Rooflight condensation, or skylight condensation as homeowners often search for it, is the film of water you find on the glass or frame on a cold morning, and it is a signal worth reading rather than just wiping away. Sometimes it is harmless and sometimes it points to a failed unit, a cold frame or a room that cannot shed its moisture. This page helps you tell which, explains what causes each kind, and says what actually reduces it. It sits under our energy efficient rooflights page.
Three kinds of condensation, three different causes
Before you change anything, work out where the water is. The position tells you almost everything.
On the inside face of the room-side pane
This is the common one. Warm, damp air from the room meets a glass surface colder than the dew point of that air, and the moisture drops out as droplets. It is worst in kitchens, bathrooms, utility rooms and bedrooms, and worst on cold, still mornings. It is a moisture and temperature problem, not a fault in the glass.
On the outside face of the outer pane
A film on the outside of the glass on a clear, cold night looks alarming but is a sign that the unit is doing its job. A well-insulated pane keeps the heat inside, so the outer surface gets colder than the air outside and dew forms on it, exactly as it does on a car windscreen. It clears as the sun comes up. It does not mean anything is wrong.
Between the panes
Moisture or a cloudy film trapped inside the sealed unit means the edge seal has failed and humid air has got into the cavity. You cannot wipe it off because it is not on a surface you can reach. A unit in this state cannot be repaired. The glass is replaced, and we explain how we assess one on misted and failed rooflights.
Why a rooflight is more prone than a wall window
A rooflight sits at the top of the room, which is exactly where warm, moist air collects. Steam from a kettle, a shower or a pan rises and meets the coldest large surface in the room, which is often the glass overhead. Add to that the fact that the glass faces the open sky, which loses heat fast on a clear night, and you have conditions that suit condensation.
The frame and the surroundings matter as much as the pane. Heat travels through metal far more readily than through glass or timber, so an older aluminium frame without a thermal break can run cold along its whole length. Water often shows up first on the frame and on the plaster around it, long before the centre of the glass mists. A kerb or upstand that has little insulation, or a light well lined with bare plaster, does the same. If you want to see how the frame and glass combine into one figure, read rooflight U-values explained.
| Where you see it | Likely cause | What helps |
|---|---|---|
| Room side of the glass | Damp air on a cold surface | Ventilation, warmer glass |
| Frame and surrounding plaster | Cold frame or uninsulated kerb | Thermal break, insulated kerb |
| Outer face on clear nights | Efficient glass losing heat to the sky | Nothing, it clears by itself |
| Between the panes | Failed edge seal | Replace the unit |
What you can do about moisture in the room
Most of the cure sits in the room, not in the rooflight. Cutting the moisture or removing it quickly does more than any change to the glass.
- Run the extractor fan in the kitchen or bathroom while you cook or wash, and leave it on for a while afterwards.
- Close the door of a steamy room so the moisture is not carried into a cooler space under a rooflight.
- Open the rooflight, if it opens, for a few minutes after steam is produced. Warm air leaves at the highest point.
- Avoid drying washing on radiators in a room under a rooflight.
- Use a dehumidifier in a room with persistent moisture, particularly in a flat with little through ventilation.
A rooflight that opens is the best passive vent you can have in such a room. Our page on electric and opening rooflights shows the options, including models with a rain sensor that close themselves when it starts to rain.
What a better rooflight changes
A replacement unit cannot fix a room that has far too much moisture in it, but it moves the odds in your favour in three ways.
First, warmer glass. A modern sealed unit with a low emissivity coating, argon fill and a warm edge spacer keeps the inner pane warmer than an old double unit or a single pane. A warmer surface needs more moisture in the air before it condenses on it. The spacer is the part that gets overlooked, and we give it its own page: what is a warm edge spacer. The old aluminium spacer made the edge of the glass the coldest strip, and that is where the first droplets formed.
Second, a frame with a thermal break. Warmer metal stays dry. A frame designed for current Building Regulations is much less likely to sweat than a thirty-year-old one.
Third, triple glazing, where it suits the roof. The inner pane is warmer still. We weigh this in double or triple glazed rooflights and in is triple glazing worth it for a rooflight.
Insulation around the opening
A warm pane in a cold surround simply moves the problem to the edge. The kerb or upstand of a flat roof rooflight, or the lining of a roof window opening, should be insulated all the way round and sealed so that warm air cannot find its way into the cold layers of the roof build up. When that is missing, you see damp patches or mould along the edge of the plasterboard splay, and sometimes a dark line on the ceiling.
The splay itself, the angled reveal that leads from the glass down to the ceiling, matters too. A deep splay in a thick roof is a cold chimney of air if it is not lined with insulation. Our flat roof rooflights and roof windows pages explain how those openings are put together. On a roof lantern, the base frame and upstand do the same job, and any gaps there show up as damp on the inner face of the brickwork or the ceiling around it.
Condensation in Brighton and Hove homes
Coastal air is damp, and the city has plenty of homes where moisture has few ways out. Tall terraced houses with a bathroom added under a flat roof, converted flats where the only vent is a rooflight, and loft rooms built into a roof void all hold on to steam. Sea mist on a still morning also leaves the outside of the glass wet, which is the harmless kind described above.
In a converted building the ventilation often belongs to the whole house rather than to your flat. An extractor that discharges into a shared roof void, or a duct that someone else controls, may sit outside your lease. If that is your case, tell us at the survey and we will say what can be done within the roof opening itself.
The most useful single habit is to ventilate quickly after steam is made, not hours later. Condensation forms in minutes, and so does the cure if the air has somewhere to go.
Sun tunnels and condensation
A sun tunnel can collect moisture inside the tube, particularly if the diffuser at the ceiling is not sealed and humid room air travels up. A sealed ceiling diffuser and a tube that is insulated through the loft keep it dry. You can read more on sun tunnels.
When condensation points to a bigger problem
Occasional droplets on a cold morning are normal. Some signs are not.
- Standing water or streaks on the glass that return every day, even in mild weather
- Black mould growing on the plaster or the sealant around the frame
- Water running down the inside of a splay and staining the ceiling
- A cloudy or misty look between the panes that never clears
These point to either a failed unit, a cold bridge in the frame or kerb, or a leak that is being mistaken for condensation. A leak and condensation can look alike. A rule of thumb is that condensation follows the weather inside, appearing after cooking or showers or on cold mornings, while a leak follows the rain outside. If staining appears only after rain, the cause is more likely water getting in than moisture getting out. We assess the whole opening on the survey, and where a unit or its surround is the cause, the answer is replacement. We do not paper over it.
Replacement and the rules
If the answer is a new unit, removing an entire rooflight or roof window and fitting an identical one still counts as building work. That makes it notifiable, with Part L applying to what goes in. The target for a flat-roof rooflight or lantern is 2.2 W/m²K. For a roof window it is 1.4 W/m²K or Window Energy Rating band B. Fitting new glass into a frame that is sound is not treated as a controlled fitting. When a notification is due for the replacement, we send it to Building Control ourselves. The detail is in the Building Regulations guide, and what the numbers mean is explained in what U-value does a rooflight need.
Condensation is not a reason to avoid an efficient rooflight. The reverse is true: a colder, older unit sweats more. If you worry that a new rooflight will make the room feel cooler, read do rooflights make a room cold.
Solar control and condensation are separate things
A tinted or coated pane that cuts the sun's heat does nothing for moisture. If your trouble is a hot room in summer rather than a wet one in winter, see solar control glass for south-facing rooflights. The two problems can sit in the same room, and the fix for one does not solve the other.
Getting to the roof, and what the visit involves
Replacing a failed unit means getting glass and fitters to the roof safely. In a tall terrace or a mansard that may mean a scaffold on the street, and in a converted building it may mean the freeholder's consent. The access plan is a written page in every quote. Its contents are the way up to the roof, the person applying for any scaffold licence and bay suspension, notice to neighbours, the Building Control route and the price. Seethe access plan and scaffold licences and parking suspensions. If you are a leaseholder, rooflights in flats and leasehold buildings explains the lease side, and you should check your own lease.
If the replacement involves a larger opening or a change to how the roof looks, planning may come into it. Flats have no permitted development rights, and conservation areas with an Article 4 direction can remove them for houses. The planning guide sets out the questions to ask the council. Finished work is covered by the 10-year workmanship guarantee on our guarantee page, and the quote is a fixed written figure that follows a free survey. What moves that figure is covered on costs.
A short checklist before you call
- Locate the water. Room side, frame, outer face or between the panes.
- Check the weather link. Does it follow cooking and showers, or rain?
- Test the extractor. Make sure it moves air and vents outside.
- Open the rooflight. If it opens, ventilate after steam.
- Book the survey. If water sits between the panes or damp keeps returning, we assess the unit and its surround.
If the checklist leaves you unsure, request a free survey and the glass, the frame and the room get looked at together. For the wider subject, go back to the energy efficient rooflights hub, or look at whether rooflights overheat in summer for the opposite problem.
