Why is my rooflight misted between the panes?

What is happening inside the glass

A sealed double-glazed unit is two sheets of glass held apart by a spacer bar, with a sealant running round the edge. The gap between the sheets is filled with dry air or a gas, and the spacer holds a drying agent that soaks up any trace of moisture left from manufacture. While the seal is sound, the gap stays dry and the glass looks clear.

When the sealant cracks, shrinks or lets go of the glass, damp air moves in and out of the gap as the temperature changes. The drying agent in the spacer fills up, and after that the moisture has nowhere to go. It condenses on the cold inner faces of the glass as a cloud, a film of droplets or a line of tide marks, and it often shows first along the lower edge or in one corner.

A rooflight is harder on a seal than a wall window. It faces the sky, so it takes the full heat of the sun on a summer afternoon and the coldest night sky in winter, and water sits on or runs across it for longer than it does on a vertical pane. A skylight that has had many seasons of that will often show mist before the frame itself looks tired.

Mist between the panes or condensation on the glass

Two different problems look similar from the room, and the fix for each is different. A quick test settles which one you have: wipe the glass on the inside with a dry cloth and look again.

Surface condensation on a sound unit is a ventilation question more than a glazing one, and our answer on rooflight condensation goes through what reduces it. Mist that stays put when you wipe is the case this page is about.

Why seals fail

Age is the usual cause, because sealant hardens and loses grip over many cycles of heating and cooling. A few other things bring it forward.

• Standing water. A rooflight at a shallow pitch, or one with a blocked drain channel, keeps the edge of the unit wet for long stretches, and wet sealant ages faster.
• Heat build-up. Dark frames and south-facing roofs run hot, and the gap pressure rises and falls with every sunny spell, which works at the seal.
• Movement. A frame that has flexed, or a unit that was not packed and supported properly when fitted, puts strain on the edge.
• A manufacturing fault. A unit that mists early in its life may still sit inside the maker's own warranty, so keep any paperwork from the original fitting.

Clean glass and a sound frame can sit around a misted unit, which is why many misted rooflights do not need to come out whole.

Cleaning does not remove it

The mist is inside a closed space, so no cleaning product can reach it. Some firms offer to drill the glass and treat the gap, but a unit that has lost its seal has also lost the insulating performance it was bought for, and a sealed gap cannot be made sound again with the edge left as it is. The lasting answer is a new sealed unit made to the exact size of the old one, or a new rooflight where the frame has aged as well.

Mist does not make a rooflight unsafe in itself. It dims the view and lets more heat pass through the glass, and the unit tends to look worse over time as more of the gap clouds over. Glass that is cracked or loose in its frame is a different matter and is better looked at promptly.

What replacing it involves

There are two routes, and a survey tells you which one fits your roof.

A new unit in an old frame is not notifiable, while a whole new rooflight is. Our guide to replacing a rooflight and the Building Regulations explains the difference, and our hub on misted rooflight replacement brings the whole subject together.

The free survey takes in the frame, the seals, the upstand or kerb and the state of the roof covering round the edge, and we then write up both routes, with makers compared where the roof suits more than one. Book a free survey and the written quote puts the options next to each other.