Mist between the panes means the sealed unit has failed. Mist on the room side is condensation, which points to air, heat and moisture rather than a faulty window.
A misted roof window can mean two quite different things, and the fix depends on which. Mist trapped between the glass is a failed seal. Mist on the room-side surface is condensation. In the tall, often converted houses of Cotham, where top-floor rooms get warm and moist, both turn up regularly.
Mist between the panes
If the fog or droplets sit inside the glass, where you cannot wipe them, the sealed unit has failed. The gas or air seal has broken and moist air has got in. Cleaning will not cure it. The unit, or sometimes the whole window, needs replacing, and that is a job for a roofer rather than something to try yourself. The glass itself is rarely the whole story, so we check the frame and flashing too.
Mist on the room side
Condensation on the inside surface is a different problem. Warm, damp room air meets cold glass and drops its moisture. Top-floor rooms, loft bedrooms and bathrooms are the usual places. The window is behaving normally; the room is simply wetter than the glass can cope with, so the cure is airflow and moisture control. Converted flats and loft rooms often have little ventilation, which makes this more likely.
When the surround is at fault
Sometimes the glass is fine but the lining, insulation or vapour control around the opening is poor. Cold bridges form, moisture collects at the frame, and stains appear on the reveal. A leak at the flashing or the roof covering nearby can look similar. The things we check are the same in each case. We look at these: Each points to a different fix.
- Whether the mist is inside or outside the glass
- Flashing and sealing around the frame
- Lining, insulation and vapour control at the opening
- Staining or damp on the surrounding timbers and plaster
What we do about it
For a failed unit we advise replacing it, matching the window and where possible reusing the frame. If the cause is condensation, we explain how to improve ventilation and check the surround. If the flashing or roof covering is at fault, we repair it. Replacing old windows can be done as a proper job rather than a quick patch. We tell you which of these applies before any work is agreed.
What you can do meanwhile
Open the window slightly or use the room's extractor to clear moisture, wipe the inside surface dry, and avoid drying laundry in that room. Take a photograph of the mist so we can see where it sits. Do not go onto the roof; call us for a proper look.
Frequently Asked Questions
Can a misted double-glazed unit be repaired?
Generally not. Once the seal has broken, the moisture is inside for good and the unit is replaced. We check the frame and surround at the same time so you do not fit a new unit into a faulty opening. It is better done once.
Is the mist a sign of a leak?
Not always. Mist between the panes is a failed seal and on the room side it is condensation. A leak tends to show as drips, stains or damp around the frame, not as fog on the glass. We can tell the difference on a visit.
Will replacing the window stop condensation?
Not on its own. A new window helps with insulation but condensation also depends on how humid the room is and whether air moves. We look at the lining and ventilation too. We check the ventilation as well.
Talk to our team about VELUX and roof window installation in Cotham
Every roof is different — the fastest way to get an accurate answer is a quick chat with our team.