How Can You Tell If Your Rooflights Are Losing Heat?
You can tell a rooflight is losing heat with three simple checks: feel for a cold downdraught underneath it on a winter evening, look for morning condensation on the glass or a black line of mould at the reveal, and run the back of your hand around the perimeter on a windy day to feel for draughts. A thermal image confirms it visually, showing the unit and the junction as cold patches against the warmer ceiling. None of these needs a professional to start, and together they tell you whether the loss is in the unit, the upstand or the perimeter.
Knowing which of the three it is matters, because a cold unit, a bridged upstand and a leaking perimeter feel similar from underneath and need different fixes.
The homeowner tests, no kit needed
| Check | What you do | What it points to |
|---|---|---|
| Downdraught | Stand under the rooflight on a cold evening, feel for cool air falling | A cold, poorly insulating unit |
| Condensation | Look for misting on the glass in the morning, or mould at the reveal | Cold glass, or a thermally bridged upstand |
| Draught | Move the back of your hand slowly around the reveal on a windy day | A leaking perimeter |
The cold downdraught
A cold unit chills the air against its inner surface, that air gets denser and sinks, and you feel it fall as a draught even though nothing is leaking. Stand under an old rooflight on a January evening and you can feel it on the back of your neck. A steady coolness that does not change with the wind points to the unit itself being cold. That is a replacement job, because the weakness is the glazing.
Condensation and mould
Misting on the room side of the glass in the morning means the inner surface is dropping below the dew point of the room air overnight, which is a cold unit doing its worst. A black line of mould at the reveal, with no misting on the glass, is different: that is usually a thermally bridged upstand running cold around the opening, covered on thermal bridging at rooflight junctions. Both are heat problems, but one is the unit and one is the junction.
The draught check
On a cold, breezy day, move the back of your hand slowly around the reveal a few centimetres from the surface. A moving line of cold air is a leak, not just a cold surface. A lit incense stick or a smoke pencil held near the joints will pull sideways where air is moving. A draught that gets worse in wind points to a leaking perimeter, covered on air leakage around rooflights. Often you find both a draught and a cold surface, because the same corner-cutting causes both.
Separate the leak from the cold. A leak follows the wind and gets worse in a gale. A cold downdraught is steady and depends on temperature, not wind. A cold reveal that stains but does not drip after rain is condensation, not a leak. Getting this right before you call anyone saves you paying for the wrong fix.
What a thermal image reveals
A thermal camera shows surface temperature as colour. On a cold day, pointed at a rooflight from inside, it reveals what your hand can only guess at:
- The unit as a cold block if the glazing is poor, cooler than the ceiling around it.
- A cold line around the perimeter if the upstand is thermally bridged, the tell-tale of a bare timber kerb.
- Cold streaks where air is leaking, chilling the surface along a joint.
- Rows of cold dots where fixings bridge from the cold outer structure to the warm inner face.
You do not need a survey-grade camera to see the obvious cases. A clip-on phone thermal camera or a cheap handheld one, used on a cold morning with the heating on, shows a bad rooflight clearly. What it will not do is tell you the U-value, only where the cold is. For that, and for the fix, a survey turns the picture into a specification.
What to do with the result
If the unit reads cold and mists, it is past its life and the answer is replacement, not resealing. If the perimeter is a cold line, the upstand needs insulating. If a joint leaks, the perimeter needs sealing. Usually a survey finds more than one, because the same job that fitted a cheap unit also skimped the upstand and the sealing. The full ranking of what to fix first is on how you reduce heat loss through rooflights. If the rooflight is genuinely sound and only fifteen years old, the honest advice is to leave it.
How can I tell if my rooflight is losing heat without a thermal camera?
Three checks. Stand under it on a cold evening and feel for a cold downdraught, which points to a cold unit. Look for morning misting on the glass or a black line of mould at the reveal, which points to cold glass or a bridged upstand. And run the back of your hand around the reveal on a windy day to feel for draughts, which points to a leaking perimeter. Together they tell you whether the loss is in the unit, the junction or the seal.
Is condensation on a rooflight a sign it is losing heat?
Usually yes. Misting on the room side of the glass means the inner surface is dropping below the dew point of the room air, which is a cold, poorly insulating unit. Mould at the reveal without misting on the glass points to a thermally bridged upstand running cold around the opening. Both are heat problems. Water appearing only after rain, or a tide mark, points to a leak instead, which is a different fix.
Get it surveyed and priced
We confirm whether the loss is in the unit, the upstand or the perimeter, and price the fix as one job. A 10-year workmanship guarantee, and installers with 25 years on rooflights.
Request a fixed quote, or start from energy efficient skylight installation.
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.