Argon vs Krypton: Which Gas Fill Is Worth It?

The gas sealed inside a double-glazed unit is not just air; it is an inert gas chosen because it conducts heat far less readily than plain air. Argon has been the industry standard for two decades and is present in the vast majority of energy-efficient windows sold in the UK. Krypton is the denser, rarer and more expensive alternative, better suited to narrow cavities than wide ones. Understanding which gas belongs in which type of unit is useful before you accept an upsell, so below we compare both across every dimension that matters. If you want to see how the glass spec as a whole affects your bills, the full glazing cost guide gives the wider picture, and our explanation of U-values and Window Energy Ratings covers how these gas fills feed into the thermal performance numbers.

The headline comparison

Here is the side-by-side picture before the detail. The figures come from the thermal conductivity data for each gas and the cavity-width performance characteristics cited by insulated glazing researchers.

Conductivity figures from published thermal data. Cost differential cited as approximately 40% more for krypton.

What the gas actually does

Inside a sealed double-glazed unit, heat moves from the warm inner pane to the cool outer pane by three routes: conduction through the gas, convection currents within the cavity, and radiation across the gap. Low-emissivity (Low-E) glass tackles radiation. The gas fill tackles conduction. Plain air has a thermal conductivity of about 0.0262 W/m·K. Argon, at about 0.016 W/m·K, conducts heat roughly 35% less readily than air. The practical effect is meaningful: a well-made argon unit can raise a window's energy efficiency by up to 30% compared with an equivalent air-filled unit. That improvement, combined with Low-E glass and a warm-edge spacer, is what gets modern double glazing over the Window Energy Rating band B threshold that Part L requires.

Krypton goes further: its thermal conductivity is about 0.0088 W/m·K, roughly half that of argon. On pure conductivity numbers it looks like a clear winner. The complication is that cavity width changes the story considerably.

Worth it Both gases beat air, but cavity width dictates which gas should be in which unit.

Argon: the standard fill

Argon is the workhorse of the industry and has been for good reason. It is abundant in the atmosphere, inexpensive to extract, non-toxic and stable inside a sealed unit. Its sweet spot is a wide cavity of about 12 to 16 mm. At those widths, the denser argon molecules do an excellent job of suppressing convection currents inside the gap, limiting heat transfer. The vast majority of energy-efficient windows sold in the UK are argon-filled, and a good unit should lose no more than about 5% of its gas over 25 years, meaning the performance you buy on day one is essentially the performance you keep throughout the window's life.

For standard double glazing on a typical semi-detached or terraced house, argon is not a premium add-on. It is the baseline, and any reputable installer includes it as standard alongside Low-E glass and a warm-edge spacer. If a quote leaves argon out to look cheaper, the unit will struggle to meet Part L, which is the legal minimum for replacement windows. You can check how your current glazing compares using the A-rated vs A+ glass comparison.

Best Buy Argon is the Best Buy for standard double glazing. It does the job at no meaningful premium.

Krypton: the niche upgrade

Krypton is a genuinely better insulator on a conductivity basis, about 0.0088 W/m·K against argon's 0.016 W/m·K. The problem is that it is rarer, far more expensive to extract and bottle, and its performance advantage only materialises in a narrow cavity of about 9 to 12 mm. In that narrower space, krypton's denser molecular structure is better at suppressing convection than argon. In a standard 12 to 16 mm gap, the advantage shrinks and krypton can actually underperform argon because the convection dynamics shift in wider cavities.

Krypton is cited at roughly 40% more expensive than argon at the unit level. US studies comparing krypton triple-pane windows against argon double-pane windows show krypton units costing 200 to 300% more overall (those figures are from US market data and reflect the full unit cost difference, not the gas premium in isolation). In the UK, the premium tends to show up as part of the overall cost of a slim-profile or triple-glazed specification rather than as a line item.

The legitimate home for krypton is triple glazing, where all three panes create two gas cavities that are necessarily narrower than a standard double-glazed unit, and slim-profile windows where the frame depth forces a narrower cavity. In those contexts, krypton earns its cost. In a standard double-glazed casement, it does not. See our full double vs triple glazing comparison for context on when the extra pane is worth having.

It depends Krypton: worth it in triple glazing and slim units, but an unnecessary upsell on standard double glazing.

Why cavity width decides it

The cavity width is the single most important factor in choosing between these two gases, so it is worth understanding why. Inside a sealed glazing unit, the gas does not just sit still. Temperature differences between the warm inner pane and cool outer pane set up convection currents, warm gas rising on the inner side and falling on the outer side. These currents transfer heat across the cavity, partly defeating the insulating gas.

In a wide cavity, the gas has room to form those convection rolls efficiently. Argon's properties are well matched to suppressing them at 12 to 16 mm widths. Below about 12 mm, the narrower gap actually changes the convection pattern, and krypton's denser molecules are better suited to that geometry. Go narrower still and xenon becomes interesting, but xenon is prohibitively expensive for mainstream windows.

This is why you cannot simply swap krypton into a standard double-glazed unit and expect a gain. The cavity was designed for argon. Fitting krypton into that unit may bring no benefit at all, and some engineers note a performance drop in very wide gaps because the heavier krypton molecules can set up stronger convection in a geometry not suited to them. When a salesperson offers a krypton upgrade on a standard double- glazed window, the cavity width should be your first question.

Skip it Skip the krypton upsell on any standard double- glazed unit with a 12 mm or wider cavity.

Cost and value

Argon fills are included as standard in virtually all energy-rated double glazing sold in the UK. There is no line-item surcharge because it is part of the base specification. Krypton carries a meaningful premium: roughly 40% more than argon at the gas level, and that cost flows through into the finished unit price. On a triple-glazed or slim-profile specification, where krypton genuinely improves performance, the premium may be justified as part of a package that already costs more than standard double glazing. On a like-for-like double-glazed unit with a standard cavity, there is no performance return to justify the extra spend.

If you are comparing window quotes and want to understand what drives the cost differences, our double glazing cost guide breaks down where the money goes, from glass specification to installation labour. The gas fill is a small part of that picture, but it illustrates a broader principle: premium components need the right context to deliver premium results.

Best Buy Argon represents the best value in standard double glazing. Krypton only justifies its premium in the narrow-cavity applications it was designed for.

I have been fitting windows for over twenty years and krypton is one of the most reliable upsells I see from less scrupulous reps. A salesperson tells a customer that krypton is twice as good as argon, which is sort of true on conductivity numbers, and charges them 40% more for it, which lands in a unit with a 16 mm cavity where krypton does nothing useful. Save the krypton conversation for triple glazing or a slim-profile system. On a standard casement, argon is the right gas and it is already in there.

Tom Bradley, FENSA-registered installer

The verdict

Argon is the Best Buy and the correct standard fill for the vast majority of UK double glazing. With a thermal conductivity of about 0.016 W/m·K and the ability to raise energy efficiency by up to 30% over air, it does exactly what a gas fill needs to do at no meaningful premium. It suits the 12 to 16 mm cavities found in standard double-glazed units and retains its performance reliably over a 25-year window life.

Krypton is a legitimate choice only where the cavity is narrow, typically 9 to 12 mm, as found in triple glazing and some slim-profile aluminium systems. In those applications it genuinely outperforms argon because the narrower gap plays to its molecular properties. In a standard double-glazed unit with a wide cavity, it does not earn its roughly 40% cost premium, and you may see no performance benefit at all.

The practical rule: if a salesperson offers you a krypton upgrade on ordinary double glazing, ask what the cavity width is. If the answer is 12 mm or more, decline and keep the money. If you are specifying triple glazing or a slim-profile system, krypton is worth discussing. For a broader look at the glazing options that genuinely affect thermal performance, our guide to U-values explained covers the full picture.