Argon and krypton gas both improve window insulation, but they differ in cost and performance
When you buy new windows in the UK, the space between the glass panes is almost always filled with an inert gas rather than plain air. This gas fill is a key part of how modern glazing keeps heat inside your home. The two main options are argon and krypton, and they are not interchangeable.
Argon is the standard and most cost-effective gas fill for UK windows, offering good insulation at a low price. Krypton improves U-value by 0.1 to 0.2 W/m²K but costs 3 to 5 times more. That makes it only worthwhile for slim-profile or heritage windows.
- Argon is the default gas fill for 95% of new UK windows.
- Krypton improves U-value by 0.1 to 0.2 W/m²K over argon.
- Krypton costs roughly 3 to 5 times more than argon per window.
- Use krypton only for slim-profile or heritage windows under 12mm gaps.
- Check whole-window U-values from BFRC ratings, not just gas fill.
- Argon and krypton gas both improve window insulation, but they differ in cost and performance
- The U-value difference between argon and krypton gas fills is small
- Argon is the cheaper and more widely available option for UK homeowners
- Krypton is useful only where the glazing cavity must be very narrow
- Quick numbers, argon vs krypton for a typical UK home
- For almost all UK homes, argon glazing offers the best balance of cost and energy savings
- Any gas-filled window you buy must be certified and installed by an accredited professional
Argon is the standard gas fill used in most double and triple glazing sold in the UK, while krypton is a denser, more expensive gas that allows a narrower gap between panes, often used in slim-profile or heritage windows. The Glass and Glazing Federation (GGF) confirms that argon is the default choice for the vast majority of new windows sold in the UK (GGF, 2026). Both gases are inert, odourless, non-toxic, and colourless, and neither poses a safety risk inside the home. The key trade-off is simple: argon offers good insulation at a low price, while krypton offers slightly better insulation at a significantly higher cost.
The U-value difference between argon and krypton gas fills is small
The performance of a window is measured by its U-value, which tells you how much heat passes through it. A lower U-value means better insulation. For a standard double-glazed unit with a 16mm gap filled with argon, the centre-pane U-value is typically around 1.2 W/m²K, according to data from the British Fenestration Rating Council (BFRC) (BFRC, 2026). The same unit filled with krypton can achieve a U-value of approximately 1.0 to 1.1 W/m²K based on BFRC ratings.
The improvement from argon to krypton is roughly 0.1 to 0.2 W/m²K. For a typical home, this is a marginal gain. For triple glazing, the difference narrows further: both gases can achieve U-values below 0.8 W/m²K, making the choice between them even less impactful on overall performance (Energy Saving Trust, 2026). In practice, the U-value of the whole window including the frame is what matters, and that depends more on the frame material and glass coatings than on the gas fill alone.
Argon is the cheaper and more widely available option for UK homeowners
Cost is where the two gases diverge sharply. Argon-filled double glazing costs roughly £50 to £100 per window more than standard air-filled units, based on industry pricing from the GGF (GGF member pricing surveys, 2026). Krypton-filled units typically add £150 to £300 per window, and availability is limited to specialist manufacturers. Argon is the default fill for the vast majority of new windows sold in the UK. That makes it easy to source and compare quotes.
For a typical three-bedroom semi-detached house with 10 windows, upgrading from air to argon adds about £500 to £1,000. Upgrading to krypton adds £1,500 to £3,000. The Energy Saving Trust notes that argon is the standard gas fill used in most A-rated double glazing sold in the UK, and it is readily available from all major window suppliers (Energy Saving Trust, 2026).
Krypton is useful only where the glazing cavity must be very narrow
Krypton’s higher density means it insulates better in gaps as small as 6 to 10mm. This is why it is used in heritage or slim-profile windows where a standard 16 to 20mm gap cannot fit. Argon loses effectiveness in gaps smaller than 12mm, so krypton is the only practical gas fill for very thin units. The GGF technical guidance on gas fills confirms that argon performs poorly in narrow cavities, while krypton maintains its insulation value (GGF, 2026).
For standard cavity widths of 16 to 20mm, argon performs nearly as well as krypton. In that scenario, krypton becomes an unnecessary expense. Unless your home has particularly slim window frames such as in a listed building or a conservation area, you do not need krypton. The BFRC cavity-width performance data shows that at standard widths, the U-value difference between the two gases is negligible (BFRC, 2026).
Quick numbers, argon vs krypton for a typical UK home
| Type of fill | Typical U-value (centre-pane) | Cost per window (installed) | Annual energy saving vs single glazing | Payback time vs single glazing |
|---|---|---|---|---|
| Air (no gas) | 1.4–1.6 W/m²K | £0 (baseline) | £0 (baseline) | N/A |
| Argon | 1.2 W/m²K | £50–£100 | £195 | 5–10 years |
| Krypton | 1.0–1.1 W/m²K | £150–£300 | £205–£215 | 15–30+ years |
Figures sourced from BFRC ratings, GGF member pricing surveys, and Energy Saving Trust cost guides for a gas-heated semi-detached house in England, 2026 (BFRC, 2026; GGF, 2026; Energy Saving Trust, 2026). The table shows that argon provides roughly 90% of the insulation benefit of krypton at about a third of the cost.
For almost all UK homes, argon glazing offers the best balance of cost and energy savings
This is the direct answer to the question of argon vs krypton glazing. The Energy Saving Trust states that upgrading from single glazing to A-rated double glazing, which is typically argon-filled, saves a typical semi-detached house £195 per year on energy bills based on 2026 figures (Energy Saving Trust, 2026). Switching from argon to krypton would save an additional £10 to £20 per year at most, based on the U-value difference.
With a payback period of roughly 5 to 10 years for argon versus 15 to 30-plus years for krypton, argon is the cost-effective choice for standard window replacements. The Department for Energy Security and Net Zero (DESNZ) data on energy consumption in the UK confirms that the average household spends around £1,000 per year on heating, and window upgrades are one of the most effective measures to reduce that cost (DESNZ, 2026). For almost all UK homes, argon glazing delivers the best balance of upfront cost and long-term energy savings.
Any gas-filled window you buy must be certified and installed by an accredited professional
All replacement glazing in England and Wales must comply with Building Regulations Part L, which requires a certified installer or a Building Control application (GOV.UK, 2026). Look for installers registered with FENSA, CERTASS, or the GGF’s Competent Person Scheme. These bodies verify that the windows meet the required U-values and gas-fill retention.
The gas fill itself is sealed at the factory. You cannot verify the gas type after installation, so insist on a written warranty and a BFRC energy rating certificate. The MCS (Microgeneration Certification Scheme) is not relevant for glazing. Use FENSA or CERTASS for window installations (FENSA, 2026; GGF consumer guidance, 2026). If you are replacing windows as part of a larger home improvement project, you may also want to check whether any energy efficiency grants or local authority schemes apply current energy grants and eligibility criteria for homeowners.
For most homeowners, the choice is straightforward. Argon gives you the performance you need at a price that makes sense. Krypton is a niche product for narrow frames, not a general upgrade. If your window frames are standard depth, stick with argon and put the money you save into better glass coatings or a higher-rated frame how to choose the best window frame material for energy efficiency.
Frequently Asked Questions
Argon is better for most UK homes because it provides good insulation at a low cost. Krypton offers a slightly lower U-value (0.1 to 0.2 W/m²K better) but costs 3 to 5 times more, so it is only recommended for slim-profile windows where argon cannot fit, according to the Glass and Glazing Federation (GGF, 2026).
Krypton gas adds roughly £30 to £80 per window compared to argon, depending on size and glazing type. The British Fenestration Rating Council (BFRC) notes this premium is rarely justified for standard double glazing (BFRC, 2026).
No, you cannot mix argon and krypton gas in the same sealed unit. Each cavity must be filled with a single inert gas, and mixed fills do not improve performance, as stated by the Glass and Glazing Federation (GGF, 2026).
Krypton improves the centre-pane U-value by roughly 0.1 to 0.2 W/m²K compared to argon in a standard 16mm gap. For triple glazing, the difference is even smaller, both achieving below 0.8 W/m²K, according to the Energy Saving Trust (2026).
Yes, krypton is worth it for heritage windows that require a slim gap under 12mm, where argon performs poorly. The Energy Saving Trust recommends krypton in this case to maintain insulation without altering the window profile (Energy Saving Trust, 2026).
