Installing a vapour barrier

A standard vapour barrier is 0.20 mm PE film with a diffusion resistance corresponding to S_d ≥ 95 m, CE-marked to EN 13984. Check that the product's declaration of performance suits the construction — S_d (the equivalent air layer thickness) is the figure that tells you how well the film holds back water vapour.

The vapour barrier must be on the warm, inner side of the insulation. If it is positioned incorrectly — on the cold side — water vapour will condense inside the structure. This is a fixed rule in moisture design (the Glaser method / SBi-anvisning 224).

Always the warm side. A vapour barrier on the cold side collects moisture in the structure.

Roll out the sheets with an overlap of at least 100 mm at all joints. Plan the direction of the sheets so you get as few joints as possible and so that the overlaps fall over a solid surface behind (batten/rafter) that you can press them against.

Seal all overlaps and joints with approved vapour barrier tape or compriband. The tape must be pressed against a firm backing — a taped joint that only holds film to film in mid-air will open up again. Use the tape/adhesive the film supplier specifies for their system.

Electrical conduits, ventilation ducts, pipes and other penetrations are the most common leakage points. Use ready-made collars or tight tape all the way round each penetration. One unsealed penetration can carry large amounts of moisture up into the structure — treat each one carefully.

Seal each penetration individually. Leaks collect moisture precisely where you cannot see it.

Bring the membrane tight to all adjoining surfaces — floor, walls, ceiling/roof — and seal the connection with compression tape or tape against a solid backing. The building envelope must be unbroken all the way round; it is the joints between surfaces that most often leak.

Ideally, add a service layer (furring battens/lining wall) on the inside of the vapour barrier so electrics and other services can be run without perforating the membrane later. Screws and holes in a finished vapour barrier are hard to find and seal afterwards.

Airtightness is documented by a pressure test (blower door) at 50 Pa. The BR18 limit for an ordinary detached house is indicatively around ≤ 1.0 l/s per m² heated floor area — the exact value and area reference depend on the BR18 version and must be checked against the current text before use. Low-energy/stricter classes have a lower limit.

Most roof structures require moisture documentation under BR18 § 287 and SBi-anvisning 224 — the Glaser method or a hygrothermal simulation. It is a design task, but as the contractor you should know the requirement and fit the membrane so that the structure actually meets the documented solution.