Why Is My Rooflight Misted Between the Panes?

What is happening inside the glass

A double glazed rooflight unit is two sheets of glass held a short distance apart by a spacer bar. The gap is filled with dry air or gas, and a sealant runs all the way round the edge to lock that dryness in. The spacer contains a desiccant, a drying material that soaks up any trace of moisture left from the factory.

Misting between the panes means that closed space is no longer closed. Humid air from the room or the roof has found a way in. The desiccant fills up, and after that the moisture has nowhere to go. It condenses on the glass whenever the temperature drops, which is why the haze is often worst on a cold morning and lighter in the afternoon.

Why the seal gives way

A rooflight faces the sky. In summer the glass can get very hot in direct sun, and on a clear winter night it loses heat fast and gets very cold. The two panes do not move by quite the same amount, so the edge seal is stretched and squeezed through every cycle. It is a flexible joint doing a hard job.

Water is the second enemy. A rooflight on a low pitch or a flat roof holds rain and meltwater against its lower edge, sometimes for hours. Sealant that sits wet for long periods softens and lets go sooner than the same sealant on a wall window would.

The frame can be the real cause

Sometimes the glass is only the messenger. A frame that has bowed, a timber sash that has swollen, or an upstand that has settled by a few millimetres can put a constant twist on the unit. The seal was never made to resist that.

A useful clue is where the mist starts. Haze creeping in from one corner or one edge points to a local fault, often a frame or drainage problem on that side. Haze that appears evenly across the pane points more towards general ageing of the seal.

Why you cannot wipe it off

The film is on the two inner faces of the unit, sealed away from any cloth or cleaner. Wipe the room side and then the outside. If the haze stays exactly where it was, it is inside. Dirt, by comparison, moves or smears when you touch it.

Ordinary condensation is a different thing and sits on the room face of the glass. It appears on a cold morning after a shower or cooking, and it wipes away. If you want to compare the symptoms, our guide to the signs a rooflight has failed sets them out side by side.

Does misting mean the rooflight is leaking?

Not by itself. A misted unit has lost its inner seal, but the frame and the weathering round it can still be keeping the rain out. The two faults are separate, and one does not prove the other.

It is still worth looking at the ceiling reveal and the upstand for stains or soft plaster at the same time. Misting that comes with damp marks tells you the water is finding another route as well, and that the frame needs looking at, not just the glass.

What to do about it

Misting does not get better by itself. The unit can carry on doing its basic job for a good while, since the glass still keeps weather out, but the view is spoilt and the insulation value of the unit has fallen. The sealed gap no longer insulates as designed, and the glass is working harder than it should.

The answer is new glass. Whether that means a new sealed unit in the existing frame or a whole new rooflight depends on the state of the frame, and we help you weigh it up in new sealed unit or a whole new rooflight. If you are wondering about shortcuts, can a misted rooflight be cleared deals with that directly.

What we do at the survey

On arrival we study the rooflight from inside and out, note where the mist begins, and check the frame for movement, corrosion or cracking. We test the upstand and the weathering and look for signs of water in the reveal. The quote that follows sets a new unit beside a whole new rooflight, priced line by line, so the comparison is easy to read.

Our page on misted and failed rooflights in York explains the whole job, from the first visit to the handover.

Questions worth asking yourself first

Before anyone visits, a few minutes of looking helps. Note which rooflights in the house are misted and which are clear. If only one has gone, the fault is probably particular to that unit or its position. If several have clouded at once, they may share an age, a maker or a batch, and the others are worth watching.

Look at how the haze behaves through a day. A film that thickens overnight and thins by afternoon is typical of a unit with a failed seal. A film that never changes is more likely to be staining on the glass faces inside the unit, which also means the seal has been open for some time.

Keep a phone picture of the glass taken early in the day, with the blind open. It gives whoever visits something to compare against, and it helps if you are asking more than one firm for a quote.

Does the room make a difference?

A kitchen or bathroom puts a lot of warm, wet air into the space under a rooflight, and that air presses against the inner pane all day. A well sealed unit shrugs that off. A unit with a weak seal lets a little of that humidity creep into the gap over time, so rooms with steam are often where a failing seal first shows.

Good ventilation in the room helps the plaster and the paintwork, and it takes load off the glass, but it will not bring a failed unit back to life. If the seal is open, the gap will keep drawing moisture in.