The short answer
Condensation on a flat rooflight forms when warm, damp room air meets a surface colder than the dew point of that air. It is usually on the inside face of the glass or around the frame, and it is a sign of humidity, cold bridging or a failing seal rather than a hole in the roof. Working out which one it is decides the cure.
Condensation or a failed glass unit?
Flat rooflight condensation comes in two kinds, and telling them apart is the first job. Moisture that appears on the room side of the glass, clears as the day warms and returns on cold mornings is ordinary condensation. It is water from the air landing on a cold surface. Moisture or a milky film trapped between the panes, which never wipes away, means the sealed unit has failed and let humid air in. Skylight condensation of the first kind is a house problem. The second kind is a glass problem, and it needs the glass unit replaced.
If you can wipe it off with a cloth and it returns, it is on the surface. If you cannot reach it, it is inside the unit. Our page on why rooflights mist up covers the sealed-unit failure in detail.
Why the room side of the glass gets wet
Warm air holds water vapour, and cooking, bathing, drying clothes and breathing all add to it. When that air touches a cold surface, the vapour turns to water. A rooflight is often the coldest surface in the room, because glass insulates less well than the roof around it and loses heat to a clear night sky.
- High humidity in kitchens, bathrooms and utility rooms puts more vapour in the air.
- Poor ventilation lets that vapour linger instead of leaving the room.
- A cold frame or kerb conducts heat away and chills the edge of the glass.
- Glazing with a low thermal performance has a colder inner surface.
Rooflights over kitchens and bathrooms are the most likely to show it, since those rooms generate the most moisture.
The kerb and the cold bridge
Condensation that collects at the edge of the glass, or leaves a damp ring on the plaster around the opening, often points to the kerb. If the kerb is not insulated well, the cold from the roof side travels through it into the reveal and chills the surface. Warm room air then condenses on that cold ring. This is called a cold bridge, and it can show up as mould on the lining in time.
The cure lies in how the kerb and reveal are built. An insulated kerb, an insulated and splayed reveal where the roof depth allows, and a plastered finish keep the inner surface warm. That construction is described in our guide to kerbs and upstands.
What you can do about it
Some of the answer is in how the room is used, and some is in the rooflight.
- Run the extractor fan during and after cooking and bathing, and keep internal doors closed so moisture leaves through the extract rather than into other rooms.
- Open a window or an opening rooflight for a short time on damp days to let humid air out.
- Wipe condensation away so it does not run onto the frame and the plaster.
- Keep furniture and curtains from blocking air movement under the rooflight.
An opening rooflight is a practical help, because it vents the warm, damp air at the highest point in the room where it collects. If your rooflight is fixed and the room suffers regularly, replacing it with an opening unit, or adding an extractor, is a sensible conversation to have at the survey.
When replacing the rooflight is the answer
An old rooflight with single glazing, or with a poor unit and a cold frame, will condense far more than a modern one. A new unit with better glazing and a thermally broken frame keeps the inner surface warmer. A whole new unit is building work that must meet Part L, which sets 2.2 W/m²K as the limit for a flat-roof unit. Notifying Building Control falls to us when it applies, and this answer on Building Regs explains why. Where only the sealed glass unit has failed in a sound frame, changing the glass alone is not a controlled fitting.
Condensation, mould and the ceiling
A little morning condensation that clears is a nuisance. Moisture that stays wet for long periods is more serious, because it can darken the plaster, lift paint and encourage mould along the lining. Treat a recurring damp patch as a message that the room is too humid, the opening is too cold, or both. Wipe the surface, clean any mould with a suitable product, and deal with the cause so it does not return. If the plaster itself has been soaked repeatedly, it may need to be made good when the cause is cured.
How we find the cause
At the free survey we look at the rooflight, the kerb, the reveal and the room. We check whether the moisture is on the surface or in the unit, how the room is ventilated and how the kerb is insulated. Then we give you a fixed written quote for whatever the cause calls for, with the reveal made good and plastered inside once the work is done, and a 10-year workmanship guarantee on our installation.
