Vapour control layers sealed to the frame
Every roof that keeps a room warm has a layer on the warm side of the insulation whose job is to stop humid indoor air reaching the cold construction above. On a plain roof that layer is easy: a sheet, lapped and taped, running from wall to wall. At a rooflight it has to be cut, taken up a kerb or a trimmer, and joined to a manufactured frame made of something entirely different, in a corner, at height, usually on the same day the covering is coming off. That junction is where vapour control layers fail, and because it is buried under insulation and plasterboard within days, nobody looks at it again for twenty years. This page is about that junction in full: what the layer does, what the numbers mean, what seals to what, and what happens when the connection is left loose. The wider thermal picture is on the energy efficient rooflights hub, and the kitchen version of the same problem, where the room’s own moisture load is the driver, is on roof lantern condensation in a kitchen.
It is an air barrier first and a vapour barrier second
The name describes the smaller half of the job. Water vapour does move through building materials by diffusion, slowly, driven by the difference in vapour pressure between inside and outside, and a control layer slows that down. But the volume of moisture that diffusion moves is modest.
What actually wets a roof build-up is bulk air movement. Warm humid room air, driven by the stack effect and by wind pressure, finds a gap and flows through it carrying its moisture with it. Widely cited comparisons put the moisture carried by air leakage through a small gap at hundreds of times the moisture that diffuses through an equivalent area of intact membrane. The membrane is doing its second job perfectly while the gap beside it undoes the whole thing. This is why the tape line matters more than the sheet.
The number on the roll, and what makes something a control layer
Vapour resistance is quoted in MNs/g, meganewton seconds per gram, and higher means more resistant. The same property is often expressed as an sd value in metres, the thickness of still air that would offer the same resistance, and again higher means more resistant.
Ordinary 500 gauge polythene sheet sits somewhere in the low hundreds of MNs/g. A purpose-made reinforced vapour control layer, often foil-faced, is higher again, and the foil-faced boards used in warm deck build-ups carry their own resistance in the facing. A material is usually described as a vapour control layer once it is above about 200 MNs/g, and as a vapour barrier well above that. For a domestic rooflight it is rarely the choice of sheet that decides the outcome. It is whether the sheet is continuous.
The rule of five, and keeping the cold side open
A roof build-up is a sandwich, and any vapour that gets past the inner layers has to be able to leave through the outer ones. The long-standing design principle is that the vapour resistance of everything on the warm side of the insulation should be at least five times the resistance of everything on the cold side.
Get that ratio the wrong way round and the construction traps moisture: vapour enters faster than it can escape and condenses inside the insulation, where it is invisible and where it destroys the insulation’s performance long before it does anything visible. This is the reason a warm deck flat roof, which has a genuinely impermeable waterproof membrane on the outside, needs a properly sealed control layer underneath rather than a nominal one. The outside cannot breathe at all, so the inside has to be closed.
Warm deck flat roofs: the layer under the insulation
In a warm deck, the commonest modern flat roof, the deck is laid on the joists, the vapour control layer goes directly onto the deck, the insulation goes on top of that, and the waterproofing goes on top of the insulation. Everything structural stays on the warm side, which is why it works.
At a rooflight the layer is cut to the opening, then taken vertically up the inside face of the kerb, over the top of the kerb, and sealed to the underside of the frame. It is not trimmed at the deck and left to end there. Where the kerb is insulated through its depth, as it should be, the layer runs on the room side of that insulation the whole way up. The finished detail is a continuous, unbroken envelope from the field of the roof, up the kerb, and onto the frame, with no free edges anywhere.
Cold decks and inverted roofs, and what changes
A cold deck puts the insulation between the joists with a ventilated void above it, and the control layer goes on the warm side of the insulation, under the ceiling. It is the harder arrangement to make work, because the layer is interrupted by every joist, every downlight and every service run, and because the ventilated void above depends on cross-ventilation that a new kerb can easily block. Current guidance discourages new cold deck flat roofs for exactly these reasons, and where one exists and a rooflight is going into it, the ventilation path around the new opening has to be maintained deliberately.
An inverted, or protected membrane, roof puts the insulation above the waterproofing, with ballast or paving over it. There the waterproof membrane is itself the vapour control layer, sitting directly on the deck, and the junction at the kerb is a waterproofing detail rather than a taping detail. Walk-on units frequently sit in this arrangement, covered on walk-on rooflight installation.
Pitched roofs and rooms in the roof
In a loft conversion the insulation runs between and usually under the rafters, and the control layer sits on the room side of it, behind the plasterboard. Above the insulation there should be a clear 25mm ventilation gap to the underlay unless the build-up is specifically designed without one.
A roof window interrupts the layer across two to four rafter bays. The trimmed opening has a doubled trimmer at the head and cill and a rafter or a trimming rafter each side, and the membrane has to be dressed around all four, into the reveal, and sealed to the frame. All four of the main manufacturers supply a vapour barrier collar sized to their own frames, which is a pre-formed sheet with a taped edge and a hole the right shape. It costs very little, it is the correct product, and it removes almost all the improvisation from the detail. Ordering it with the unit is the single most useful thing on the schedule.
The junction: taking the layer onto the frame, not near it
The failure mode is nearly always the same. The membrane is cut generously, taken up the kerb, and then trimmed off level with the top because the frame is going on next and the loose flap is in the way. What is left is a free edge sitting somewhere in the void between the insulation and the frame, doing nothing.
The correct sequence is to leave the membrane long, set the frame, then dress the surplus up onto the frame face and tape it there. On a kerb-mounted unit that means the membrane lands on the vertical outer face of the aluminium frame, above the level of any insulation, with a continuous tape line all the way round including the corners. On a roof window it means the collar is taped to the outer frame, not to the sash, because the sash moves and the tape will part within a season.
Tapes, primers, and what actually sticks to a powder coated frame
Tape choice is not incidental. A general purpose duct tape has an adhesive that creeps and a carrier that shrinks, and a joint made with it will be open in five years. What is wanted is a purpose-made airtightness tape, usually acrylic or butyl, rated for permanent internal use.
Surfaces matter more than the tape. Powder coated aluminium is smooth and takes tape well provided it is clean and completely dry, and a wipe with a solvent cleaner to remove release agents and handling grease is worth the two minutes. Bare or rough sawn timber is porous and dusty and needs a primer, or a liquid-applied airtightness paste rather than a tape. PVC-U takes acrylic tapes well. Wet or frosty surfaces take nothing at all, which is a genuine constraint on a Gloucestershire roof in January and a reason the detail is planned rather than improvised.
Corners, folds and the 100mm that decides it
A rooflight perimeter has four internal corners, and a flat sheet cannot turn a three-dimensional corner without either a cut or a fold. Cutting is faster and leaves a hole. Folding leaves a bulky pleat that the tape has to bridge, and a pleat taped only across its face has an open channel running through it.
The methods that work are to fold rather than cut, dress the pleat flat in one direction, tape across it, and then dress a separate corner piece over the whole thing, or to use liquid-applied paste at the corner where geometry defeats tape entirely. Both take a few minutes each corner. Neither can be done after the insulation is in.
The service void, and the downlight fitted in 2031
The most reliable way to keep a vapour control layer intact is to keep it away from the people who will drill into the ceiling later. A service void, a batten zone of 25 to 50mm between the control layer and the plasterboard, gives cables and light fittings somewhere to live on the room side of the membrane.
Without it, every downlight installed in the reveal or the ceiling around a rooflight is a hole cut clean through the layer, usually a hole 70mm across with a warm updraught behind it. Four downlights around a lantern reveal is a substantial deliberate air path into the roof build-up, and it is created by an electrician with no reason to know the membrane is there. Building the void, and saying what it is for, is worth more than any grade of membrane.
Interstitial condensation risk, and the assessment behind the detail
The reason all of this is specified rather than guessed is that the risk can be calculated. A condensation risk assessment takes the build-up layer by layer, applies internal and external temperature and humidity conditions, and works out whether the temperature at any interface falls below the dew point of the air reaching it. The classic steady-state method is set out in the relevant British and international standards, and the broader guidance on managing moisture in buildings sits in BS 5250.
The output is a straightforward answer: whether condensation is predicted, where in the build-up, and whether it dries out over a year. It is the kind of check that matters on an unusual build-up, a deep cold deck, or a room with a high moisture load such as a bathroom or a utility. For a standard warm deck with a properly sealed control layer, the answer is comfortable, which is exactly why the standard detail is the standard detail.
Build-ups, and where the layer sits in each
| Roof type | Where the layer sits | What it seals to at the opening | Main risk |
|---|---|---|---|
| Warm deck flat roof | On the deck, directly under the insulation | Up the kerb, onto the outer face of the frame | Membrane trimmed level with the kerb top and left open |
| Cold deck flat roof | Under the joists, on the room side of the insulation | Into the reveal, onto the frame from below | Interrupted at every joist, and the vent path blocked by the new kerb |
| Inverted roof | The waterproofing itself, on the deck | Dressed up the kerb as part of the waterproofing | Insulation above the membrane left unrestrained around the kerb |
| Pitched roof, room in the roof | On the room side of the rafter insulation | Manufacturer’s collar taped to the outer frame | Taped to the sash instead of the frame |
| Pitched roof, cold loft with a sun tunnel | At ceiling level, on the warm side | Sealed around the tube at the ceiling diffuser | An open tube acting as a chimney into a cold loft |
| Lantern on a warm deck | On the deck, under the insulation | Up all four kerb faces and onto the lantern base frame | Four corners, each needing a folded and taped detail |
What failure looks like, and how long it takes to show
Nothing happens in the first winter. Moisture accumulates in the insulation and the timber slowly, and the construction dries back a little each summer, so the early years look fine.
What appears eventually is a brown-edged mark on the ceiling near the reveal that is worse in February than in November, insulation that is visibly compressed and darkened when someone finally lifts a board, timber with a raised moisture content and a soft surface, corrosion on fixings and on any steel in the build-up, and a measurable drop in the thermal performance of insulation that is now partly wet. None of it is dramatic and all of it is expensive, because getting back to it means taking the covering off. The distinction between this and the water you can wipe off the glass is set out on condensation, ventilation and rooflights.
Sequence on site, and why this is a one-chance detail
The control layer goes in while the roof is open, between forming the kerb and laying the insulation, and it is complete before anything covers it. On a straightforward flat roof unit that is perhaps forty minutes of work, most of it in the corners. It cannot be revisited from inside without taking down the ceiling, and it cannot be revisited from outside without taking off the covering.
That is why it belongs with the other buried details: upstand height, falls, flashing laps and where the covering terminates. Those four are what the ten-year workmanship guarantee is written against, and the vapour control junction sits alongside them for the same reason, which is that nobody can inspect it once the job is finished.
Gloucestershire winters, and the roofs where this matters most
The vale is a humid place. Gloucester, Tewkesbury, Longford and the villages along the Severn sit low, beside a tidal river, on ground that holds mist, and external relative humidity through a still winter night is frequently at or near saturation. That reduces the drying potential of any construction, which means a build-up that gets damp in December has less opportunity to dry in January than the same build-up would in a drier county.
The Forest of Dean is wetter again, and its stone cottages around Coleford and Mitcheldean often have thick walls, small windows and high internal humidity from wood burners and drying clothes. In the Stroud valleys the conversions that most need rooflights are the ones cut into a bank with no other elevation, which puts a warm damp room directly under the roof. In all three, the difference between a taped junction and a trimmed one shows up sooner than it would elsewhere, and the flat roof detailing that carries it is on flat roof rooflight installation.
Next on this subject.
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