01Short answer
Yes, they can, and it tells you something
Flat rooflight condensation is a sign that the glass or its edge is colder than the room around it, so the fix lies in how the rooflight is built, not in wiping it.
Warm air carries moisture. Cooking, showers, kettles and breathing all add it. When that air touches a surface cold enough, the moisture turns to water droplets. A flat rooflight is often the coldest surface in the room, so it is where you notice it first. A bit of fogging on a freezing January morning is normal for almost any glazing. Streaming water that drips onto the worktop is not, and it points to a rooflight that is losing too much heat.
That is why we only talk about condensation as a symptom. The question to ask is not "how do I stop the damp" but "why is this glass so cold?" Answer that and the damp usually takes care of itself.
02Cold glass
Why the surface ends up cold
Condensation forms where a surface drops below the temperature at which the room air can no longer hold its moisture, and three parts of a flat rooflight set that temperature.
The first is the glass itself. Older twin-skin domes and early double glazing let heat flow out quickly, so the inner face stays cold. A modern sealed unit with a low-e coating (a thin invisible layer that bounces heat back into the room) and argon gas between the panes keeps the inner face much closer to room temperature.
The second is the edge. The spacer is the strip that holds the two panes apart. An aluminium spacer conducts heat, so the rim of the glass runs colder than the middle and the droplets gather in a line round the edge. A warm-edge spacer made of low-conductivity material cuts that out, and our sealed units are made with one routinely.
The third is the frame and the kerb, the raised upstand the rooflight sits on. If the kerb is thin, uninsulated or joined badly to the roof insulation, the frame gets cold and the plaster lining around the opening can darken as moisture collects there. You can read more on the kerb in our guide to rooflight kerbs and upstands.
| Where it shows | Likely cause | What changes it |
|---|---|---|
| Whole pane, most mornings | Glass losing heat too fast | Low-e, argon-filled double or triple glazing |
| Ring round the edge | Metal spacer acting as a cold bridge | Warm-edge spacer |
| Frame and plaster reveal | Cold kerb or frame | Insulated kerb joined to the roof build-up |
| Between the panes | Failed sealed unit | New sealed unit or a whole new rooflight |
03Inside or between
Droplets inside the room, or fog between the panes
Where you see the moisture matters more than how much, because the two cases have different causes.
Moisture on the room side of the glass is ordinary condensation. It wipes away, and it comes from the air in the room meeting a cool surface. Moisture trapped between the two panes is a different thing. It means the seal of the sealed unit has let go and the gas has gone, so the unit no longer insulates the way it should. That cannot be wiped off from either side. We explain the signs on our page about why rooflight glass mists.
On the room face
Clears when the room warms and the air dries. Points to cold glass, a cold edge or a damp, steamy room. Better glazing and better ventilation both help.
Between the panes
Stays whatever the room does. Points to a failed seal. The sealed unit is replaced, or the whole rooflight if the frame is tired too.
04What we specify
Building a flat rooflight that stays dry
A warm rooflight is a set of choices made together, and each one raises the temperature of the coldest surface in the room.
We start with the glazing. A good double glazed rooflight can reach around 1.4 W/m²K (the U-value, a measure of heat loss where lower is warmer). Triple glazing goes lower still. Our page on rooflight U-values sets out what the numbers mean in practice.
Then the edge, with a warm-edge spacer as standard. Then the kerb, insulated and joined to the roof insulation so there is no cold path from the outside into the frame. Our warm-edge spacers and thermal breaks page covers how that joint works.
- We measure the opening and look at the existing kerb and roof build-up at the free survey.
- We choose glazing, spacer and kerb insulation as one package, and put it in the written quote.
- We fit the unit, join the kerb to the roof insulation and finish the lining.
- If the job needs notifying, we send the Building Control notification ourselves.
If you are swapping an older unit, replacing an old rooflight shows how that process runs. Our flat rooflight installation and replacement work sits under flat roof rooflights.
05Air in the room
Ventilation still counts
Even the warmest skylight cannot cope with a room that holds on to every drop of steam.
Good glazing raises the surface temperature, but the moisture still has to go somewhere. An extractor over the hob, a fan in the bathroom and a way to open the room up on a still, damp day all lower how much water the air holds. An opening rooflight is one way to do the last of those, and it also lets warm, wet air rise out at the highest point of the room.
- Use the cooker extractor while you cook, and leave it running a few minutes after.
- Keep internal doors closed when drying washing indoors.
- Open a window or an opening rooflight briefly rather than leaving it ajar for hours.
- Keep furniture and blinds from sitting tight against the glass, as they trap cold air against the surface.
If you are planning a new rooflight in a steamy room, tell us at the survey. We will choose the glazing and the opening option with that in mind.
Damp glass on a flat roof: what is normal?
Is a little condensation on a skylight normal?
Yes. On a cold, still morning almost any glazing can fog briefly, especially over a kitchen. It should clear as the room warms. Water that runs down the glass or drips daily is a sign the rooflight is too cold.
Will a new rooflight stop it completely?
We cannot promise that, because the moisture comes from the room as well as the glass. A warm-edge, low-e unit on an insulated kerb makes it far less likely, and good ventilation does the rest.
Does water between the panes mean I need a whole new rooflight?
Not always. If the frame and kerb are sound, a new sealed unit may be enough. If the frame is worn or the kerb is cold, a whole new rooflight is the better route. We look at both at the free survey. Replacing the whole rooflight is notifiable under Building Regulations, and we handle that notification.
Does triple glazing help?
It makes the inner face warmer, so it helps. It also makes the unit heavier, so we weigh it against the frame and the opening at survey.
