Guide · Energy Efficient Rooflights
Vapour control layers sealed to the frame
A vapour control layer only works if it is continuous. Around a rooflight it almost never is, because the layer runs across a ceiling, arrives at an opening, and then has to be turned up a reveal and bonded to a manufactured frame made of something entirely different. That junction is where most roof moisture problems on an otherwise sound installation begin, and it is closed in about an hour with tape and a primer if somebody thinks about it before the plasterboard arrives.
This page is about that hour. What the layer is doing, which side of the insulation it belongs on in each build-up, how it terminates against a timber, polyurethane or aluminium frame, and what the sequence has to be for it to be possible at all. Room side condensation and ventilation strategy are handled separately from the energy efficient rooflights hub.
What the layer is holding back
Warm indoor air carries water vapour. In a heated house through an English winter the inside air holds considerably more moisture per cubic metre than the outside air, and that difference creates a vapour pressure pushing outwards through the fabric all winter long. It does not need a draught. Vapour diffuses through solid materials.
A vapour control layer is a sheet with a high resistance to that diffusion, placed on the warm side of the insulation so the moisture never reaches a surface cold enough to condense on. Everything it does depends on it being unbroken, because vapour will happily go around an obstacle. A layer that is ninety five per cent complete does not perform ninety five per cent as well.
Which way the moisture is actually moving
The direction of vapour drive is what puts the layer on the warm side rather than the cold. Through a British heating season it is outwards, from the room to the roof, for the great majority of hours. In high summer on a dark flat roof under strong sun there are periods of reverse drive, where a hot wet membrane pushes moisture inwards, which is one reason bituminous build-ups behave differently from single ply.
For a domestic rooflight in Essex the winter case dominates and the layer goes inside. What varies is where inside means, and that depends entirely on where the insulation sits. Getting the layer on the wrong side of insulation is worse than having no layer at all, because it traps moisture against a cold plane instead of holding it back.
Interstitial condensation, and why it is the dangerous one
Condensation on the room face of the glass is visible, annoying and harmless to the building. It is a temperature and humidity matter and it is covered on the hub’s condensation page rather than here.
Interstitial condensation happens inside the construction, on a cold surface within the roof build-up where nobody can see it. Timber that stays above about twenty per cent moisture content for extended periods loses strength and eventually decays. Mineral wool that gets wet slumps and stops insulating, which makes the surface colder and the condensation worse. The failure is slow, it is silent, and by the time it shows on a ceiling the timber has usually been wet for several winters.
Vapour resistance, and the five to one rule
Vapour resistance is measured in MNs/g. Plasterboard sits around 0.05 to 0.1. A polythene sheet is 250 to 500. A proprietary reinforced vapour control membrane is typically in the hundreds. Roofing underlays vary enormously, from near vapour tight bitumen felts to breather membranes deliberately made low.
The design rule of thumb in a ventilated pitched build-up is that the resistance on the warm side should be at least five times the resistance on the cold side, so that any vapour that gets past the layer can escape outwards faster than it arrives. That ratio is why pairing a good vapour control layer with an old impermeable bitumen underlay is a poor combination, and why breather membranes changed pitched roof detailing.
Warm deck flat roofs, and the layer under the insulation
In a warm deck the insulation sits on top of the structural deck and the waterproofing goes over it. The vapour control layer belongs directly on the deck, beneath the insulation, and it is often a self adhesive bituminous sheet or a reinforced membrane bonded down.
At a rooflight the layer has to run across the deck, turn up the inside face of the kerb and terminate under the frame, so that the whole upstand is enclosed on the warm side. What frequently happens instead is that the membrane is stopped neatly at the edge of the opening, leaving four cut edges facing an open timber box. The kerb then becomes the drying route for the entire roof, and it is the coldest timber in it.
Cold deck roofs, and the ceiling line membrane
In a cold deck the insulation lies between or over the ceiling joists with a ventilated void above. The vapour control layer sits immediately above the ceiling finish, below the insulation, and the ventilated void does the drying.
A rooflight in a cold deck means an insulated shaft rising through the ventilated void from the ceiling plane to the underside of the unit. The layer therefore has to leave the horizontal ceiling plane, travel up the outside of that shaft with the insulation, and seal to the frame at the top. It is the most demanding of the three build-ups and it is common in Essex on garage conversions and older rear additions, which is exactly where it is most often skipped.
Pitched slopes, and the sheet behind the plasterboard
In a rafter level insulated slope, a loft conversion being the usual case, the vapour control layer sits behind the plasterboard on the warm face of the rafters, either as a separate membrane or as foil backed board with the joints taped.
At a roof window the sheet has to be cut, folded into the reveal on all four sides and bonded to the frame. Manufacturers supply an insulation collar and a vapour control apron sized to the window for exactly this, and the collar is not an accessory to be left in the van. Fitting the window without them, and stuffing loft quilt into the gap instead, is the standard route to a marked ceiling line around a roof window three winters later.
The junction at the frame itself
This is the detail the whole page exists for. The membrane has to be mechanically and permanently bonded to the frame, all the way round, with no gaps at the corners. Corners are where it fails, because a flat sheet cannot turn a three dimensional corner without being cut or folded, and a cut needs a separate piece taped over it.
The order of work is: dress the membrane up the reveal with slack in it rather than stretched tight, form and fold the corners, tape the folds, then bond the free edge to the frame with a continuous run of tape. Slack matters. A sheet pulled tight across a corner tears at the fixing when the timber moves, and it will move seasonally for the life of the building.
Corners fail first, so every corner is folded and taped as a separate operation rather than trusted to the run of tape.
Tapes, primers and what actually sticks
Tape choice is not a detail. An acrylic airtightness tape will bond permanently to a clean, dry, dust free surface and will not bond at all to a dusty timber kerb, a chalky old frame, a damp surface or a powder coated aluminium profile in cold weather.
Two things fix that. A primer, which is a thin liquid rolled onto porous or dusty substrates to give the tape something to key into, and temperature, because most tapes need a substrate above about five degrees to develop full adhesion. Both are trivial and both get skipped on a cold February afternoon when the plasterer is due. The other honest point is that this is a permanent bond being made once, so a good tape is worth its cost against the labour of doing it twice.
| Substrate | What it needs | Common failure |
|---|---|---|
| Clean planed timber | Tape direct, brush the dust off | Sawdust film under the tape |
| Rough sawn or wet timber | Primer, and a dry day | Bonding to a loose fibre layer |
| Polyurethane or uPVC frame | Solvent wipe, then tape | Release agent left on the profile |
| Powder coated aluminium | Wipe and warmth, dedicated tape | Cold surface, tape lifts at a corner |
| Plaster or blockwork reveal | Primer, always | Tape pulling a dust layer away |
One membrane, two jobs
The same sheet is usually doing airtightness work as well as vapour control, and it is worth being clear that these are different mechanisms. Vapour control resists diffusion, which is slow and driven by concentration. Airtightness stops bulk air movement, which is fast and driven by pressure.
Air movement carries far more moisture than diffusion does, by an order of magnitude, which is why the seal matters more than the sheet. A pinhole in a membrane barely affects its vapour resistance and completely defeats its airtightness. That is the argument for taping every joint and every penetration rather than relying on overlaps held down by the plasterboard.
The penetrations added afterwards
A layer sealed perfectly on the day gets holes punched through it later by people with entirely reasonable intentions. Downlighters are the main offender: a ring of six recessed fittings in a ceiling next to a rooflight is six holes through the vapour control layer, each one a chimney carrying warm moist air into the roof build-up.
The answer is airtight rated fittings with an integral gasket, or surface mounted fittings, or a service void formed below the membrane so cabling and lights sit on the room side of it and never cross it. A service void of 25 to 50mm on battens is the tidiest solution and it costs almost nothing at first fix. Extract ducting from a bathroom or a kitchen is the other regular penetration, and it needs a proper sealed grommet rather than expanding foam.
Sequencing, and the hour that has to happen first
The junction can only be made while the frame is set and the lining is still open. That is a window of a few hours in the middle of a job, and if it passes the work becomes a strip out rather than a task.
So the sequence on a rooflight install runs: form and trim the opening, build or prepare the kerb, insulate it, set the unit and weather it externally, then close the vapour control layer and the air seal internally, then board the reveal, then skim. Boarding before sealing is the single most common ordering mistake, and it happens when the plasterer arrives on a different day from the glazier and nobody owns the junction between them.
Loft conversions in older Essex stock
Rafter level insulation in a Victorian or interwar roof brings a particular version of the problem. The original underlay is usually a bitumen felt with a high vapour resistance, which means the cold side of the build-up cannot breathe, and the five to one ratio is inverted.
Where the covering is coming off anyway, replacing the felt with a breather membrane resolves it properly. Where the covering is staying, the answer is a ventilated air gap of at least fifty millimetres between the insulation and the underlay, ventilated at eaves and ridge, plus a genuinely well sealed vapour control layer on the warm face. Every roof window in that slope becomes a junction in both the vapour layer and the ventilation path, and both have to be maintained past it.
Reading the signs that the layer was never closed
Failures at this junction present consistently. A grey or brown shadow appearing on the plaster in a line around the reveal, worst at the head. A musty smell in the roof void that was not there before. Damp staining on the underside of the boarding at one corner only, which points at an air path rather than water from outside.
The distinguishing test is timing. Water from a weathering failure appears during or shortly after rain and tracks from a specific point. Moisture from a failed vapour control layer appears through a long cold spell without any rain at all and is worst in the coldest weeks. Photographing it with the date and the weather is genuinely useful, because the pattern over a winter identifies the mechanism.
Checking it before the lining goes on
Two checks take five minutes each and both happen before the plasterboard. The first is visual and tactile: run a hand along the taped edge all the way round, pressing, feeling for a lifted corner or a section that has not been rolled down. Tape needs pressure to bond, and a roller does what a thumb does not.
The second, where the house is being pressure tested anyway on a new build or a large extension, is a smoke pencil held at the perimeter under depressurisation. Any remaining path shows immediately and can be taped there and then. On a routine extension nobody tests, so the visual check plus a photograph of the completed seal is the record worth having.
What belongs on the specification
The junction gets built properly when it is written down and allocated to somebody. A specification that closes the question names the membrane and its vapour resistance, states which side of the insulation it sits, names the tape and primer system, and says explicitly who is responsible for the seal at the frame and at what point in the programme.
That last item is the one that decides the outcome. On a job with a separate builder, roofer and glazier, this junction sits precisely on the boundary between two trades, and unallocated work at a boundary does not get done. Sunspire sets the responsibility out in writing at survey stage so the seal is closed before the reveal is boarded, and the workmanship behind it carries the ten year guarantee. The related airtightness and insulation continuity questions are set out on the energy efficient rooflights page.
Keep reading
More on energy efficient rooflights
Solar gain, overheating and shading
Read the guide →Condensation, ventilation and rooflights
Read the guide →Triple glazing in a rooflight and when it pays
Read the guide →Warm-edge spacers and where condensation starts
Read the guide →Low-emissivity coatings and argon fills
Read the guide →
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