Slowly. A well made sealed unit loses a small percentage of its argon fill each year through the perimeter seal, which is normal rather than a defect. The effect on performance over a normal service life is modest. What actually matters is whether the seal fails outright, because that admits moisture and ends the unit.
Why argon is there
Because it conducts heat less readily than air does.
Filling the cavity with a heavier, less conductive gas reduces the heat carried across it, improving the U-value by a worthwhile amount at very little cost.
It is one of four things that make a modern unit perform, alongside the coating, the spacer and the frame.
How much it contributes
Meaningful and not transformative.
The low emissivity coating does most of the work in a modern unit. The gas fill improves on that by a smaller increment, and the spacer and frame account for the rest.
Which is why a unit that has lost some of its fill still performs respectably.
The rate of loss
Small, and standards exist for it.
A well made unit loses a low single figure percentage of its fill each year. Over fifteen or twenty years that accumulates to a noticeable proportion, and the unit is still performing far better than an air filled one.
What it means in practice
Very little for a household, and it is worth saying plainly.
Nobody notices a rooflight becoming imperceptibly less efficient over two decades. What people notice is a unit misting, which is a different failure.
The failure that matters
The perimeter seal, and it is what ends a sealed unit’s life.
The seal keeps gas in and moisture out. When it fails, moist air enters, the desiccant in the spacer absorbs it until saturated, and then the unit mists internally.
That misting cannot be wiped away because it is inside the glass, and the unit is finished.
Why a rooflight is a harder duty
Worth knowing, because it explains why rooflight units are built differently.
A rooflight sits close to horizontal, takes more direct sun, swings through greater temperature ranges, and holds water on its surface rather than shedding it quickly.
All of that works the perimeter seal harder than a vertical window does, which is why a unit made for a window is not a substitute.
Drainage at the perimeter
A specific reason rooflight units fail early when they are wrong.
Water standing against the edge of a sealed unit attacks the seal, so a rooflight frame has to drain the perimeter properly rather than letting water sit in the rebate.
A frame that does not is the commonest reason a good unit fails in five years rather than twenty-five.
Whether a unit can be refilled
Not practically, and it is worth saying because people ask.
There are processes for it and they are not a domestic proposition. Where a unit has genuinely degraded, replacing the sealed unit in a sound frame is the sensible answer.
How to tell whether it has gone
You largely cannot without instruments, and that is fine.
Gas loss is invisible and gradual. There is no symptom to look for, no change in appearance, and no practical home test.
Which is another reason not to worry about it: the thing you can see, misting, is the thing that matters.
Krypton and other fills
Available, and rarely worth it on a domestic rooflight.
Krypton performs better than argon in a narrow cavity and costs considerably more, so it appears in triple glazing and slim units where the cavity is too narrow for argon to work well.
On an ordinary double glazed rooflight, argon is the sensible fill.
Air filled units
Still made, and worth checking you are not buying one.
A unit with no gas fill performs measurably worse for a small saving. On a cheap product it is one of the corners cut, alongside an aluminium spacer.
Both are on the specification if anybody asks for it.
What a guarantee usually covers
Misting rather than gas concentration, and it is worth reading.
Sealed unit guarantees are generally written against visual failure, meaning internal misting within a stated period, rather than against a measured fill level.
Which is reasonable, because misting is what a household experiences.
Typical service life
Long, on a well made unit properly installed and drained.
Rooflight sealed units routinely last decades. Where one fails early, the cause is almost always water standing at the perimeter or a unit not designed for overhead use.
What to ask a supplier
Three questions rather than a debate about gas.
Whether the unit is made for overhead glazing, whether the frame drains the perimeter, and what the sealed unit guarantee actually says.
Those decide service life. The fill is a detail by comparison.
Replacing a failed unit
Frequently possible without replacing the rooflight. Where the frame is sound, the glazing can often be changed on its own, which also brings modern coatings and a warm edge spacer.
What the spacer does is on what is a warm edge spacer.
The desiccant, and what it is buying
The component that decides how long a unit lasts after the seal starts to admit anything.
The spacer is filled with a drying agent that absorbs stray moisture, and it works quietly for years. Misting begins when it is saturated and can absorb no more.
Which means a unit does not fail on the day the seal degrades. It fails some years later, when the desiccant gives up.
What we always specify on the glazing
We specify units made for overhead use with properly drained frames, because that decides how long a sealed unit lasts far more than the fill does.
The whole picture is on the energy efficient rooflights page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.