Rooflight misting between panes is a symptom with one cause: the sealed cavity inside the glass is no longer sealed. This guide explains what sits inside that cavity, how the edge seal, the desiccant and the gas fill give way in turn, why roof glass is so much harder on them than a wall window, and why the fog cannot be wiped, dried or polished away from either side.
First, check which side the moisture is on
Condensation on the room side of a skylight is a ventilation question; moisture trapped between the panes is a failed unit. A dry cloth and a cold morning tell you which you have.
On a cold, still morning, a kitchen or bathroom skylight can run with water on its inner face. Cooking, showers, drying laundry and simply sleeping in a closed room all put water vapour into the air, and the warm air rises to the ceiling, where the glass is the coldest surface it meets. That moisture sits on the room face of the inner pane. Wipe it with a dry cloth on a pole and the glass comes up clear. It will return tomorrow if the room stays humid, but it is on a surface you can reach, and better ventilation, an extractor or a trickle vent in the frame will reduce it.
Condensation on the outside face is also normal, and on high-performance glass it is actually a sign the unit is insulating well. On a clear night the outer pane radiates heat to the open sky and cools below the air temperature, so dew forms on it, exactly as it forms on a car roof. It burns off in the first hour of sun.
Moisture between the panes behaves differently. A cloth on either face does nothing. You may see a faint grey haze across the whole pane, a band of fog along the lowest edge, beads of water that run and collect in a line at the bottom of the glass, or, in a unit that has been failing for years, a whitish bloom that stays even on a hot dry afternoon. If you can see it but cannot touch it, it is inside the sealed unit.
A simple test: breathe on the inner face to fog it, then wipe that fog off. If a second layer of haze is still visible behind where your cloth has just been, it is in the cavity. On a roof window you can open, swing the sash round and do the same on the outer face, which rules out both surfaces in a minute.
What is inside a sealed rooflight unit
A sealed unit is a small, closed box of dry gas held between two or three sheets of glass. Four components keep that box dry, and each has a limited working life.
Around the edge of the glass runs a spacer bar, a hollow strip of aluminium, stainless steel or a plastic composite that fixes the gap between the panes, usually somewhere between 12mm and 20mm in a double glazed rooflight. Older units tend to have plain aluminium spacers; newer ones often use a warm-edge spacer that conducts less heat and keeps the edge of the inner pane warmer.
The spacer is filled with desiccant, typically a molecular sieve: small beads with pores sized to trap water molecules. They are there to take up any water vapour that was in the cavity on the day the unit was made, plus the tiny amount that creeps through the seal over the years. The spacer has pinholes or a slotted face towards the cavity so the beads can do their work.
The spacer is bonded to both panes by a primary seal, a thin bead of butyl (polyisobutylene) that is the real vapour barrier. Around the outside of that sits a secondary seal of polysulphide, polyurethane or silicone, which is the structural glue holding the unit together and protecting the butyl.
Finally, the cavity holds a gas fill. Better units are filled with argon, occasionally krypton in thin triple glazed cavities, because these heavy gases carry heat across the gap more slowly than air. Some older and budget units were filled with dry air only.
- Outer toughened pane: heat-strengthened to take hail, foot traffic during maintenance and thermal stress.
- Solar-control coating (optional) on the inside face of the outer pane, cutting heat gain on south and west roofs.
- Argon-filled cavity: a denser gas than air, so less heat crosses the gap.
- Warm-edge spacer: a low-conductivity edge that keeps the units’ edges warmer and cuts condensation at the frame.
- Low-E coating: a microscopically thin metal layer on a cavity face that reflects room heat back inside.
- Inner laminated pane: two sheets bonded to an interlayer, so if it ever breaks, the glass stays in place overhead.
- Triple glazing only: a third pane and a second argon cavity, usually with a second low-E coating. Warmer, heavier, and it lets in slightly less light.
Add a low-emissivity coating on one of the inner faces and, on roof glass, a laminated inner pane for overhead safety, and you have the full stack. When people ask why rooflight misting between panes cannot be dealt with from the room, this is the answer: every part of that stack is sealed away from the room and from the weather, which is exactly what it was designed to be.
How the unit fails: seal, desiccant, then gas
Misting is the last of three failures to become visible. By the time you can see it, the seal has been passing moisture for a while and the desiccant has run out of capacity.
No seal is perfectly vapour-tight. Butyl is very good at stopping water vapour, but it is a finite barrier around a long edge, and a small amount of moisture moves through it every year. On a healthy unit the desiccant takes that in its stride. The trouble starts when the seal is stressed or degraded, because the rate of moisture entry rises while the desiccant's capacity stays fixed.
The sequence usually runs like this:
- The edge seal weakens. Repeated flexing, heat, ultraviolet light or standing water loosen the bond of the secondary seal, which then lets stress and moisture reach the thin primary butyl. Tiny paths open up at a corner or along one edge.
- Gas starts to escape. Argon atoms are small and the same paths that let vapour in let argon out. This is invisible. The glass looks perfect while its insulating value quietly falls.
- The desiccant saturates. The beads keep absorbing the incoming moisture until there is no free capacity left. Up to this point the cavity still looks clear.
- Vapour reaches the cavity air. With nothing left to absorb it, water vapour builds up in the gap and condenses on whichever pane is colder: the outer pane on a winter night, sometimes the inner pane on a hot day with air conditioning below.
That order matters for anyone deciding what to do. The fog you see is not the fault itself; it is the late, visible evidence of a unit that has already lost the two things that made it worth having, its dry cavity and much of its gas. How that affects your heating is covered in the answer on whether a misted rooflight loses heat.
Why roof glass works its seals harder than a window
A skylight lives in a harsher position than any wall window in the same house. It faces the open sky, sheds water across its surface, and swings through a wider range of temperature every day.
The gas in a sealed unit expands when it warms and contracts when it cools. Because the cavity is closed, that movement pushes the panes very slightly outwards on a hot afternoon and pulls them inwards on a cold night. Glaziers call it pumping, and every cycle flexes the edge seal. A vertical window in a wall sees a moderate version of this. Roof glass sees the extreme version, for two reasons.
By day it faces the sun more squarely. At Cambridge's latitude the midsummer sun climbs to a little over 60 degrees at noon, so a flat or shallow-pitched rooflight takes that light almost head-on for hours, while the wall windows below take it at a glancing angle. The outer pane and the frame get properly hot.
By night it faces the sky. On a clear, still night, glass that looks straight up loses heat by radiation to a cold sky, which is why a flat rooflight can frost over when the car on the drive is only damp. The flat, open country around Cambridge, with little high ground to shelter a roof, gives plenty of those clear nights between October and April.
Put the two together and a south-facing flat rooflight can go from hot to below freezing within the same day in spring, and repeat that hundreds of times a year. Water adds to the load. Rain runs across roof glass rather than falling past it, and on a unit set with too little fall it can lie against the bottom edge for days, soaking the very corner where the seal is under most strain. Ultraviolet light is the last factor: a secondary seal left exposed to direct sun, because the frame or capping does not cover it, slowly hardens and cracks.
How misting progresses, and what each stage means
A failed unit rarely goes from clear to opaque overnight. It moves through recognisable stages, and knowing which one you are looking at helps you plan the replacement at a sensible time.
| What you see | What is happening | What it means |
|---|---|---|
| Faint haze on cold mornings, gone by midday | Desiccant just saturated | Seal has failed; plan ahead |
| Fog or droplets most days | Vapour building in the cavity | Light and view clearly reduced |
| Water pooling at the lower edge | Liquid water trapped inside | Coatings and spacer at risk |
| White bloom that never clears | Mineral residue on the glass | Glass permanently marked |
The first stage is easy to dismiss because the glass looks fine in the afternoon. It clears as the sun warms the unit and drives the moisture back into vapour, then condenses again overnight as the glass cools. The unit is not recovering; the water has simply changed state.
At the later stages the moisture starts to leave marks. Water that condenses and evaporates repeatedly on the same spot leaves behind the traces it picked up from the seal and spacer, and over time those deposits etch the glass surface or attack a soft coating. That is the white or rainbow-tinged bloom that sometimes shows on very old units. Once it is there, the glass is marked for good.
Skylight misting between panes is not a safety hazard in itself, and misting alone does not mean the glass is about to break. The answer on whether misted rooflight glass is dangerous covers the cases where it comes with cracks or loose glass. For the wider list of symptoms that point towards replacement, such as cold frames, draughts and stiff openers, see the signs a rooflight needs replacing.
Why the fog cannot be wiped or dried away
The moisture is inside a sealed box, so nothing done to either outer face can reach it. Opening the box to dry it leaves a unit without its seal, its desiccant or its gas, which is not the unit you paid for.
From the room you can clean the inner face of the inner pane. From the roof you can clean the outer face of the outer pane. The misting is on the two faces in between, and they were closed off at the factory. Heating the glass with a hairdryer, running a dehumidifier or opening the window for a week can shift the balance for a few hours, which is why a failed unit sometimes seems to clear on a warm dry spell, but the cavity remains open to the next damp night.
Some firms offer demisting: drilling small holes through the glass, washing and drying the cavity, then plugging the holes with vents or caps. On a vertical window this can improve the look for a while. On roof glass it runs into problems that come straight from the physics above. It does not restore the edge seal, so vapour keeps arriving. It does not replace the saturated desiccant. It cannot put back the argon, so the unit stays at air-filled performance at best. It cannot remove etching or coating damage already done. And on an overhead pane, drilling may compromise a laminated or toughened pane that is there for safety. The answer on whether misted rooflight glass can be cleared goes through this in more detail.
The honest conclusion is a positive one: a new sealed unit, or a new rooflight, restores all four components at once, with a fresh seal, dry desiccant, a full gas fill and clean coatings, and it gives you the chance to choose better glass than the original.
What misting does to the light in the room
A clear skylight gives you a sharp sun patch, a view of the sky and light that reaches well into the room. A misted one turns that into a dull grey glow that stops closer to the ceiling.
Moisture droplets and haze in the cavity scatter light in every direction. Some of it is bounced back out through the roof; more of it arrives in the room as a soft, grey diffusion rather than a clear beam. On a north-facing slope, where the light was soft to begin with, the room simply feels dimmer and flatter. On a south-facing slope you lose the defined band of winter sun that crossed the floor, and in summer the glass can glow white and glary without giving you any more useful light.
You also lose the sky. Much of the pleasure of a rooflight over a kitchen table in a Romsey terrace or over a bed in a Chesterton loft is seeing clouds move and knowing what the weather is doing. A fogged cavity replaces that with a blank ceiling panel, and it tends to make the room feel lower.
This is where a replacement becomes an upgrade rather than a like-for-like swap. Once the old unit is coming out, the glass can be chosen for the slope it sits on: solar control glass for a hot south or west slope, a high light transmission low-E unit for a gloomy north one, a laminated inner pane for safety, and a warm-edge spacer so the inner edge stays warmer and drier. Our light planning page sets out how orientation drives those choices, and the answer on whether a new rooflight will be brighter than the old one covers what changes and what solar control glass gives up.
From a misted unit to a clear one
Once you know the cavity has failed, the question is not whether to replace but what to replace: the sealed unit alone, or the whole rooflight with its frame.
If the frame is sound, square and of a design that accepts a new sealed unit, fitting new glass into it can be the neat answer. If the frame is old, cold, distorted, or bonded to the glass as in many frameless flat rooflights, the whole unit usually goes, often onto the existing kerb or upstand. The guide on glass only or full unit replacement weighs those routes, and pitched roof windows have their own options in replacing a misted roof window.
Replacement is also the moment to ask whether the rooflight is in the right place and the right size for how you now use the room, and whether the new glass meets the current Part L limit for rooflights. Where a replacement is notifiable, we handle the Building Control notification and install to current Building Regulations, and the Building Control for replacement rooflights guide explains when that applies.
A light survey looks at the failed unit, the frame and kerb, the slope and orientation, and the room below. Every quote comes with a written light plan setting out the glass, size and position we recommend and why, and the installation carries our 10-year workmanship guarantee. You can read more on the misted and failed rooflight replacement service, or book a light survey or message us on WhatsApp with a photo of the glass.
Questions people ask
Can a misted rooflight clear up on its own?
It can look clear on a warm, dry day, because the trapped moisture turns back into vapour, but it returns when the glass cools. The seal and desiccant do not recover, so the unit will not stay clear for good.
Is misting between the panes a sign of a roof problem?
Not necessarily. Misting between panes comes from the sealed unit itself. Water on the ceiling or around the frame is a separate matter, and the answer on whether a misted rooflight can cause damp explains how to tell the two apart.
Does triple glazing mist the same way?
Yes. A triple glazed unit has two cavities, each with its own spacer and seal, and either can fail. You may see misting in one cavity while the other stays clear, and the replacement is still a new sealed unit or a new rooflight.
Will a new unit mist again?
Every sealed unit has a working life, but a well-made unit on a kerb with proper fall, with its edge seal shaded from direct sun, should give many years of clear glass. The answer on how long rooflight glass lasts covers what shortens and extends that life.
