Does a rooflight need a vapour barrier?

Barrier or control layer, and why the word matters

The trade still says vapour barrier. The correct term is vapour control layer, and the change of word carries the whole idea. A barrier suggests something that stops vapour absolutely, which no practical construction does. A control layer slows the rate at which water vapour diffuses into the structure to something the structure can dry out faster than it takes in.

That distinction matters because it explains why continuity beats material. A 500-gauge polythene sheet with a 50mm slit in it performs worse than a much less resistant membrane that is unbroken, since most vapour travels with moving air through gaps rather than by diffusing through intact sheet. Sealing the layer is a bigger lever than upgrading it.

What the layer is stopping

Warm indoor air holds a great deal of water vapour. It is also at slightly higher pressure than the cold air outside during winter, so it pushes outwards through the ceiling wherever it can. As it travels through the roof build-up it cools, and at the point where it drops below its dew point the vapour becomes liquid inside the construction.

In a cold roof that point is in the rafter void or on the underside of the sarking. In a warm roof it is at the underside of the deck. Either way you get water forming on timber, on insulation and on fixings, in a place where nobody will see it and where there is very little air movement to dry it out. Interstitial condensation is the term, and its consequences are soft insulation, corroded fixings, mould and eventually rotted timber.

The rooflight is where the layer gets cut

To fit a rooflight you cut a hole through the ceiling, the insulation, the vapour control layer and the deck. Everything else in the roof continues past the opening. The vapour control layer does not: it is severed on all four sides.

Left as cut, you have created a slot around the whole perimeter through which warm humid air can pass straight into the coldest part of the roof structure. Worse, that slot is beside a timber trimmer that is already conducting heat outwards, so the air arrives at a cold surface immediately rather than gradually. Rooflight surrounds fail before the middle of a roof does, and this is why.

Sealing to the frame, in the order it has to happen

The sequence is fixed by access. The layer must be dressed to the frame before the plasterboard reveal goes on, because afterwards there is no way to reach it.

The existing layer is cut with a generous upstand rather than flush, so there is material to work with. It is then turned up the inside face of the trimmed opening and lapped onto the inner flange of the rooflight frame, taped with a purpose-made vapour-tight tape rather than a general-purpose one. Corners are the weak point, so they are formed with pre-made corner pieces or folded and taped rather than cut and butted. Where the frame material will not accept tape reliably, a compatible airtightness sealant is run as a bead between the layer and the frame and clamped by a batten. Only then does the insulation get worked in behind the reveal and the plasterboard fixed.

Fixings through the layer after it is sealed are avoided where possible and taped where not. Our page on vapour control layers sealed to the frame goes through the detailing in depth.

Where the layer sits in each roof build-up

In a warm flat roof the insulation sits above the deck and the vapour control layer sits on top of the deck, below the insulation, so the whole structure stays warm and dry. The rooflight sits on an upstand kerb rising through that insulation. The layer has to be turned up the inside face of the kerb and sealed to the rooflight frame at the top, and the kerb has to be insulated on its outer face so the timber does not sit cold.

In a cold pitched roof, insulation sits between and under the rafters with a ventilated void above it, and the layer sits on the warm side of the insulation directly behind the plasterboard. Here the sealing is done at the ceiling plane, up the reveal, to the frame. The ventilated void above must also stay ventilated past the opening: a rooflight trimmed into a cold roof frequently blocks the cross-flow between rafter bays, and counter-battening or a rafter tray is needed to carry the air path around it.

What happens when it is left short

Nothing, for about two winters. Then dark staining at the corners of the reveal, mould on the plaster where it meets the frame, and a musty smell in the room. The homeowner concludes the rooflight is leaking, which it is not.

Above the ceiling, meanwhile, the insulation around the trimmer has been slumping as it takes up water, the fixings are corroding, and the trimmer itself is at high moisture content. By the time it shows through the plaster, several years of damage have already happened. This is the failure that a ten-year workmanship guarantee has to be written to cover, and it is why we treat the perimeter as the job rather than the unit.

Airtightness and vapour control are the same job here

They are separate functions in theory and the same piece of work in practice at a rooflight. Air leakage carries vapour hundreds of times more effectively than diffusion does, so any gap that admits air is also admitting moisture at a rate that no membrane specification can offset.

That means the test of a good detail is whether it would hold air, not whether the material has a high vapour resistance. If you can feel a draught around a rooflight reveal on a windy night, the vapour control at that point is failing regardless of what is written on the membrane.

The reveal that hides the problem

A plasterboard box built out from the frame, with the boards nailed to battens and a void behind, is the standard shortcut. It squares up a reveal quickly, it decorates beautifully, and it leaves an uninsulated, unsealed cavity connected to the room at every board joint.

The correct reveal follows the insulation line, with insulation taken tight to the frame and the vapour control layer sealed to the frame behind the board. It takes longer and it costs more, and it is invisible once painted, which is exactly why it gets left out. Ask any installer to describe how they will form the reveal before you accept a price. Our page on rooflights and insulation explains what the surround costs you thermally when it is done the quick way.

Retrofitting into a roof already built

Where a rooflight is being replaced, the perimeter is opened anyway and the layer can be brought to the new frame properly as part of the work. Where the rooflight is staying and only the ceiling is being upgraded, the layer has to be lapped and sealed onto the existing frame, which is achievable from inside if the reveal is taken off.

Where the roof cannot be opened, an internal vapour control layer applied to the room face of the ceiling and sealed to the frame is second best but genuinely useful. What is not acceptable is stopping the new layer at the edge of the old reveal and boarding over it, which is the arrangement that fails.

We build the layer to the frame on every job and we do not leave a roof open overnight. Fifteen years in the trade, Oxford and twenty five miles by road, new installations and replacements only, with a ten-year workmanship guarantee. Call 01865 704245 for a range by email, then a fixed price after a survey.