The short answer
Mist on one pane is a clue, not a puzzle. If the fog sits between the two sheets of glass, the sealed unit has failed and the whole unit needs replacing, even though only one face looks cloudy. If it sits on the room side or the roof side, it is ordinary condensation on a surface, and the unit may be perfectly sound.
Four places mist can sit, and what each one says
A rooflight unit has four glass faces, and the face that is fogged tells you whether the seal has gone. Start by working out which surface you are actually looking at.
A standard sealed unit is two panes with a cavity between them. Count the faces from the room outwards: the inner face of the inner pane, the cavity side of that pane, the cavity side of the outer pane, and the outer face of the outer pane. Triple glazed units add two more faces. Only the cavity faces are sealed away from the weather and the room. When people say "one pane is misted", they usually mean one of these four surfaces looks worse than the rest, because the light catches it differently.
| Where the mist is | What is going on | Does the unit need replacing? |
|---|---|---|
| Inside the cavity | The edge seal has let moist air in and the drying agent is used up | Yes, the unit is replaced |
| Room side, inner face | Warm damp room air meeting cold glass | No, but check the room's ventilation |
| Roof side, outer face | Dew forming on glass that is holding its heat well | No, it clears as the day warms |
| Cavity, with a tide mark | A seal that has failed and dried out several times | Yes, and the mark shows how long |
The first row is the only one that calls for work. The next two are weather and habit. You can tell them apart with a dry cloth and a little patience, which is what the next section covers.
The cloth test and the clock test
Two simple checks, both done from inside the house, sort a failed unit from a fogged surface in a few minutes.
First, the cloth. Wipe the room-side face of the glass with a dry microfibre cloth. If the mist wipes away and stays away, it was on that surface. If you wipe and the fog is still there, looking just as grey, it is behind the glass. A fogged cavity cannot be reached with a cloth, and no amount of rubbing changes it. Please do not climb onto the roof to try the outer face. Leave that to the survey, where it is done from safe access.
Second, the clock. Note the time of day when the mist is worst. Room-side condensation appears after showers, cooking and drying washing indoors, and fades as the room is aired. Roof-side dew appears on clear, cold mornings and burns off as the sun reaches the glass. Cavity mist behaves differently: it may thin on a dry, warm afternoon and thicken again in damp weather, but it never goes entirely. That slow, weather-following haze is the signature of a blown unit.
A third sign is shape. Cavity mist often starts at one corner or along one edge, where the seal gave up first, then creeps across the pane over weeks. Surface condensation tends to gather evenly, or along the coldest edge of the glass. For a wider look at how a sealed unit loses its seal, see why rooflights mist between the panes.
Why the failed face is not always the one that looks worst
The cloud you see is only moisture on whichever cavity face is coldest, and a sealed unit fails as a whole even when one face shows it.
Moist air has entered the cavity through the edge seal. Water condenses on the coldest surface it can find, and in a rooflight that is usually the outer pane's cavity face, because that pane is nearest the sky. So the outer pane can look fogged while the inner pane seems clear. Yet both share the same cavity and the same failed seal. Replacing "just the cloudy pane" is not a real option, because the two panes are bonded into one factory-sealed unit with a spacer bar between them.
That matters for the decision. A glass unit is made to order to fit your frame, and what we fit is a new sealed unit, not a single sheet. The wider choice between a new unit and a new rooflight is set out in replacing the glass unit or the whole rooflight.
When one unit is misted and its neighbours are clear
With a lantern, a run of roof windows or a pair of rooflights, one misted unit and several clear ones is common, and each unit is sealed on its own.
Each unit has its own edge seal, so each fails on its own timetable. A lantern might have eight panels, and one misted panel says nothing certain about the other seven. The misted one is replaced by itself. The lantern stays, the bars stay, and a new unit goes into the same opening. Our page on misted roof lantern panels goes through how that is done, and a single-unit question is answered at replacing one misted pane in a lantern.
There is a sensible reason to look at the neighbours, though. Units of the same age, made with the same spacer and sealant, and sitting under the same weather, have had the same life. A seal that has gone on the south-facing panel, which takes the most sun and the biggest swings in temperature, may be on its way out on the others. At the survey we look at every unit and record its condition. You then choose: replace the one that has failed now, or renew several together and have the scaffold, access and making good done once.
A second reason to replace together is matching. A new unit next to old ones can look slightly different, because the old glass has been weathering for years. If the lantern is over a dining table, a single brighter panel may catch the eye. Replacing all the panels in one go gives an even finish.
Using the replacement to improve the glass
A failed unit has to come out, so the new one can be specified better than the old, rather than copied.
The replacement is sized to the existing frame, but its glass build is ours to choose. For a loft bedroom under the Gatwick routes, or a kitchen where rain drums on a lantern, a laminated acoustic inner pane is a sensible upgrade. For a room that overheats, a solar control coating earns its place. For a cold room, a warm-edge spacer and a modern low-emissivity coating cut the heat escaping through the glass. The choice goes into the written quiet spec that comes with every quote, so you can see what each change is for before anything is ordered. The options are covered in upgrading the glazing when a rooflight unit fails.
If you are unsure which of the four faces is the problem, send us a couple of photographs, taken in daylight from inside, through the quote form or on WhatsApp. A photo showing the mist and its shape is often enough to say whether the unit has failed.
Questions people ask about one misted pane
Can the mist between the panes be cleaned away?
No. It is inside a sealed space that cannot be opened without destroying the unit. The moisture will return, and the failed seal is the reason. The answer is a new sealed unit, and why demisting does not last explains the detail.
Will the clear unit beside it mist as well?
Only if its own seal fails, which depends on its age, its position and the weather it faces. A clear unit beside a misted one is not doomed, but it is worth a look at the survey.
Is one misted unit covered by an old guarantee?
It may be, depending on who made it and when it was fitted. Check any paperwork from the original installation, then read whether a misted rooflight is covered by its original guarantee. Our own replacement work carries a 10-year workmanship guarantee, described on our guarantee page.
Is it safe to leave one misted unit for a while?
A misted sealed unit is not a danger in itself. It cuts the light a little and loses some insulation, and the seal will not recover. If you can see a crack, that is different, and is a misted rooflight dangerous explains what to look for.
When you are ready to go ahead, read our parent guide to misted and failed rooflight replacement in Crawley.