The invisible gas sitting between your panes, and why it matters more than most homeowners realise.
Air is a reasonable insulator but ordinary air also allows a degree of convection within the cavity of a sealed unit, circulating slightly and carrying heat across the gap. Filling that cavity with an inert gas instead reduces this convective movement because the gas is denser and more sluggish than plain air, slowing heat transfer between the outer and inner panes and improving the overall thermal performance of the window.
Argon is the gas fill used in the vast majority of UK double and triple glazed units. It is colourless, odourless, non-toxic and significantly denser than air, giving a meaningful improvement in thermal performance for a relatively low cost. Because it is abundant and inexpensive to produce, it has become the practical default across the industry and is what most homeowners will receive as standard within a sealed unit.
Krypton gas is denser again than argon and offers a further improvement in thermal performance, particularly useful in narrower cavity widths where there is less room for argon to perform at its best. However, krypton is far more costly to produce, which is why it is rarely specified for standard residential glazing in the UK and tends to appear only in specialist, very slim sealed units where cavity space is limited.
The benefit of any gas fill depends on the width of the cavity it sits within. Argon performs best in cavity widths of around sixteen to twenty millimetres, which aligns well with typical UK double glazed unit specifications, meaning most homes are already using argon in a cavity width suited to it without needing to consider krypton at all.
Gas fills are sealed within the unit at the point of manufacture using a robust edge seal, and a well-made unit should retain the vast majority of its gas fill for its working life. Gradual gas loss over many years is a known and expected part of sealed unit ageing, occurring slowly rather than suddenly, and quality manufacturing and spacer systems are designed to minimise that loss rate.
For most UK homes, confirming that argon gas fill is included as standard within a double or triple glazed unit is sufficient, since krypton is generally unnecessary outside of specialist slimline applications. If you have a specific narrow cavity requirement, such as a secondary glazing retrofit, it is worth raising this directly with your installer.
Yes, argon is the standard gas fill used across the vast majority of UK double and triple glazed units, chosen for its strong thermal performance relative to its low production cost.
Krypton offers a further improvement in thermal performance, particularly in narrow cavities, but it is significantly more expensive, so it is rarely used outside specialist slimline glazing applications.
Sealed units are designed to retain their gas fill for their working life, though a very gradual loss over many years is a normal part of ageing. Quality edge seals are designed to minimise this.
You can request it, but for typical UK cavity widths argon already performs very well, and krypton's extra cost is usually only justified in narrow specialist units, such as some secondary glazing systems.
Understanding the invisible coating that turns an ordinary window into an energy-saving one.
The component around the edge of the glass that quietly affects comfort, condensation and durability.
Two figures worth understanding before you sign off on new windows.
A plain-English look at the frame material behind almost every replacement window in the UK.
Weighing up an extra pane of glass against the cost, weight and payback for a typical UK home.
Understanding the two safety glass types and where the rules say each one belongs.
Book a free survey and we will confirm exactly which gas fill and cavity width is specified for your sealed units, backed by a fixed written quote.
