Rooflight misting between panes looks like a smear on the inside of the glass, except that the smear is out of reach. You wipe the room side and nothing changes. You wipe the outside, and still nothing changes. The haze is sealed in the gap between the two sheets of glass, and it has got there because a part of the unit that was meant to last has stopped working. This guide explains what that part is, why it gives up, and how to tell misted skylight glass from the ordinary condensation that gathers on any window in a steamy house.
What is inside a sealed unit
A modern rooflight, whether it is a flat glass unit over a kitchen extension, a pane in a roof lantern or the glass in a pitched roof window, is made of sealed units. A spacer keeps two sheets of glass, or sometimes three, a few millimetres apart to make each unit. Around the spacer, where it meets the glass, sits a ring of sealant. Everything depends on that ring staying whole.
The spacer is usually a hollow bar bent into a frame, and the hollow is packed with a drying agent, called desiccant, in the form of tiny beads. Its job is to take up the little moisture trapped in the gap when the unit was made, and to keep taking up the trace of vapour that slips past the seal over the years. The gap often holds a heavier gas than air, such as argon, which slows the passage of heat. A thin coating on one pane does the same job another way.
Put simply, the unit is a small sealed room with its own dry atmosphere. While that room stays shut, the panes stay clear whatever the weather does outside. The sealant is normally applied in two layers: a thin one against the glass that acts as the vapour barrier, and a thicker one outside the spacer that holds the sandwich together. Both stay flexible, because the glass moves.
How the edge seal fails
No edge seal is perfectly airtight. Water vapour moves through even a good one very slowly, and the desiccant deals with that. The unit fails when vapour arrives faster than the desiccant can absorb it, or when the seal opens enough for damp air to get in directly.
The first sign is rarely dramatic. A corner hazes after a cold night. A line of tiny droplets forms along one edge and runs down when the glass warms. Later the whole pane carries a pale film. Each of these is the same thing at a different stage: the desiccant has reached its limit, the gap has become damp, and moisture now condenses on the inner faces of the glass whenever they are colder than the air trapped with them.
Once water has got in, the unit cannot dry itself out. The seal that let the moisture in will let more in, and the desiccant does not recover. Some firms advertise drilling a hole in the glass and fitting a vent. That treats the symptom and leaves the insulation behind, because the gas fill has gone and the seal is still open. A sealed unit that has failed has to be replaced as a unit.
The glass itself did not fail. The panes are almost always whole, and the rooflight goes on keeping the weather out. What you have lost is the clear view and the warmth the unit once held back.
The glass is rarely the fault. The fault is the thin ring of sealant at its edge.
Why a roof is harder on glass than a wall
A unit in a wall window lives a gentle life. A unit in a roof takes the weather square on, looking at the sky, and that shows in how long the sealant keeps its grip.
Heat and cold come first. On a clear winter night the outer pane of a rooflight drops below freezing, because it looks straight up at a cold sky and loses heat fast. In July sun the same pane may be too hot to touch, and a dark frame passes that heat to the glass edge. Every cycle makes the glass and the spacer grow and shrink by a tiny amount, and the sealant has to stretch and recover with them thousands of times. It hardens a little each year under ultraviolet light, and a hardened sealant cracks or lets go of the glass.
Movement is the second pressure. Wind flexes a large pane, and on a lantern the long bars carry that movement into every glazed panel. A frame that has shifted on its kerb, or timber that has swollen and shrunk, squeezes the unit in a way it was never designed to take. A unit packed on the wrong blocks, or pinched by a bead fitted too tight, starts its life under strain.
The third pressure is water that stays put. A pitched roof window sheds rain quickly. Flat rooflights are set on a kerb over a roof with only a gentle fall. If the covering holds a puddle against the frame, the edge of the glass ends up standing in water. Sealant is made to cope with rain, not with weeks of immersion. On a misted flat rooflight, one side often shows the cloud first, and that is usually the low side where water collects. Drainage holes in the frame clogged with paint, moss or leaf litter have the same effect from inside the rebate.
Across the Wirral and along the Mersey, rain arrives from the west and exposed sites take salt-laden wind, so a rooflight that stays damp has less chance to dry out between spells. None of that causes misting on its own, but it shortens the odds.
Age, build quality and plain time
Every sealed unit has a finite life, and the only honest thing to say about how long is that it depends. The factors behind lifespan have their own answer, how long rooflight sealed units last, and this guide sticks to the cause.
What age does is accumulate. Sealant dries, takes up heat and light and loses the springiness that lets it follow the glass. The desiccant absorbs a little vapour every day and has only so much capacity. In an old unit both are near their limits, and one hard winter or one patch of standing water is enough to tip it over. It looks sudden. In practice the unit was running down for years.
Build quality counts as well. Spacers, sealants and desiccant vary between makers and between generations of product, and a unit built with less sealant or less desiccant has less in reserve. That is one reason two identical rooflights on neighbouring terraces can fail a good while apart. Fitting adds its own share: a pane on setting blocks in the right places is supported evenly, while one that is wedged or touches the frame at a corner carries a point load, and point loads open edges.
Misted, dirty or condensing: telling them apart
Cloudy glass has more than one cause, and only one of them means the unit has failed. Moisture or dirt can sit in three places: on the outer face, on the room-side face, or between the panes.
Dirt and algae are on the outside. On a flat roof, moss, bird droppings and grime build up and make the glass look cloudy from below. They wipe off, though you should not climb up to do it; look from an upstairs window. A cloudy pane that clears with a wash was never misted.
Condensation on the room side is a humidity issue. Cooking, showers and drying clothes all put steam into the air, and when it meets the inner pane, often the coldest surface in the room, it turns to water. It wipes away with a finger and comes and goes with how the room is used. Our guide to rooflight condensation explains what makes it better or worse. A failing unit can make it more likely, since a unit that has lost its insulation runs colder on the inside.
Dew on the outside is often good news. On a clear still night the outer pane of a well-insulated unit can fall below the dew point of the outside air, because little heat is escaping from the room to warm it. It burns off once the sun reaches it. A misted unit stays hazy whatever the weather.
| Where | What you see | Wipes off? | What it means |
|---|---|---|---|
| Outside face | Grime, moss or dew | Yes | Needs washing, or it is dew |
| Room-side face | Droplets, steam | Yes | Humidity in the room |
| Between the panes | Haze, beads, film | No | The sealed unit has failed |
- Touch the room-side glass: if the droplets come away on your finger, it is surface condensation
- Look from both sides: haze that shows from outside as well is inside the unit
- Check on a dry, mild day: dew and steam are gone, a failed seal still shows
- Shine a torch along the edge for a pale line, a gap or a shrunken strip of sealant
What the cause tells you about what comes next
A failed seal changes the question from why it happened to what the frame can take. If the frame is square, solid and dry, a new sealed unit made to the same size can sometimes go into it. If the frame has corroded or rotted, or the kerb below has failed, a new unit would be asked to survive the conditions that killed the first. Misting is often the first visible sign of water trouble around the frame, which is why the cause matters. The deciding questions, one at a time, are on the page about swapping the glass or the whole unit. A lantern with a single failed pane is the subject of a separate page on lantern glazing.
It also tells you how fast to move. A failed unit is rarely a leak, so there is time to arrange a survey. But if you see staining on a ceiling or reveal, soft paint round the frame or a dark mark beside the rooflight, put the job higher up the list.
What you should not do is clean the glass more often, smear sealant round the frame or wait for the cloud to clear. A failed seal does not recover, and extra sealant on the outside only traps more moisture. Photograph the unit from inside and from a window, note anything you can see around it and have it looked at properly. Cost follows the route the survey recommends, and what a misted rooflight unit costs to replace spells out the link. We never name a figure before surveying.
Any new unit must meet the thermal limit for its type. For existing dwellings in England the limits sit in Approved Document L (2021 edition, volume 1), Table 4.2: 2.2 W/m²K for rooflights, and either 1.4 W/m²K or band B for roof windows. Notifiable work is reported to Building Control by us, and the installation comes with a written 10-year workmanship guarantee. In Flintshire and Wrexham the Welsh rules apply, and the surveyor will confirm the route for your address. From measuring to handover, the whole visit is described on the main page for misted and failed rooflight replacement. Get a free quote
Still looking at a hazy rooflight?
Can misted skylight glass clear up by itself?
It can look better in dry warm weather, because the moisture in the gap evaporates a little and condenses again when the glass cools. The seal is still open, so the cloud returns, and over time it settles into a permanent film.
Is a misted rooflight leaking?
Not usually. The haze is moisture inside the glass unit, and the unit still sheds rain. Check the area round the frame for stains, which would point to a separate problem at the kerb or flashing.
Why does only one pane in my lantern mist?
Each pane is its own sealed unit with its own edge. The one that gets the most sun, or sits lowest where water collects, tends to go first. The others may follow, as they are the same age and have had the same weather.
Does misting mean my rooflight is losing heat?
It does. A unit that has taken in damp air has lost the dry layer between the panes, and usually the gas that went with it, so it insulates less than it did.