To stop condensation on a flat rooflight, break the cold surface it forms on and cut the moisture in the air. That means insulating the kerb continuous with the roof, fitting a thermally broken frame and a warm-edge glass unit, ventilating the room so warm moist air can escape, and reducing humidity from cooking and drying washing. Surface condensation on the reveals or glass is manageable this way. Condensation fogged between the panes of a sealed unit is different: the unit has failed and needs replacing, and no ventilation will clear it.
The distinction is the whole answer. Water on the surface is about temperature and humidity you can control. Water inside the glass is a broken unit. Get that right and you know whether you are managing a room or replacing a rooflight.
Why condensation forms
Warm air holds moisture, cold surfaces do not warm the air touching them, and where warm moist air meets a surface below its dew point the moisture condenses out as water. A kitchen at 20 degrees and 60 per cent humidity has a dew point around 12 degrees, so any surface below 12 degrees in that room runs wet. On a poorly built rooflight the coldest surfaces are an uninsulated kerb and the inner edge of a frame with no thermal break, which is exactly where the condensation appears. The building physics in full is on cold bridging and damp around flat rooflights.
The four things that stop it
1. Insulate the kerb
The biggest single cause is a bare timber kerb running cold from the warm room to the cold roof. Carrying rigid insulation up the outer face, continuous with the roof insulation and taped at the joint, keeps the inner face above dew point so it stops wetting. This is construction, not something you can add from inside, which is why we insulate every kerb we build as standard.
2. A thermally broken frame and warm-edge glass
A frame with a polyamide thermal break keeps its inner profile warm, so it does not condense along the edge. A warm-edge spacer around the sealed unit runs warmer than the old aluminium type and cuts condensation at the very edge of the glass. Both are specification choices made when the unit is ordered.
3. Ventilate the room
Moisture that leaves the room cannot condense in it. An extract fan in a kitchen or bathroom, vented outside, removes moisture at the source. Better still on a flat ceiling is an opening rooflight vent, because warm wet air collects at ceiling level where the rooflight is, and venting from that point clears it directly. That option is on opening and hinged flat rooflights.
4. Cut the humidity
- Do not dry washing under the rooflight. A single load puts litres of water into the air, and it rises straight to the coldest surface.
- Use lids on pans and run the extract fan while cooking.
- Crack a window or use a night-vent setting so humid air does not build up overnight, which is why condensation is worst on winter mornings.
If it only ever appears in cold weather and never during a summer storm, it is condensation, not a leak. Resealing, re-flashing or re-bedding will change nothing, because the water is forming on the inside from room air. The fix is a warm surface and drier air, which is why we build every kerb insulated from the start.
When it means the unit has failed
There is one kind of condensation you cannot manage, and it tells you the unit is finished. If the fog is between the panes of the sealed glass, inside the unit where you cannot wipe it, the seal has failed and moisture is trapped in the cavity. No amount of ventilation, insulation or heating will clear it, because the water is sealed in. That unit needs replacing.
The same applies to an old multi-skin polycarbonate dome whose internal flutes have filled with condensation and dust that cannot be cleaned out. That is a degraded unit at the end of its life, covered on yellowed and brittle dome rooflights. In both cases the answer is a new unit, not a repair. We do not offer resealing, and on a failed sealed unit it would achieve nothing anyway.
| Where the water is | Cause | Fix |
|---|---|---|
| On the reveals, evenly, in cold weather | Cold, uninsulated kerb | Insulate the kerb, ventilate, cut humidity |
| On the inner face of the glass | Cold room surface, high humidity | Warm-edge glass, ventilation, lower humidity |
| Fogged between the panes | Failed sealed unit | Replace the unit |
| Inside a dome’s flutes | Degraded multi-skin dome | Replace the dome |
Suffolk and north Essex specifics
Older flat-roofed extensions across Ipswich, Kesgrave and Chantry almost all have uninsulated kerbs, because insulation was not standard on a 1980s or 1990s job, so surface condensation on the reveals is common and it is nearly always the kerb. On the coast at Felixstowe and Harwich, a fogged sealed unit near the sea can also show early edge-seal failure, because salt-driven weathering is harder on the perimeter seal, which is one more reason we specify marine-grade units there. The moisture side is the same everywhere: kitchens and bathrooms generate it, and a flat ceiling collects it.
Common questions
Why does my rooflight only condense in winter?
Because condensation needs a cold surface, and the kerb and frame only run cold enough in cold weather. In mild weather the surfaces stay above dew point and stay dry regardless of rain. That seasonal pattern is the clearest sign it is condensation from a cold bridge rather than a leak, which would follow rain at any time of year.
Can I stop condensation without replacing the rooflight?
If it is surface condensation, ventilation and lower humidity help immediately, though a genuinely uninsulated kerb needs the insulation built in, which means work at the kerb. If the fog is between the panes, the sealed unit has failed and only replacement clears it. So it depends on where the water is: on the surface you can manage it, inside the glass you replace the unit.
Will an opening rooflight help with condensation?
Yes. Warm moist air collects at ceiling level where the rooflight sits, so an opening vent there clears it directly, better than a wall window. Combined with an insulated kerb, a thermally broken frame and an extract fan, it is one of the best defences against a damp, condensation-prone room, especially in a kitchen or bathroom.
Get a damp rooflight sorted
Tell us where the water appears, whether it stops in mild weather, and whether it is on the surface or inside the glass. We will survey it, confirm whether it is a cold kerb or a failed unit, and give you one written fixed price. Every kerb we build is insulated as standard, and every installation carries a 10-year workmanship guarantee.
Request a fixed quote in Ipswich, Suffolk or north Essex. The wider job is on flat roof 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.