Air leakage around a rooflight is uncontrolled air moving through the perimeter of the opening, past the lining, through perished gaskets, or through a vapour control layer that was cut and never resealed. It loses heat directly, and because the rooflight sits at the top of the room where warm air collects and pressure is highest in winter, it leaks harder than a gap lower down would. Insulation does nothing about it, because insulation slows heat conducting through material, not air moving through gaps. Airtightness is a separate job, achieved during a replacement with tape and membrane bonded to the deck and carried up to the frame, not with expanding foam.
On rooflights we survey after a failure, uncontrolled air movement is often doing more damage than the U-value ever did. It costs heat, it drives draughts, and it carries moisture into the construction where you cannot see it.
Why the top of the room leaks hardest
Warm air rises and collects under the ceiling. In winter that warm air is at higher pressure than the cold air outside, and the pressure difference is greatest at the highest point of the room. The rooflight sits exactly there. So it behaves like the top of a chimney: warm air pushes out through any gap in the perimeter, and cold air is drawn in low down to replace it. A gap of the same size in a skirting board leaks far less, because it sits where the pressure difference is smallest.
This is why a leaky rooflight perimeter feels worse than its size suggests, and why sealing it is worth doing properly. Every litre of warm air that escapes at the ceiling is a litre of cold air pulled in at floor level, and the heating runs to make up the difference.
Where the air actually gets through
Cutting an opening in a roof creates a perimeter, and the perimeter is a stack of joints, each of which either got sealed or did not:
- Plasterboard to reveal. The joint where the ceiling board meets the reveal lining. Often filled and skimmed for looks, which hides a gap rather than sealing it.
- Reveal to upstand. Where the internal lining meets the timber upstand. A common route, because it is out of sight during finishing.
- Upstand to deck. Where the base of the upstand meets the roof deck. This is the joint that matters most, because it is the boundary between inside and the cold roof build-up.
- Perished gaskets. On the unit itself, the rubber gaskets that seal the opening light to the frame harden and shrink with age and UV. An old opening rooflight often leaks air around a gasket that has lost its spring.
- Service penetrations. Wiring for an electric rooflight or a light punched through the lining and left unsealed.
The unsealed vapour control layer
Every warm roof has a vapour control layer on the warm side of the insulation. Cut a hole in the roof for a rooflight and you cut a hole in that layer. Reinstating it around the opening is a short job that is skipped far more often than it is done. When it is left open, it is both an air leak and a moisture path, which is how air leakage and interstitial condensation turn out to be the same failure seen from two angles. Warm moist air leaking through the gap carries its moisture into the cold construction, where it condenses and rots timber out of sight.
How leakage is identified
| Method | What it shows | Who does it |
|---|---|---|
| Hand or damp-back-of-hand check | Draughts at the reveal on a windy or cold day | Homeowner, no kit needed |
| Smoke pencil or incense stick | Air movement traced along the perimeter joints | Homeowner or surveyor |
| Thermal imaging on a cold day | Cold streaks where leaking air chills the surface | Surveyor with a camera |
| Blower door test | Whole-dwelling leakage rate, pinpoints paths under pressure | Air-test engineer, usually on new work |
For a homeowner the simplest test needs nothing: on a cold, breezy day, run the back of your hand slowly around the reveal a few centimetres from the surface. A moving line of cold air is a leak. Hold a smoke pencil or a lit incense stick near the joints and watch the smoke pull sideways where air is moving. Draught you can feel plus condensation on the surface usually means both a leak and a cold surface, which is common, because the same poor detailing causes both. Our simple homeowner checks are on how to tell if your rooflights are losing heat.
Draught or cold downdraught? A leak moves air from outside to in, and you feel it as a directional draught that gets worse in wind. A cold unit creates a downdraught, where room air is chilled by the cold glass and sinks, which you feel as a steady coolness under the rooflight regardless of wind. They feel similar and need different fixes. A leak needs sealing. A cold unit needs replacing. Both often need doing together.
How airtightness is achieved during a replacement
Airtightness at a rooflight is a detail, not a product you spray on at the end. The sequence we work to:
- Expose the perimeter cleanly. With the covering back and the opening open, we can see every joint rather than sealing over guesswork.
- Bond the membrane to the deck. Airtightness membrane and tape bonded to the roof deck around the opening, lapped to the existing layer so it is continuous, not patched.
- Carry it up the upstand. The membrane is dressed up the full height of the upstand and terminated at the frame, so there is a sealed line from deck to frame with no break.
- Seal the frame junction. The unit is bedded and the frame-to-upstand junction sealed, so the opening light gasket is the last line rather than the only one.
- Reinstate the vapour control layer. Lapped, taped and continuous with the roof layer, terminating at the frame, so it is airtight and vapour-tight together.
What we do not use is expanding foam as an airtightness measure. Foam fills a gap and looks convincing, but it is not an airtightness product, it does not stay bonded to timber through a decade of seasonal movement, and it cracks away from the joint it was meant to seal. It has a place as a backing to a taped joint. It has no place as the seal itself.
What it means for your job
If your rooflight draughts, mists and stains together, you almost certainly have a cold unit, a leaking perimeter and probably a bridged upstand, because the same corner-cutting causes all three. A replacement done properly fixes all three at once: a better unit, a continuously insulated upstand, and an airtight, vapour-controlled perimeter. Doing one without the others leaves the room half-fixed. We set the priorities on how you reduce heat loss through rooflights, and the related bridging failure is on thermal bridging at rooflight junctions.
Can air leakage around a rooflight be sealed without replacing it?
The visible reveal joints can sometimes be improved from inside, but the joint that matters, upstand to deck, is under the roof covering. Reaching it means lifting the covering around the perimeter, which is most of the work of a replacement. If the unit is old or cold as well, it usually makes sense to replace and detail the whole junction together. If the unit is genuinely sound and only the internal joints leak, we scope that honestly rather than pushing a full replacement.
Is expanding foam enough to stop draughts around a rooflight?
No. Expanding foam fills a gap but it is not an airtightness product. It does not stay bonded to timber through years of seasonal movement, it cracks away from the joint, and it lets air track around it. Proper airtightness uses membrane and tape bonded to the deck and carried up to the frame, continuous with the roof layer. Foam can back up a taped joint, but on its own it is a temporary look rather than a seal.
Get the perimeter surveyed properly
We check the unit, the upstand and the perimeter for both heat loss and air leakage, tell you which you have, and price a replacement that seals the junction rather than skinning over it. One fixed quote, a 10-year workmanship guarantee, and installers with 25 years on rooflights.
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Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.