Gas fill
Argon vs krypton gas-filled units
Fill the cavity of a sealed unit with plain air and heat crosses it fairly freely. Fill it with a heavier, less mobile inert gas and those tiny convection currents slow right down — so less warmth reaches the cold outer pane. The two gases you will see on a spec sheet are argon and krypton. This guide explains how each works, why argon is the sensible default, and the narrow set of cases where krypton earns the extra cost.
Why an inert gas beats air
Both argon and krypton are colourless, non-toxic and denser than air. That density is the point: heavier gas molecules move more sluggishly, which dampens the convection loop that would otherwise ferry heat from the warm inner pane to the outer one. They are also chemically inert, so they will not react with the glass, the coating or the seal over the life of the unit. Swapping air for argon typically shaves a useful amount off a unit’s U-value without changing anything you can see.
Argon: the sensible default
Argon does most of the heavy lifting in UK homes. It is abundant, inexpensive and works best in the standard cavity widths — roughly 14 to 16 mm — that most double-glazed units are built to. Because there is so little cost penalty, a modern energy-efficient unit will almost always be argon-filled as standard. If a quote does not mention the gas fill at all, that is worth querying; it should be argon at the very least.
Krypton: for slim cavities
Krypton is denser still, and it performs better than argon when the cavity is narrow — typically below about 12 mm. That makes it the go-to for slim sightlines, heritage frames and triple glazing, where three panes have to share a slimmer overall unit and each gap is small. In those tight spaces krypton reaches its best insulation across a shorter distance than argon can. The catch is supply: krypton is far rarer and considerably more expensive, so it only makes sense where the cavity genuinely rules argon out.
In a standard-width unit, paying for krypton buys you little over argon — the wider gap already lets argon do its job. That is why the honest answer to “which is better?” is “it depends on the cavity”, and why it pays to understand cavity width and performance before choosing.
Does the gas leak out?
A well-made unit loses only a very small fraction of its fill per year through the seals, and quality manufacturers fill to a high concentration precisely to allow for that. The bigger risk is a failed seal, which lets the gas escape faster and can leave misting inside the cavity. The warm-edge spacer bar and the double seal around it are what keep the gas where it belongs, which is why gas fill and edge sealing are best thought of as one system rather than two options.
What to specify
For most homes, argon-filled units with a low-E coating and a warm-edge spacer are the value sweet spot. Reserve krypton for slim or triple-glazed units where the cavity is too narrow for argon to shine. When you gather prices, it helps to know what to look for on a glazing quote and to match the tech to your frame material, since a slim heritage frame and a chunky modern one call for different gas-and-cavity choices. For the ratings side you can read up on U-values and window energy ratings explained, and if cost is the hurdle you may be able to spread the cost with funded glazing, subject to eligibility and a home survey.
Any efficiency figures here are indicative typical ranges from Energy Saving Trust and manufacturer data, not guarantees — the exact gain depends on your existing windows, the unit you choose and your home.