Why mist and a leak are different things
A double-glazed skylight has two panes with a thin gap between them, closed at the edge by a spacer and a sealant. That sealed gap is meant to stay dry. When the edge seal weakens, humid air creeps in over the seasons, the drying agent in the spacer fills up, and the moisture turns to a haze on the cold inner faces of the glass. The glass is cloudy, but the water is only vapour and condensation. It stays inside the unit.
A leak is liquid rain or meltwater getting past the rooflight into the building. It travels through a gap in the weatherproofing: round the edge of the frame, through a failed flashing, under a lifted piece of roof covering, or along a cracked kerb. It arrives as a drip, a stain or a damp patch.
The unit's seal and the roof's weatherproofing are two separate systems. One keeps the gap between the panes dry. The other keeps the rain out of the house. A misted unit is the first system failing. A leak is the second. The first is taken apart in the guide on fog inside double glazing.
How to tell which one you have
You can sort the signs from the room below and from the outside, without going on the roof.
- Haze between the panes that wiping will not shift: a failed sealed unit.
- Droplets on the room-side face that wipe away: ordinary condensation.
- A brown stain on the ceiling around the frame: water getting in.
- Drips after rain, none in dry weather: a leak at the roof.
- Damp, peeling paint on the splay: a leak, or heavy condensation.
Condensation on the inside face is the one that confuses people. Warm, moist air from a kitchen, bathroom or a houseful of people meets cold glass and drops its water on it. That sits on the room side, runs down, and wipes off. It says something about the room's ventilation and the glass temperature, not about the sealed unit. Mist trapped between the panes does not wipe off, from either side.
When a misted unit and a leak turn up together
They can share a cause. A rooflight that sits in standing water on a flat roof can have a seal that has been wet for years, and the same roof can have a kerb joint that has started to let water through. A frame that has moved can open the unit's edge and the weatherproofing joint at once. Age does it too: sealant and gaskets harden at about the same rate.
So a stain on the ceiling by a rooflight that is also misted does not mean the mist is the cause. It may be, or the water may be coming from another point, and the mist is a coincidence. Someone has to get eyes on the frame, the upstand and the roof beside it.
If you see a stain, a drip or damp plaster, treat it as water coming in. Put a bowl under any drip, and keep the area clear of anything that water would harm. Do not climb onto the roof to look. A skylight and the area round it may be slippery, and a plastic or glass roof panel will not carry your weight.
What mist costs you if you leave it
Mist on its own does not let water into the house. What it does is slowly spoil the rooflight's job.
- Light. The fog cuts the daylight the rooflight passes. A skylight that once lit the room now gives a grey wash.
- Warmth. A sealed unit that has let air in has lost its gas fill, and the pane insulates less well. The room below feels the difference in a cold spell.
- Looks. Water marks dry on the inner faces and stay there. The haze gets worse, not better.
- Judgement. A rooflight that is misted is also old, and it is worth checking the frame, the seals and the kerb at the same time, since they have had the same years of weather.
None of this is an emergency in the way a roof leak is. It is a good reason to plan the replacement rather than leave it. If you can, plan it for a dry spell, since the fitting is simpler and the room is easier to keep clean.
What we do about it
We replace the failed part. For a framed rooflight with a sound frame, that is a new sealed unit, made to the opening. For a plastic dome, or a frame that has aged out, it is the whole rooflight. Which of the two applies is worked through on the page about swapping only the glass.
If water has been getting in, we say so at the survey. We check the frame, the upstand, the flashing and the ceiling beneath, and the written quote covers the rooflight replacement and the weathering of the new unit into the roof. A new rooflight is fitted with fresh seals and weathering, so the point where water was getting in is dealt with as part of the swap.
The survey is free and the written quote that follows is fixed. We deal with Building Control for you where the job needs it, and the fitting is backed by a ten-year workmanship guarantee. To get it moving, ask for the survey, or send a photo of the rooflight and the ceiling below to us on WhatsApp. The whole job is described on replacing misted rooflights.