it reaches the cold zone where it could condense. Its position is part of the wall design, not incidental.
Related guides
- Moisture resistance: H1, H2 and H3 — What's the difference between H1 and H2 plasterboard?
- Thermal conductivity (λ) of plasterboard — What is the λ value on a plasterboard data sheet?
- EN 520 board types explained — What do EN 520 Type A, D, E, F, H, I, P and R mean?