Does a rooflight need a vapour barrier?

The layer belongs on the warm side, and it has to be continuous

The rule is simple to state. Insulation keeps the warm side warm. Anything on the room side of the insulation stays above dew point, so vapour reaching it does not condense. Anything on the cold side of the insulation is below dew point in winter, so vapour reaching it does. The control layer therefore goes on the warm face of the insulation, and the roof is built so that the cold side can dry outwards.

Continuity is the whole point. A layer that covers 95 per cent of a ceiling and stops short at one frame is not 95 per cent effective, because moisture takes the shortest available route and a 5mm slot at a perimeter will carry more water in a season than the entire remaining area passes by diffusion. That is why the junction at a rooflight is worth more attention than the flat run either side of it.

Vapour barrier, vapour control layer, and why the words differ

Homeowners say vapour barrier, drawings say vapour control layer, and the distinction is real. A true barrier stops vapour completely. A control layer resists it by a stated amount, measured as a vapour resistance in MNs/g or as an equivalent air layer thickness in metres, and the number is chosen so the construction dries in the right direction rather than being sealed on both faces. A polythene sheet at 500 gauge is one product. A reinforced foil-faced membrane on a warm deck flat roof is another, with a far higher figure. An intelligent membrane that changes its resistance with humidity is a third. The unhelpful outcome is a roof sealed tight on the cold side too, which traps whatever moisture is already inside.

Air leakage carries far more water than diffusion

This is the part that surprises people. Water vapour diffusing slowly through an intact membrane is a small flow. Warm moist air pushed through a gap by the stack pressure of a heated house is a much larger one, by a factor most estimates put in the hundreds. A single unsealed corner behind a plasterboard reveal can deliver more moisture into a roof over one winter than the whole ceiling passes by diffusion in a decade.

So the layer is an air barrier first and a vapour barrier second, and the test of the work is whether it is sealed rather than whether it is present. Present but loose is the common finding when an old rooflight comes out.

Where it runs at a kerb-mounted unit on a flat roof

On a warm deck the stack from the inside out is ceiling, service void, control layer, insulation, waterproofing. At the opening, the layer comes across the deck, turns up the inside face of the timber kerb, runs to the top of the kerb and is taped onto the underside of the rooflight frame where it lands. The insulation then wraps the outside face of the kerb and the waterproofing goes over the top of that. Done in that order the warm side stays sealed and the timber kerb sits inside the insulated envelope rather than outside it. Kerb build-ups are covered on upstands, kerbs and warm roof build-ups.

A cold deck is harder, because the layer sits under the joists while the insulation sits between them and there has to be a ventilated void above. The junction still gets made, and the ventilation path past the opening has to be kept open at the same time. That case is on flat rooflights on a cold roof build-up.

A loft conversion, where the layer sits behind the plasterboard

Where a room has been formed in a roof, the layer is either a separate membrane stapled across the rafters or the foil face of an insulated board with every joint taped. At a roof window it has to be dressed into the opening, along the reveal and onto the outer frame of the unit, which means the plasterer cannot be the first person to think about it. Manufacturers supply an insulation collar and a vapour apron sized to their own frames precisely for this junction, and they cost very little relative to the work of opening a finished ceiling later.

Where the roof is a cold pitched roof with insulation at ceiling level and the rooflight lights a landing below, the layer belongs at ceiling level and the loft above stays ventilated. Getting the two mixed up is how a warm damp loft appears in a house that previously had a dry one.

What the guidance asks for

Approved Document C requires that construction is designed so that interstitial condensation does not damage the fabric, and the standard behind it is BS 5250, which sets out the assessment and the detailing. Approved Document L brings the same junction in from the other side, because insulation continuity and air tightness at a controlled fitting are part of conserving fuel and power rather than an optional extra. For a replacement, the Part L position is on rooflights and Part L for a replacement. Nothing in either document is satisfied by a layer that stops 50mm from the frame, and the way the junction feeds into the whole thermal picture is on the energy efficient rooflights hub.

What it looks like when the layer was left short

The failure is slow and it does not announce itself. In year one there may be nothing. By year three there is a faint grey shadow in the plaster at the head corners of the reveal, worst in February. By year six the plasterboard is soft in one corner, screws are showing rust spots through the paint, and there is black spotting where the ceiling meets the reveal. Open it up and the insulation behind is damp and compressed, the timber has surface staining, and the membrane is visible, cut square, 80mm from the frame it was supposed to reach.

None of that is water coming in from the roof. It is warm air from the room, arriving at a cold surface it should never have reached.

Existing rooflights, and what can be done without opening the roof

Where a unit is already in and the junction is suspect, the reveal is the way in. Taking the plasterboard off the reveal exposes the perimeter, the insulation can be reinstated tight and continuous, and a new membrane or a vapour-tight tape can be lapped from the ceiling layer onto the frame before the reveal is rebuilt. It is a day of work in one room, it does not touch the covering, and it is a good deal cheaper than doing it after the plaster has been renewed twice. The insulation half of that job is on adding insulation around an existing rooflight.

Severn Vale humidity, and the roofs where this is not optional

Gloucestershire gives this detail a hard time. The low ground along the Severn and the Avon runs at high outdoor humidity for weeks at a time in winter, so the outward drying that a construction relies on is slower here than on a dry exposed site. Cold air draining down the Stroud and Chalford valleys after dark drops the deck temperature quickly. Cotswold stone slate roofs frequently have no underlay at all, or a mortar torching in place of one, so the outer layers are unusually open and the inner layers carry all the responsibility.

The build-ups where the junction earns its money most are warm deck rear extensions across Gloucester and Tewkesbury, loft conversions in the interwar semis at Longlevens and Charlton Kings, and any shower room or kitchen lit from above. Those are rooms producing a lot of vapour with a cold frame directly overhead, and the layer taped onto that frame is the thing standing between the two.