Why rooflights mist between the panes

A sealed unit is two sheets of glass kept a few millimetres apart by a spacer bar running round the edge. A bonded seal joins the spacer to each sheet. Between the sheets sits dry air or, in some units, a gas picked by the maker, and the spacer holds a drying agent that soaks up the small amount of moisture trapped at manufacture.

That design only works while the edge stays closed. The glass itself is not the weak point. The seal is a thin line of bonding material, and everything that goes wrong between the panes begins there, with a path for damp air to enter or with water sitting against the edge long enough to find its way in.

Once the drying agent has taken all it can, any further moisture has nowhere to go. It condenses on the cold inside face of the outer pane, and that is the grey haze or the beads of water you can see from the room.

The same construction runs through every sealed rooflight, whether it is a roof window on a loft stair, a flat rooflight over a kitchen extension or one pane of a lantern. Only the size, the weight and the surroundings change.

Seal age. The bonding material is flexible, and with years of heat and cold it can lose its grip on the glass. It need not fail all at once. A hairline gap opens, damp air moves in and out as the temperature changes, and the drying agent slowly fills.

Sun on the edge. A rooflight faces the sky, so direct sun can reach its glass for long spells. Every round of heating and cooling works the edge seal, because the glass and the bonding material move by different amounts, and over the years the seal is the part that can give.

Water at the frame. A roof window sits at an angle, so rain and meltwater run down the glass and gather at the bottom edge. Where the frame drains well, the water leaves. Where a channel is blocked, or a bead holds a pool against the seal, the edge sits wet for long periods and the seal wears from the outside in.

More than one of these can act on the same unit, which is why two windows of the same age on the same roof do not always cloud together.

The route matters because it tells you whether the fault lies in the glass alone or in what holds it. A unit that has simply reached the end of its seal is a straightforward swap. A unit that failed because water kept standing at the frame points at the frame.

Not every cloudy skylight has a failed unit. Warm, damp air meeting cool glass leaves a film on the inner face, and that is normal in bathrooms and kitchens. It wipes away and returns when the room steams up again.

The test takes two minutes. Wipe the room-side face of the glass with a dry cloth and see whether the cloud moves. Then look at the outer face from a safe place. If the glass clears on the room side or the outer side, the cause is surface moisture, and using a vent, running an extractor fan or opening the window helps with it. If the cloud stays exactly where it was after both wipes, it is inside the unit.

One trap is a unit that looks clear on a dry day and cloudy on a humid one. The seal has not mended in those dry spells. The air in the gap has simply held less moisture.

Condensation can sit on the outer face too, on a cold clear morning, and burns off as the glass warms. That is also surface moisture and also no sign of a failed seal. Only a cloud that ignores both wipes and every change of weather belongs to the sealed gap.

Much of the advice about misted double glazing is written for windows in walls, and it does not carry over to the roof. A wall window stands upright, so rain runs off its face and the bottom edge stays comparatively dry. A rooflight is tilted or flat. Water lies on it, snow and leaves settle on it, and the glass sees the full sun for longer.

The frames differ too. A pitched roof window, from VELUX or another maker, is built into flashing and an opening with its own drainage path, and a flat rooflight stands on a kerb. A failed unit can therefore be tied up with the frame, the flashing or the kerb as well as the glass, which is why a roof survey takes in the window's surroundings as well as the pane.

The glass is also made for the job. A rooflight pane is sized and specified for its place in the roof, and that is part of what the replacement has to match. Choosing between a new pane and a new window follows on from that point, and the pane-or-window comparison sets it out.

Lanterns and flat rooflights add a further twist. Their units can be larger, and a big pane can put more weight on its edge seal and flex more with temperature than a small one. A lantern with several panes can show cloud in one while the rest stay clear, since each one is a sealed unit of its own.

A clouded pane is easy to read as a sign the roof is letting in rain. The two are different faults. A leak shows up as stains on the lining round the opening, drips from the frame after rain, or damp plaster beside the window. Cloud between the panes shows up as a haze or droplets on the glass and nothing else.

A window can have both. A leaking flashing can soak the lower edge of the unit and bring on the cloud, and in that case replacing the glass alone leaves the cause in place. This is one reason we look at the lining and the roof edge at the survey, as well as at the cloud.

If you have stains as well as cloud, mention both when you get in touch. Send a picture of each to WhatsApp 07880 284133 if that is easier.

Light goes first. A hazed pane lets less of it through, and a loft room or a kitchen addition designed around its rooflight loses much of its purpose. The view of the sky goes next, which matters to anyone who put the window over a bed or a reading chair for exactly that reason.

The cloud also changes how the window behaves when the sun is on it. The room can feel duller on a bright day, with the light spread flat across it. A blind covers the cloud, but a blind cannot bring the clarity back.

What the cloud does not do is change how the window opens or closes. The frame, the handle, the hinges and the flashing carry on as before. If the room is only sometimes used, that may be reason enough to plan the work for a convenient time. What it does to warmth is covered under how much heat leaks.

Nothing you do indoors will dry a sealed gap, and a dehumidifier does not reach it. What you can do is manage the room side, so condensation on the inner face does not confuse the picture. Opening the window after a shower, and leaving the blind open while the room dries, both help.

Outside, keep the glass clear of leaves and debris. Where the window has drainage channels at the bottom, a blocked one keeps water against the seal. A flat rooflight is washed from below with a pole carrying a water feed, or by ladder from the eaves, while a centre-pivot window rotates so you can reach its outer face from the room. Walk-on rooflights are the only ones made for standing on.

Neither habit guarantees a unit will never fail. They simply stop you making a bad situation worse.

The visit costs nothing and the price waits until after it. We look at which of the three routes fits what you have, judge whether the frame is still sound and see whether water is sitting where it should not. Replacing only the glass, or a window in its existing frame, is not providing a controlled fitting, so the energy limits do not apply to that work. Cause and frame together decide whether a new sealed unit is enough.

The price then follows as a fixed written quote, while the cost answer explains what moves it. The daily side, such as opening, blinds and cleaning, comes up at the survey, and before we leave we show how the new window opens, shades and gets cleaned.