01Lifespan
A good unit lasts for years, not for ever
How long do sealed units last? There is no fixed expiry date. Quality, exposure and how the unit is fitted all move the number.
This page sits inside our guide to misted and failed rooflight replacement. Sealed units, whether in a skylight or a larger rooflight, are commonly quoted as lasting somewhere between 10 and 25 years. That is a wide spread, and we would be wary of anyone who gave you a single figure. A well made unit with a good edge seal, fitted square in a sound frame, sits at the long end. A cheap unit, or one left to stand in water, sits at the short end.
The honest way to think about it is this. A sealed unit is a sandwich of glass with a seal round the rim. Everything depends on that rim staying airtight. While it does, the gas stays in, the damp stays out and the glass does its job. When it lets go, you see mist between the panes, and that is the end of the unit's useful life. Our page on why rooflight glass mists up explains how it happens.
Those figures are a guide, not a promise. A unit in a shaded roof with good drainage can outlast one in full sun, and the reverse can happen too. Treat the range as a sense of what is normal, and let the glass itself tell you when it is time.
- 10 to 25years, the commonly quoted life of a sealed unit, depending on quality and exposure
- 10year workmanship guarantee on our fitting
02What wears them out
The things that shorten a unit's life
Four things do most of the damage, and most of them are about where and how the glass sits.
Rooflights have a harder life than wall windows. They face the sky, so they take more sun and bigger swings in temperature, and the seal flexes each time the glass heats and cools. A south-facing pane on a dark roof will work its seal harder than a shaded one.
Standing water is the second. If rain or condensation sits against the edge of the glass, it keeps the seal wet for long stretches. A rooflight at a very shallow pitch, with little fall to clear water, is more exposed. Quality of manufacture is the third, since the spacer, the desiccant inside it and the sealant all differ from maker to maker. Fitting is the fourth. A unit that is squeezed, loaded unevenly or sat on the wrong packers can stress the seal from day one.
| Factor | Helps the unit last | Shortens its life |
|---|---|---|
| Sun and heat | Shade or a lighter surround | Hot, south-facing pane on a dark roof |
| Water at the edge | Drained glazing bars and a good fall | Water that sits on the seal |
| Build quality | Warm-edge spacer, sound edge seal | Budget unit with a weak edge |
| Fitting | Square, correctly packed, even load | Squeezed or unevenly supported glass |
03Spotting the end
How you know a unit is on its way out
The signs are easy to see once you know them, and they come in a fairly clear order.
The usual first sign is a faint haze or a few droplets near one corner, which can look like dirt that has got between the panes. It may come and go with the weather, clearing on a dry spell and returning on a damp morning. After that, the mist spreads and stays, sometimes with a white scale or a streaky film once the water has dried out. There is a less obvious sign too: the room under the glass starts to feel cooler, because the gas fill has gone.
When you see any of this, do not wait for the pane to look dreadful. A failed unit loses insulation steadily, and the room under it costs more to heat. Our page on signs a rooflight has failed lists the signs in order, with a note on frames and kerbs as well.
04When it goes
Making the next one last
A replacement unit gets the best chance when the spec and the fitting are right from the start.
When we replace a failed unit, we look at why it failed before we order the new one. If water has been sitting on the edge, we say so, and we check the glazing bars and the pitch. New units come with warm-edge glazing as standard, with a low-e coating and gas fill, so you get the thermal benefit as well as clear glass. Our page on warm-edge spacers and thermal breaks shows why the edge detail matters.
If the frame is the real weakness, a new unit in it will not last either. At the survey we tell you plainly whether glass alone makes sense, and our guide to sealed unit or whole rooflight sets out the choice. Our fitting is covered by a 10-year workmanship guarantee, as described on our guarantee page.
- We look at why the old unit failed.
- We check the frame, bars and fall for anything that would shorten the next one.
- We order a new unit with a warm-edge spacer and low-e coating.
- One team fits it square and correctly packed.
Misted glass and sealed units: what readers fret over
Does the glass guarantee match the likely life?
Not necessarily. Makers set their own glass guarantee periods, and the likely life can be longer or shorter. Our own 10-year guarantee covers our workmanship in fitting.
Do triple glazed units last longer?
Not by themselves. Life depends on the edge seal and fitting more than the number of panes. Triple glazing is a warmth choice, as shown in double vs triple glazed rooflights.
Can a unit fail without any mist showing?
Yes. A seal can start to leak and the gas can escape before the glass visibly clouds, so the room may feel cooler first.
Should I replace a unit before it fails?
Usually there is no need. A working unit is doing its job. Replace it when mist or cold tells you the seal has gone.
