Condensation forms on a rooflight when warm, moist room air meets a surface colder than its dew point, and the surplus moisture drops out as water. The rooflight is the coldest surface in the room, and kitchens and bathrooms are the wettest rooms, so the two meet there first. Condensation on the room side of the glass is usually normal physics on a cold morning, worse with a poor unit or high humidity. Condensation between the panes, inside the sealed unit, means the unit has failed and needs replacing. Telling the two apart decides whether you manage moisture or replace the glass.
Most rooflight condensation is a cold surface plus damp air, not a fault. But some of it is a failed unit, and some of it is a cold bridged upstand, and those need different action.
Dew point, in plain terms
Warm air holds more moisture than cold air. The dew point is the temperature at which a given parcel of air is full and can hold no more, so any further cooling forces the surplus out as liquid water. In a busy Suffolk kitchen the dew point of the room air might be around 12 to 14 degrees. If any surface in the room is colder than that, water forms on it. The rooflight is the coldest surface, because glass insulates far less than the walls and it faces the cold sky, so it is where condensation appears first.
Where the moisture comes from
Condensation needs a cold surface and moist air together. The cold surface is the rooflight. The moist air comes from everyday living, concentrated in certain rooms:
- Kitchens. Cooking, kettles and washing put a lot of moisture into the air. A recirculating charcoal extractor removes smells but not moisture, so it adds humidity with nowhere to send it.
- Bathrooms and en-suites. Showers and baths generate steam directly under the rooflight, which is the worst place for it.
- Drying washing indoors, unvented tumble dryers, and simply a full house on a cold day.
This is why the same rooflight condenses in a kitchen and stays clear in a hallway. The fix for surface condensation on the glass is therefore two-sided: a warmer unit so the surface is above the dew point more of the time, and better ventilation so the air carries less moisture. Ventilation options including powered vents are on our electric skylight installation page.
Telling normal condensation from a failed unit
| Where the water is | What it means | What to do |
|---|---|---|
| On the room side of the glass | Normal on a cold morning, worse with a poor unit or damp air | Warmer unit and better ventilation |
| Between the panes, inside the sealed unit | The unit has failed, its seal has gone | Replace the unit, wiping does nothing |
| On the reveal or as a black line at the edge | A thermally bridged, cold upstand | Insulate the upstand |
Room-side condensation
Water you can wipe off the inside of the glass is surface condensation. It is the cold surface meeting damp air, and it is normal to a degree on a cold morning, especially in a kitchen or bathroom. It gets worse with a poor unit and high humidity. Manage it with a warmer unit and more ventilation. It is not a fault in the glass.
Condensation inside the sealed unit
Misting or droplets between the two panes, which you cannot wipe because they are sealed inside, mean the unit has failed. The edge seal has let go, the gas has escaped, and moist air is now trapped in the cavity. Wiping does nothing, and there is no repair, because the sealed unit is a factory item. It is replaced. The signs of a failed unit are on how to tell if your rooflights are losing heat.
Condensation at the reveal
A black line of mould around the reveal, with the glass itself clear, is not the unit at all. It is a cold, thermally bridged upstand running below the dew point around the opening, covered on thermal bridging at rooflight junctions. Sealant does nothing. The fix is an insulated upstand.
When streaming water is urgent. Occasional morning misting is normal. Water actually running off the glass and pooling, day after day, is a sign of very high humidity, a failed unit, or both, and it can rot timber and stain plaster if left. The immediate actions are on what streaming condensation means.
Why does my rooflight get condensation and my windows do not?
Because the rooflight is the coldest surface in the room and it sits above the warmest, most humid air, which collects at ceiling level. Glass insulates far less than a wall, and a rooflight also faces the cold night sky, so its inner surface drops below the dew point of the room air before a window does. Kitchens and bathrooms make it worse by adding moisture directly beneath it. Warmer glass and better ventilation are the fix for surface condensation.
Is condensation on a rooflight a sign it has failed?
Only if the water is between the panes. Condensation on the room side of the glass, which you can wipe, is normal surface condensation from a cold surface meeting damp air, worse with a poor unit or high humidity. Misting sealed inside the unit, which you cannot wipe, means the edge seal has failed and the unit needs replacing. Mould at the reveal with clear glass is a cold bridged upstand, a third and separate problem.
Get the cause identified, not just the symptom
We tell you whether it is a cold unit, a failed unit or a bridged upstand, and price the right fix. A 10-year workmanship guarantee, and installers with 25 years on rooflights.
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