Rooflight condensation: why it happens and how to stop it

Why does condensation form on rooflights?

Air holds water vapour. Warm air holds more than cold air. When warm, damp room air touches a cold surface, it cools, and the vapour it can no longer hold turns into droplets. A rooflight is often the coldest surface in a room, because it faces the open sky, which on a clear night is colder than the surrounding air. Warm air also rises, so the moistest air in the room collects right where the rooflight is.

That is why the same rooflight can be dry all day and dripping at dawn, and why condensation is worse in winter, in kitchens and bathrooms, and in rooms where washing is dried indoors.

Is it condensation or a leak?

Condensation usually appears on cold, dry mornings or straight after cooking or showers. It forms evenly across the glass or along the frame, and wiping it away leaves a dry surface. A leak follows the rain and tends to show up at one corner or side. If you see moisture between the two panes of a double-glazed unit, that is neither: the unit's seal has failed. Our leaks guide covers how to tell them apart in more detail.

What makes it worse?

• High humidity: cooking without extraction, showers, drying washing, unvented tumble dryers and lots of people in a closed room.
• Single-skin domes and old glazing: the inner surface sits close to the outdoor temperature.
• Uninsulated kerbs: bare timber kerbs and metal frames are cold bridges, so moisture forms on the reveal and frame even when the glass is clear.
• Uninsulated reveals: plasterboard fixed straight to the kerb without insulation behind it stays cold.
• Gaps where the kerb meets the roof insulation: a strip of cold structure around the opening, often marked by a line of black mould.
• Still air: a deep, narrow reveal traps damp air against the glass.

How do you stop rooflight condensation?

Tackle both sides: less moisture in the air, and warmer surfaces for it to meet.

1. Extract at source. Use the cooker hood and bathroom extractor, and leave them running for a while after cooking or showering.
2. Ventilate. Trickle vents in windows, or a vented or opening rooflight, let damp air out. An electric opening unit with a rain sensor can be left slightly open, because it closes itself when rain starts.
3. Insulate the kerb. Adding insulation to the upstand, and making sure it meets the roof insulation without a gap, warms the frame and reveal.
4. Insulate the reveal. Insulated plasterboard, or insulation behind the lining, keeps the reveal above the dew point.
5. Upgrade the glazing. Replacing a dome or old single-glazed unit with modern double or triple glazing raises the inner glass temperature considerably.
6. Keep heating steady. Rooms that are heated and then left to go cold condense more than rooms kept at an even temperature.

When is condensation a fitting fault?

Some condensation on the coldest nights in a busy kitchen is normal, even with a good unit. It points to a fitting problem when:

• moisture or mould forms in a band around the reveal, where the kerb and roof insulation should meet;
• the kerb was left as bare timber with no insulation;
• the reveal was lined with ordinary plasterboard and nothing behind it;
• warm room air can get into the kerb or roof void through gaps at the ceiling line, where it condenses out of sight and rots timber;
• it gets worse over time, suggesting wet insulation inside the kerb.

In England and Wales, a site-built upstand on an existing dwelling is expected to reach 0.35 W/m²K, which a bare timber kerb will not. Correcting it usually means lifting the unit, insulating the kerb and re-lining the reveal.

What can you check yourself first?

1. Keep a note for a week of when the moisture appears, what the weather was doing and whether cooking, bathing or drying washing had just happened.
2. Check the extractor fans actually move air. Hold a sheet of tissue paper against the grille; it should be pulled flat.
3. Open any trickle vents in the room and check they are not painted over or blocked.
4. On a cold morning, touch the reveal and then the ceiling a short distance away. A reveal that feels much colder suggests missing insulation behind it.
5. Look for a line of mould or staining around the top of the reveal or along the ceiling edge.
6. Check whether the moisture is on the glass, on the frame or on the reveal. Each points to a different cure.

Send those notes and some photos with your enquiry. They usually show which of the fixes above is most likely to work, and whether a visit is needed at all.

Do some rooflights deal with condensation better?

A unit with a lower U-value has a warmer inner pane, so it condenses less. Thermally broken frames help at the edges, and a low-E coating on the inner pane reflects heat back into the room, keeping that pane warmer. Some plastic domes have a small condensation channel around the inside edge that collects droplets and lets them evaporate, rather than letting them drip. Vented and opening units help by moving air. None of these replaces insulating the kerb and controlling humidity, but together they can make condensation rare. For opening units see our electric opening rooflights guide.

Can condensation damage the roof?

Yes, over time. Condensation that forms where you can see it is a nuisance and can cause mould on reveals and stained ceilings. Condensation that forms where you cannot see it, inside the kerb or in a cold roof void, can soak insulation and rot joists and decking. If a ceiling near a rooflight is stained but the roof is sound, hidden condensation is worth checking. See upstand and kerb installation, rooflight replacement or send photos through the quote form.

Questions