Vapour barrier installation: how to fit it correctly

In winter, indoor air in a heated building holds far more moisture than outdoor air. As it moves into a wall, roof or ceiling, it cools inside the insulation until it reaches its dew point. The condensed water wets the insulation, lowers its performance and can eventually cause mould and rot in the timber.

A vapour barrier stops that flow. It goes on the warm side of the insulation, directly behind the interior lining. The wind barrier on the cold side does the opposite job and lets any moisture that gets in dry out, so the layers become more open to vapour from the inside out.

This applies to timber frame walls, insulated pitched roofs, ceilings under a cold loft and insulated partitions next to unheated spaces.

The usual choice is polyethylene film: inexpensive, almost vapour-tight and right for a standard build-up that dries outwards. Vapour-control membranes, often called smart membranes, let more vapour through in summer than in winter. They suit older buildings and build-ups that must also dry towards the room, but they cost more and need the manufacturer’s own tapes.

Whichever material the designer specifies, use tapes, sealants and grommets from the same system. The barrier is only as good as its joints.

• Polyethylene film: low cost and very vapour-tight; for new builds with a clear warm side.
• Vapour-control membrane: resistance changes with the season; for renovation and build-ups that dry inwards.
• Taped OSB or foil-faced boards: the vapour control layer in some systems, where the design allows it.

Start on a clean, dry surface and run the film in long sheets with as few joints as possible. Fix it to the studs or rafters with staples placed inside the lap, so the tape covers them later.

• Overlap neighbouring sheets by at least 100 mm, with each lap over a stud or rafter so it can be pressed against a firm backing.
• Tape every lap along its full length with vapour barrier tape and press it down with a roller.
• Do not stretch the film tight; leave some slack in the corners so it does not tear when the timber shrinks.
• Tape every tear, and every staple hole outside a lap, straight away.
• At the edges, leave enough film to turn onto the adjoining wall, floor or ceiling and bond it there.

Most leaks are not mid-sheet but where something passes through the film or meets another element.

• Pipes and cables: purpose-made grommets or collars, or short overlapping strips of tape; never an open slit.
• Sockets and switches in an outer wall: airtight back boxes or, better, a service cavity so the film is not cut at all.
• Windows and doors: once the gap around the frame is insulated, bond the film to the frame with window connection tape.
• Floors, ceilings and masonry: a continuous bead of the system sealant, plus a clamping batten where the system asks for one.
• Beams through the layer: seal the film around each one so air cannot travel along the timber.

A service cavity is a gap of a few centimetres between the vapour barrier and the interior board, formed with battens. Cables, sockets and small pipes run in this gap, so the electrician never needs to cut the barrier.

It also protects the barrier during finishing, because plasterboard screws go into the battens, not the film. With many sockets, built-in furniture or heavy fixings, it is the simplest way to keep the layer intact.

The same mistakes recur: film on the wrong side of the insulation, laps taped only in places, film stopping short of the floor or ceiling, open cable entries, sockets cut through the barrier and household tape that is already peeling.

Check the layer before the interior board goes on: press the tape along every lap and junction, and look closely at corners, window reveals and every cable or pipe. On larger or energy-efficient buildings an airtightness (blower door) test finds hidden leaks, so do it while the barrier is still accessible.

Photograph the finished layer, especially junctions and penetrations: once covered, the photos are the only record of the work. What Else installs and checks vapour barriers as part of its interior renovation and turnkey projects.