The short answer
A rooflight does not create moisture, but it can be the first surface where indoor moisture shows. Glass is colder than plaster, and a sloped pane sits right in the path of rising warm air. A modern sealed unit with a warm edge runs much warmer on the room side than an old one. Ventilation still has to take the moisture out.
Where the water comes from
Condensation is indoor humidity meeting a surface colder than the air can tolerate. The glass only shows you what the room is already holding.
Warm air holds more water vapour than cold air. Cooking, showering, drying washing and breathing all add vapour to the room. When that air touches a surface cool enough, the vapour turns back into droplets. Physicists call the threshold the dew point, and on a winter morning the inside face of any glazing is the coldest surface in the house.
A rooflight sits at the top of the room, which is where warm air collects. A vertical window sees the air at head height. A rooflight sees the warmest and dampest air the room has, because it rises. That is why a loft bedroom with a roof window can mist up before the ground floor does, even though the glass is sound.
So skylight condensation is usually not a fault. It is a reading. It tells you the room is humid and the glass is cold, and the fix lies in changing one of those two things.
What a modern unit changes, and what it does not
Better glazing raises the temperature of the inner pane, so the room has to be damper before droplets form. It cannot make the air drier.
Older sealed units used a plain aluminium spacer bar around the edge. Metal carries cold straight to the inner pane, so the rim of the glass stays chilly and mists first. A current unit uses a warm-edge spacer made of a less conductive material, so the rim stays nearer to room temperature. The glass itself carries a low-emissivity coating, a thin metallic layer that reflects heat back into the room instead of letting it leave through the pane. A gas fill such as argon slows the movement of heat across the cavity.
Put those together and the inner pane of a new double or triple unit is noticeably warmer than a thirty-year-old one. Building Regulations Part L sets the thermal performance a new or replacement rooflight has to meet, and the units we fit are chosen to meet it. Our page on energy efficient rooflights goes through the glazing options.
The frame matters as much as the glass. A warm pane set in a cold frame still mists along the edge. The insulation wrapped round the upstand and the detail where the frame meets the plasterboard splay decide whether the reveal stays dry. That is an installation matter, and it is why the upstand is written into the quote rather than left to chance.
Four places you can see it
Where the moisture appears tells you what is going on. The locations have different causes and different answers.
| Where | Usual cause | What to do |
|---|---|---|
| Room side of the glass | Humid room air, cold pane | Ventilate, keep blinds open |
| Between the two panes | Failed edge seal | Replace the sealed unit |
| Outside face, clear night | Pane losing heat to the sky | Nothing, it clears by mid-morning |
| Frame and splay | Cold bridge at the reveal | Better insulation round the upstand |
Moisture trapped between the panes is the one to act on, because it means the gas fill has escaped and the unit no longer insulates as it should. No amount of ventilation will clear it. A failed unit is replaced, and our service for misted and failed rooflight replacement covers that work. The page on signs a rooflight has failed helps you tell the difference first.
Ventilation is the other half
A warm pane delays condensation. Moving damp air out of the room stops it. The two jobs are separate, and a rooflight cannot do the second one alone.
Building Regulations Part F covers ventilation. Rooms where moisture is made, kitchens and bathrooms above all, need extract ventilation, and habitable rooms need background ventilation such as trickle vents. When we fit a new roof window we look at whether the room already has that provision, and we note it on the quote. Many roof windows have a built-in vent in the head of the frame, which gives a little background air without opening the sash.
An opening unit does real work here. Tilting a sash for a few minutes after a shower lets the warmest, dampest air out at the highest point of the room, exactly where it gathers. For a panel that is hard to reach, an electric opening rooflight on a rain sensor makes that routine. Closing itself when the rain starts keeps the room dry and, as a side benefit, quiet.
Habits count as well:
- Run the bathroom extract fan for a while after the shower ends, with the door shut.
- Use the cooker hood, and put lids on pans.
- Dry washing outdoors or in a room with an extractor, not on a radiator in the loft bedroom.
- Leave a blind or curtain open a hand's width at the bottom so warm air can reach the glass.
A blind pulled tight against the pane traps a layer of cold, still air behind it. That is the cold surface at its worst. Raising the blind in the morning is often enough to clear the glass.
What goes on the survey notes when the room is damp
A damp room gets a line of its own. The decision is about the room, not just the hole in the roof.
At the survey we look at how the room is used, where the moisture is made and what ventilation is there. A loft conversion used as a bedroom, a bathroom with a roof window over the bath and a kitchen lantern each get a different answer. The glazing build, the upstand insulation and the ventilation are set out in the written quiet spec that comes with every quote, next to the acoustic choices, because both come from the same inner pane and the same careful upstand. A laminated inner pane, for instance, is chosen for sound and for safety overhead, and it also behaves well thermally. You can see how the pieces fit in the page on how to choose a rooflight.
If you are replacing an old unit because it mists, the new one should be specified as a whole, not just as a pane swap. Our guide to replacing an old skylight explains when like for like is enough. The work is made good inside, and the Building Control notification is handled where the work is notifiable. Across Crawley and the thirty miles around it, Clear Skylights starts with the room. When you are ready, get a quote and tell us which room it is.
Condensation questions people ask before they replace
Is condensation on a roof window a sign the glass is faulty?
Not on its own. Room-side droplets mean the glass is colder than the air is comfortable with. Moisture between the panes is the fault, because it means the seal has gone. One is a humidity reading, the other is a failed unit.
Will a new rooflight stop condensation completely?
No unit can promise that, because the moisture comes from the room. A new rooflight has a warmer inner pane and a better frame, so it takes a damper room to mist it. Extract fans and a little daily airing do the rest.
Does a bedroom skylight mist up overnight?
It can, because people breathe out a good deal of water across a night and the glass is at its coldest before dawn. Background ventilation and a blind left slightly open at the bottom keep it in check. Our page on a skylight in a bedroom covers blinds in more detail.
Should I worry about mould around the frame?
Mould grows where a surface stays damp, and a cold reveal can do that. Clean it off, find out why the reveal is cold, and have the upstand and splay detail looked at. A survey will tell you whether the insulation round the frame is the cause.