A vapour barrier is a layer in a wall, roof or floor construction that virtually completely prevents the transport of water vapour through diffusion. It is fitted on the warm side of the insulation and has a much higher vapour resistance than a vapour control layer (VCL). In typical north-west European building physics, a true vapour barrier is mainly used in cold and freezer stores, swimming pools, saunas and certain specialised build-ups.
Difference between vapour barrier and VCL
The distinction lies in the Sd value:
- Vapour control layer (VCL) — Sd 0.5 m to 20 m, allows limited diffusion
- Vapour barrier (vapour-tight layer) — Sd > 100 m, near-total blockage
In ordinary residential construction a VCL is usually enough. A true vapour barrier becomes necessary only when there is high indoor vapour pressure, such as in:
- Swimming pools — high humidity and heat
- Saunas — extreme temperature differences
- Cold stores and freezer rooms — reversed vapour pressure
- Laundries, breweries, food processing — production halls with high humidity
Materials
A vapour barrier is usually made of:
- PE film 0.2-0.5 mm — the most common version
- Bituminous vapour barrier — used in roof build-ups on concrete decks
- Aluminium foil or aluminium-coated film — for very high vapour resistance
- Special vapour-tight coatings — on concrete floors in industrial applications
Installation and detailing
A vapour barrier only performs if it is fully sealed:
- Install on the warm side — directly behind the internal lining
- Lap the joints — at least 100 mm overlap
- Tape the laps — with vapour-tight butyl or acrylic tape
- Seal penetrations — grommets or special tape around cables and pipes
- Edge connections — finish with tape and sealant
Points to consider
- Avoid perforations — every nail or screw going through the barrier is a potential weak point
- No second vapour barrier on the cold side — if a leak does occur, moisture must always be able to escape
- Compatibility with other layers — check that the material isn’t attacked by, for example, bitumen or plasticisers
- Avoid trapped construction moisture — let any wet work dry out before sealing the construction
Application in normal housebuilding
Most houses in northern Europe do not use a true vapour barrier; a vapour control layer (Sd 2-5 m) is enough. A real vapour barrier is usually found only in:
- Flat roofs (bituminous vapour barrier on a concrete deck)
- Insulated floors above crawl spaces
- Special rooms such as saunas or swimming pools
Related terms
- Vapour control layer
- Vapour diffusion
- Vapour-permeable membrane
- Air tightness
- Building physics
- Insulation
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