Fog trapped between the panes
Misting inside a double glazed unit, where no cloth can reach it, is the most reliable sign that a rooflight has come to the end of its working life.
A sealed unit is two or three panes held apart by a spacer bar, with a band of desiccant inside the spacer and a sealant around the edge. While the edge seal holds, the air or gas in the cavity stays dry. Once it lets moisture in, the desiccant fills up and a grey bloom appears on the inner faces of the glass. It tends to come and go at first, showing on a cool morning and fading by lunchtime, then it settles in as streaks, white deposits and a permanent haze. Our guide to why rooflights mist between the panes explains the mechanism in full.
From inside the room the change is easy to underrate. A skylight over a Romsey kitchen or a Chesterton side return may be the only source of daylight at the back of the house, and a hazed pane scatters that light into a dull, flat glow. You lose the view of the sky, the sharp edge of the sun patch on the worktop and much of the brightness. The cavity has also lost its insulating value, which is covered in does a misted rooflight lose heat.
Draughts, cold frames and wet sills
If you can feel air moving under the rooflight on a windy day, or the frame is beaded with water while the glass is dry, the unit is no longer keeping the weather and the cold where they belong.
Older rooflights were built before current thermal standards. Aluminium frames from the 1980s and 1990s frequently have no thermal break, so the metal carries outdoor cold straight into the room. On a February morning that frame sits well below the dew point of the air in a kitchen or bathroom, and water forms on it, runs down and gathers on the lining or the plaster below. Stains on the ceiling around the opening, flaking paint on the reveal and a musty smell are all part of the same picture, and can a misted rooflight cause damp covers where that moisture goes.
Draughts point to tired gaskets, a sash that has dropped on its hinges, or weathering around the upstand that has shrunk away from the frame. You may notice it most as a cold column of air over a desk or a bed in a loft room. A modern unit, with a warm edge spacer, a thermally broken frame and new weathering, removes all of these at once.
Glazing that has turned cloudy, yellow or brittle
Plastic domes and polycarbonate sheets age in sunlight, and once they turn yellow or opaque the light they let through is dimmer and warmer in colour than it should be.
Many flat roof extensions from the 1970s to the 2000s were fitted with acrylic or polycarbonate domes. Ultraviolet light slowly breaks down the surface, so a clear dome turns milky, then yellow, and may craze with fine cracks. The room below takes on a jaundiced, slightly murky light, and the plastic becomes brittle enough that a hailstorm or a falling branch can split it. The guides to replacing polycarbonate rooflights and why a rooflight goes cloudy go further into the causes.
Single glazed wired glass, common in extensions of the 1960s and 1970s, is a sign in itself. It loses heat quickly, runs with condensation and falls short of current expectations for overhead safety glazing. The wired glass replacement guide explains what goes in its place. Any crack, chip or star in the glass should be treated as a reason to plan the replacement promptly.
Openers, blinds and hardware that have given up
An opening rooflight that sticks, will not close tight or has lost its motor or rain sensor has stopped doing half of its job, which is ventilating the room.
Signs include a winding handle that slips, a chain actuator that stalls part way, a rain sensor that no longer closes the sash when the weather turns, stiff hinges and friction stays, or a centre pivot roof window that drifts open on its own. Integral blinds that have jammed or faded are a smaller sign, but they matter on a south or west facing slope where the blind is your main control over heat and glare. Parts for older models can become hard to source, and once the hinges, seals and glass are all tired together, a new unit with fresh hardware is the sensible upgrade.
How the signs point to the right replacement
Each sign tells you something about what should change, from a new glass unit in a sound frame to a complete rooflight on a rebuilt upstand.
| What you see | What it usually means | Likely route |
|---|---|---|
| Misting in a sound, modern frame | Sealed unit failure | New glass unit may suit |
| Cold, wet aluminium frame | No thermal break | Full unit replacement |
| Yellowed dome or sheet | UV ageing of plastic | Replace with a glass unit |
| Draughts and stains at the kerb | Tired weathering or upstand | New unit and weathering |
| Stuck opener, no spare parts | Worn hardware | Full unit replacement |
Whether a glass swap is enough is covered in glass only or full unit replacement. Replacement is also a chance to rethink the light: a solar control coating on a south slope, a larger or better placed opening, or a flush profile in a conservation street. That is where our light planning comes in, and every quote for misted and failed rooflight replacement carries a written light plan.
Questions people also ask
How old is too old for a rooflight?
There is no fixed date, but a well made sealed unit is generally expected to last around 20 to 30 years, and cheaper or older units less. Age alone is a reason to look closely rather than a reason to replace. The guide on how long rooflight glass lasts explains what shortens that life.
Can I wait before replacing a misted skylight?
A misted unit that is otherwise intact is not usually a safety concern, so the timing is yours. In the meantime the room is darker and colder than it should be, and moisture can build around the frame. If misting comes with cracks or loose glass, plan the replacement promptly.
Will the new rooflight let in more light?
Usually, yes. Clear new glass and slimmer frames let more daylight through than a hazed or yellowed unit, although a strong solar control coating trims some brightness in exchange for less heat. See will a new rooflight be brighter for the detail.
What is the first step?
Book a light survey. We look at the unit, the upstand or rafters around it and the room it lights, then set out the replacement in writing with the glass, size and position. You can book a light survey or message us on WhatsApp.
