Does a rooflight need a vapour barrier?
Yes, and more precisely the roof around it does. The correct term in current practice is a vapour control layer, and its job is to stop warm moist indoor air travelling into the roof construction where it will meet a cold surface and condense out of sight. A rooflight punches a hole through that layer, so the detail that matters is not whether the layer exists but whether it is made continuous where it meets the frame.
This is the part of a rooflight installation that is entirely invisible at handover and entirely decisive ten years later. It costs a roll of membrane, some tape and half an hour of care. Getting it wrong puts water inside the build-up rather than on the glass, where nobody sees it until the plasterboard stains. The wider thermal picture sits on energy efficient rooflights.
What a vapour control layer physically is
A sheet, usually polythene or a reinforced foil faced membrane, laid on the warm side of the insulation and lapped and sealed at every joint. On a flat roof it is bonded or loose laid over the deck before the insulation goes down. On a pitched slope it sits directly behind the plasterboard.
Its resistance is measured in meganewton seconds per gramme, and a decent domestic layer is well above 100 MNs/g. That number matters less than the sealing, because vapour finds gaps in the way that water finds a hole. A membrane with a good resistance and an unsealed lap performs worse than a modest membrane taped properly.
Which side of the insulation it belongs on
The warm side. Always. Vapour moves from high pressure to low pressure, which in a British winter means from the heated room outwards, so the barrier goes on the side the vapour arrives from.
Put it on the cold side and it becomes a condensing surface with insulation behind it, which is precisely the failure it was meant to prevent. This is not a subtle judgement call, and it is the reason a build-up is drawn before it is built. There is also a rule of thumb for the layer’s position within the insulation itself: no more than about a third of the total thermal resistance should sit inboard of it.
Warm roof build-ups, and the straightforward case
A warm deck flat roof puts the insulation above the structural deck and the vapour control layer directly beneath it. The whole deck and the joists sit inside the insulated envelope at close to room temperature, so there is very little cold surface for vapour to find.
At a rooflight the insulation is carried up the outside of the kerb and the vapour layer is turned up its inside face and terminated against the frame. Done that way, the warm envelope stays unbroken and the kerb is not a cold bridge. Proprietary insulated kerbs make this easier, because the upstand arrives with its insulation already integrated rather than relying on somebody remembering to add it.
Cold roofs, and the harder case
A cold deck puts the insulation between the joists with a ventilated void above it, and the void is cross ventilated at the eaves so any vapour that gets through is carried away. Cutting a rooflight opening into that arrangement interrupts the ventilation path and can create a dead pocket alongside the kerb where air stops moving.
So a cold roof asks two things of the design rather than one. The vapour control layer under the insulation has to be continuous and sealed to the frame, and the ventilation route around the new opening has to be maintained, usually by keeping a clear air gap of at least fifty millimetres and by ducting around the kerb rather than into it. Where both are done, a cold roof is fine. Where the layer is torn and the void is blocked, it is the build-up most likely to hold damp timber.
The joint between the membrane and the frame
This is the whole exercise in one detail. A perfect membrane across ninety nine per cent of the ceiling with an open gap at the frame is not a vapour control layer, it is a funnel, because vapour driven by pressure difference concentrates at exactly the point where there is a route.
The correct joint turns the membrane up the inside face of the kerb, dresses it onto the underside of the frame, and holds it there with a vapour tight tape or a clamped bead of flexible sealant. It is made before the plasterboard reveal goes on, and it is not something that can be inspected afterwards. If you want to see it, ask on the day it is done.
Tapes, sealants and clamping compared
| Method | Where it suits | What limits it |
|---|---|---|
| Butyl backed vapour tape | Membrane to frame, membrane to membrane | Wants a clean dry substrate to bond to |
| Acrylic single sided tape | Laps between sheets | Poor on dusty timber or damp surfaces |
| Airtight flexible sealant | Membrane to masonry or an irregular edge | Needs a solid backing to compress against |
| Mechanical clamping batten | Membrane to a timber kerb | Wants a bead beneath it to remain vapour tight |
| Preformed collar | Standard kerb sizes on a warm deck | Only where the unit matches the collar |
Nothing exotic is required. What is required is that the method suits the two materials being joined and that somebody has decided which method is being used before the reveal is boarded.
Pitched slopes, and the layer behind the plasterboard
In a loft conversion or a room in the roof, the vapour control layer runs behind the plasterboard across the whole ceiling, and a roof window interrupts it at four edges. The window’s own insulation collar goes around the frame, and the membrane is dressed onto the frame and taped in the same way as on a flat roof.
The extra consideration on a slope is the drainage gutter formed in the underlay above the window head, which is a separate layer doing a separate job on the cold side. The two are often confused. The underlay sheds any water that gets past the tiles. The vapour control layer stops indoor air getting into the construction. Both are needed and they sit either side of the insulation.
What happens when it is missing
Interstitial condensation, which means water forming inside the construction rather than on a visible surface. Vapour passes into the insulation, reaches the cold underside of the deck or the sarking, and condenses there. In a cold deck it may be carried away by the ventilation. In a warm deck with no ventilation, there is nowhere for it to go.
The consequences build slowly. Insulation absorbs moisture and its thermal performance falls, which makes the surface colder, which produces more condensation. Timber that sits above about twenty per cent moisture content for long periods starts to decay. The first visible sign is usually a stain on the ceiling near the opening that gets blamed on the rooflight, when the unit is perfectly sound and the air behind the plaster is doing the damage. The knock-on effect on thermal performance is covered on are rooflights bad for insulation.
Retrofitting the layer at an existing opening
Where an older rooflight is coming out, the vapour control detail is one of the things worth putting right while the opening is accessible, because the labour is already committed and the covering is already lifted.
On a flat roof the existing membrane is cut back cleanly, a new piece is lapped in and turned up the kerb, and the frame is bedded onto a continuous bead. On a slope the plasterboard reveal comes off, the collar goes in, and the membrane is taped to the new frame before the lining is made good. Sunspire fits new rooflights and replaces units that have reached the end of their working life, and in both cases the layer is rebuilt as part of the job rather than left as found.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements