Why is my roof window misted up inside the glass?
Because the edge seal of the sealed unit has let go, and the cavity between the panes has taken on moisture from outside. That is the whole answer, and it is almost always the right one when the cloudiness sits between the panes and will not wipe off from either face. It is a failure of one manufactured component, not of your roof, not of your installation and not of anything you have done. The glass cannot be dried out again, so the unit is renewed rather than nursed along, and on a roof window past twenty years old the sensible unit of replacement is usually the whole window rather than the pane. The mechanism in full is at misted sealed units and why they go. What follows is the short diagnostic version, so you can be sure that is what you are looking at.
Three places moisture appears on a roof window, and only one is a failed pane
Before anything else, work out which surface the water is on. There are only three candidates and they mean three entirely different things.
| Where it is | What it looks like | What it means |
|---|---|---|
| Between the panes | Permanent haze, streaks or tide marks that never wipe off, worse when the outer pane is cold, sometimes visibly clearing in strong sun and coming back by evening | The edge seal has failed and the cavity is holding moisture. A new unit |
| On the room face | Fine droplets across the whole pane or a band around the edges, on cold clear mornings, gone by late morning | Condensation from inside the house meeting the coldest surface in the room. Ventilation and reveal geometry |
| On the outside face | An even dew across the outer pane on a clear still night, burning off after sunrise | Radiative cooling of very good glass. A sign the unit is working, not failing |
The test that settles it in ten seconds
Wipe the inside of the glass with a dry cloth. If the haze goes, it was condensation in the room. Then look at the outside face on a dry day, ideally from below with binoculars if the unit is high, and see whether the same haze is sitting there. If it is, it is dirt or bloom on the outer surface.
If both faces are clean and dry and the cloudiness is still there, it is inside the cavity, and there is no third possibility. Two further confirmations are worth having. Failed cavities usually show tide marks: faint horizontal lines low on the pane where water has evaporated and left the dissolved minerals behind. And the effect moves with the weather, thickest on a cold bright morning, thinnest after a warm dry afternoon, which is the cavity breathing through a seal that no longer stops vapour.
What is going on between the panes
A sealed unit is two or three panes held apart by a spacer bar. Inside the spacer is a desiccant, a drying agent whose job is to absorb the small amount of moisture trapped in the cavity on the day the unit was made. Around the edge are two seals: a primary butyl seal that stops water vapour crossing, and a secondary structural seal that holds the assembly together.
When the primary seal loses its grip, outside air carrying moisture starts moving in and out of the cavity as the unit warms and cools each day. The desiccant absorbs it until it is saturated, which can take a couple of years and is why the misting seems to appear suddenly rather than gradually. After that, every cold morning puts dew on the inside faces of the glass, because those faces are now the coldest surfaces available to air that has nowhere else to go.
Why a roof pane reaches that point before the one in your wall
The same unit in a vertical window will usually outlast the one overhead, and the reasons are all about position rather than quality.
- Temperature range. A roof pane faces the sun square on rather than at a glancing angle, so the seal cycles through a much wider swing between a frosty Cotswold dawn and a July afternoon.
- Standing water. The pane is close to horizontal, so rain lies on it instead of running straight off, and the edge seal spends far more of its life wet.
- Ultraviolet. Exposure overhead is simply higher, and it works on the secondary seal.
- Night sky cooling. A pane facing open sky loses heat to it on clear nights, driving a bigger daily cycle again.
Fifteen to twenty-five years is a normal span on a roof for an older unit. Where a house has several, they were glazed on the same day and they tend to reach that point within a couple of years of each other, which is why one misted window is often the first of a set. The wider service life question is at how long does a rooflight last.
How it progresses, and what each stage looks like
The sequence is consistent enough to date a failure roughly by looking at it. First a faint haze that only shows on cold mornings and clears by lunchtime, easy to mistake for room condensation. Then the haze starts appearing on days it did not before, and takes longer to clear. Then permanent tide marks along the bottom edge where water has repeatedly pooled and evaporated.
After that, running droplets inside the cavity in cold weather, visible dirt and dust drawn in with the air, and eventually a milky bloom or a fine mineral deposit etched onto the inner faces that stays regardless of the weather. At the last stage the glass has stopped being transparent in any useful sense and the daylight through it has gone noticeably grey. Nothing about this progression is dangerous, and it does not accelerate suddenly, which is worth knowing when deciding how quickly to act. That question is answered at is a misted rooflight an emergency.
Dew on the room face, which is the house talking rather than the glass
If the wipe test cleared it, what you have is condensation, and it is worth understanding because it is easy to mistake for a fault. Warm air holds moisture. A house generates a great deal of it from cooking, showering and drying clothes indoors, and that air rises. The coldest surface in a loft room is the glass overhead, so that is where the moisture lands first, sometimes while every other surface in the room is dry.
Three things control it. The vent flap in the head of the unit, which gives continuous background ventilation and is the thing most often left shut. Purge ventilation, meaning opening the window properly for a few minutes, which clears a humid room faster than anything else. And the shape of the reveal: a horizontal lining under the window lets warm air from a radiator wash straight up the face of the glass, while a splayed cill traps a pocket of still cold air against it. The full treatment is at condensation, ventilation and rooflights.
Damp in the plaster at the corners, which is a third thing again
A brown or grey mark in the plaster at the top corners of the reveal, worst in February and drying out in April, is neither of the above. It is a cold bridge: insulation stopping short of the frame, leaving a void behind the plasterboard where warm room air reaches a cold surface and gives up its moisture into the plaster.
That one is dealt with by opening the reveal, reinstating insulation tight and continuous to the frame with no voids, and taping the vapour control layer to the frame rather than trimming it short. Telling these three apart from the room below is set out at telling a failed unit from a failed installation.
What can be done about a misted unit, and what cannot
The cavity cannot be dried out and kept dry. Any product offering to do it is selling a hole drilled through a pane of glass with a vent in it, which turns a sealed unit into an unsealed one and gives up the insulating gas fill along with the argon that was doing the thermal work. The glass will look clearer and it will perform worse, permanently.
The real choice is between a new sealed unit into the existing frame and a new complete unit. Glass alone works where the frame is sound, the gaskets are supple, the hardware is right and the pane is still a stocked size, which is realistic on units under fifteen years old. Past twenty, the gaskets, the sash finish and the flashing kit are all the same age as the glass, so changing the pane leaves the rest to follow within a few seasons for the sake of one visit’s saving. That comparison is at can you replace just the glass in a rooflight.
What to send, and what a survey confirms
Four photographs make a first reply genuinely useful: the glass from inside the room in daylight, the unit from outside if you can see it from the garden, the data plate on the sash or frame if you can reach it, and the room from a few steps back so the reveal is visible. Add the address, because in this county the covering and the conservation status are often clear from a postcode.
At survey the size code is read off the old sash, the frame and hardware are checked, the trimmer is tested with a moisture meter where it can be reached, and the reveal is looked at for the cold bridge case. From there it is an indicative range by email and a fixed price after the visit. Severn Skylights Gloucester fits new roof windows and replaces units that have reached the end of their service life across Gloucester and thirty miles by road, with a ten-year workmanship guarantee. The process is at replacing a failed or misted rooflight, and the wider trade is on the skylight and rooflight installation and replacement page. Email info@severnskylightsgloucester.co.uk.
Next on this subject.
- Question Do rooflights need planning permission? Usually not. Putting a rooflight into the roof of a house is normally permitted development, which means the national rules…
- Question Do I need building regulations approval for a rooflight? Yes. Forming a new opening in a roof is notifiable work under the Building Regulations, and it is notifiable whether…
- Question Who notifies building control about a rooflight? In law the duty sits with the person having the work carried out, which means the homeowner, and in practice…
- Question How much does a rooflight cost? There is no single figure, and any site that publishes one is quoting for a job it has not seen.…