A vapour control layer (VCL) is a building-physics layer in a wall, roof or floor construction that limits the transport of water vapour by diffusion without entirely preventing it. It is the middle ground between fully vapour-open and fully vapour-tight: a layer that holds back just enough moisture to prevent condensation while letting enough through for the construction to ‘breathe’.
Sd value of a vapour control layer
Vapour resistance is expressed as the Sd value in metres:
- Vapour-open — Sd < 0.5 m
- Vapour-retarding (VCL) — Sd 0.5 m to 20 m
- Vapour-tight — Sd > 100 m
A VCL therefore has a moderate resistance. For typical north-west European conditions a layer with an Sd value of 2 to 5 metres is often used on the warm side of the insulation.
Materials that act as a VCL
- OSB boards (12-18 mm) — Sd of about 2-4 m, structural and vapour-retarding in one
- Plywood (12-18 mm) — Sd of about 5-10 m
- Kraft paper — a simple variant in older insulation batts
- PE film 0.2 mm — Sd of about 50-100 m (often still considered a VCL)
- Adaptive (humidity-variable) membranes — Sd value changes with the relative humidity (such as Intello or Vario KM)
Position in the construction
The VCL is always installed on the warm side of the insulation: in a heated building, on the inside, directly behind the plasterboard or in place of it. This stops warm, humid indoor air from diffusing through the insulation and condensing on a cold surface beyond.
Adaptive (smart) VCL
Modern adaptive membranes (also called variable VCLs) are intelligent: they have a low Sd value at high humidity and a high Sd value at low humidity. Therefore:
- Summer — moisture inside the construction can dry inwards
- Winter — moisture from indoor air is held back
Points to consider
- Fully sealed — all laps and joints must be taped with vapour-tight tape
- No perforations — cables and pipes must be sealed with grommets or collars
- No tension — don’t pull the film taut; it can tear with thermal movement
- Good air tightness — a VCL only works in an air-tight construction
Related terms
- Vapour diffusion
- Vapour-permeable membrane
- Vapour barrier
- Air tightness
- Insulation
- Building physics
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