The answer
Look at where the water sits. Condensation on the room side of the glass wipes away and is a humidity and ventilation matter. Condensation between the panes, inside the sealed unit, cannot be wiped and means the seal has failed. The first is managed with air and warmth. The second needs a new sealed unit.
The wipe test
The quickest check needs a dry cloth. Wipe the inner face of the glass, the side facing the room. If the cloudiness or the drops come away, you have surface condensation: warm, damp room air meeting a cooler pane. If nothing moves, run your eye round the edge of the glass. Moisture that stays put, with a clean look on the outside face too, is trapped inside the sealed unit.
Two more clues help. Surface condensation is worst in the morning, on cold days and after a shower or cooking, and it clears as the room warms. Mist inside the unit shows at any time, may fade in dry weather and tends to leave a tide mark where the water has dried.
Condensation on the inner face
Water on the inner face means the room is humid and the glass is the coldest surface in it. Rooflights meet this more than side windows do, because warm air rises and the glass above is exposed to the sky. It shows up in kitchens, bathrooms and loft rooms, and it is a sign of the room's air, not of faulty glass.
- Ventilation: open the rooflight if it opens, or use an extractor while cooking or showering.
- Heat: a steady, even background temperature helps more than short bursts of heat.
- Moisture: keep wet washing out of the room.
- Blinds: close a blind against the glass only if the room is warm, since a blind can trap cold air on the pane.
A modern, well-insulated unit stays warmer on its inner face, so a replacement can also lessen the problem. A rooflight that opens, perhaps with a rain sensor so it closes itself, is an easy way to give a room a fresh air path from the highest point.
Condensation between the panes
A sealed unit is two sheets of glass with a spacer and a perimeter seal, and a dry cavity between them. When that seal fails, humid air gets in, the drying agent is overwhelmed and moisture appears inside the unit. No amount of ventilation fixes this, because the water is not in your room. It is in the glass.
You will also see it described as a blown unit, a failed unit or a misted unit. They are the same thing. Our guide on why rooflights mist up between the panes explains how the seal ages, and whether glass alone can be swapped is answered in can misted rooflight glass be replaced.
The remedy is a new sealed unit, made to the size of the frame. If the frame and the weathering are in good order, only the glass changes.
What helps and what does not
People try several fixes before they call anyone. Some are sensible, and some waste an afternoon.
- An extractor fan or an opening rooflight: sensible for room-side condensation, no effect on a failed seal.
- A dehumidifier: lowers the damp in the room, but does nothing for moisture sealed inside the glass.
- Anti-mist sprays: useful for the surface of the glass, pointless between the panes.
- Drilling the unit to let it breathe: leaves the pane open to dust and damp and does not restore the insulation.
The pattern is simple. Anything that changes the air in your room helps with surface condensation. Nothing that changes the air in your room reaches the cavity, so only a new sealed unit cures a blown one.
Look, too, at where the rooflight sits. A rooflight over a shower room or a kitchen worktop meets far more steam than one over a landing, so it clouds sooner. A well-sealed, well-insulated modern unit on a ventilated roof handles that steam better, and a new unit fitted for this reason is a sensible upgrade as well as a replacement.
A table to settle it
If the two cases still look alike, this comparison sets them side by side.
| Check | Room side | Between panes |
|---|---|---|
| Wipes away? | Yes | No |
| Worst when? | Cold mornings | Any time |
| Cause | Humid room air | Failed edge seal |
| Remedy | Air and warmth | New sealed unit |
| Leaves a mark? | No | A tide line |
When it is neither
Two other things can look like condensation. Water running down the inside of the frame after rain is a leak, and it needs the flashing and the upstand checked. A yellowing or crazed plastic dome is ageing material, not moisture, and the dome is replaced whole. If you are unsure which you have, send a photo by WhatsApp on 07880 284133 or to info@vertexskylightsmiltonkeynes.co.uk, or ask for a free survey and we will look at it in person.
If the mist is between the panes, we replace the sealed unit or the whole skylight, finish the inside neatly, and back the work with a 10-year workmanship guarantee in writing. Whether it is a worry is covered in is a misted rooflight a problem, and the wider range of options sits in misted rooflights.
Fogged glass, answered
Why is my skylight wet on the inside only in winter?
The cold glass meets warm, damp air from the room. Better ventilation and steady heat reduce it, and an up-to-date unit stays warmer on the inner face.
Can condensation between the panes be dried out?
The seal has failed, so damp air will return. The lasting answer is a new sealed unit.
Does an electric or opening rooflight help?
An opening rooflight lets damp air leave from the highest point, which helps with room-side condensation. It does not cure a failed seal.
Is condensation on a rooflight worse than on a side window?
It can show up sooner. Warm air rises to the glass overhead, and the sky above it cools the pane, so a rooflight can be the first place a damp room shows itself.
