Are double glazed windows filled with gas?

Yes, the vast majority of modern double glazed windows are filled with an insulating gas, typically argon, between the two panes of glass. The UK government’s building regulations for new windows require a centre-pane U-value of 1.2 W/m²K or lower, which is almost impossible to achieve with air alone (GOV.UK, 2026). Gas filling is the standard method to meet these thermal performance targets.

The gas is sealed inside the unit during manufacture and is not something homeowners can top up or replace. Argon is the most common choice because it is denser than air, reducing convection and heat transfer. Krypton and xenon are also used, typically in thinner glazing units or for higher performance. The gas fill is part of a sealed unit; if the seal fails, the gas leaks out and performance drops, but this is a manufacturing defect, not routine maintenance.

Argon gas is the standard fill for double glazing

Over 90% of new double glazed units in the UK use argon gas as the fill. Argon is inert, non-toxic, and roughly 1.4 times denser than air, which slows heat transfer significantly. The Energy Saving Trust states that a double glazed window with an argon fill can achieve a U-value around 1.2 W/m²K, compared to 2.7 W/m²K for a single-glazed window (Energy Saving Trust, 2026). This directly reduces heat loss and lowers energy bills.

Krypton and xenon fill higher performance units

For windows requiring very low U-values, such as triple glazing or slim-profile double glazing, krypton or xenon gas is used. Krypton is about twice as dense as argon. This allows a narrower gap between panes while still insulating effectively. Xenon is the densest but is significantly more expensive. The Building Regulations Approved Document L sets a minimum U-value of 1.2 W/m²K for replacement windows, but krypton-filled units can achieve 0.8 W/m²K or lower (GOV.UK, 2026).

Air-filled double glazing is now rare in new installations

While early double glazing used dry air, it is no longer the standard for new windows because air does not insulate as well as argon. Air has a U-value roughly 0.2 W/m²K higher than argon in the same unit, meaning more heat escapes. The Glass and Glazing Federation notes that air-filled units are still found in some budget or older installations, but they do not meet current Building Regulations for new builds or replacements (GGF, 2026). If you are buying new windows, always check the specification for a gas fill.

A worked example

A typical 1930s semi-detached house in Manchester replacing 10 standard double glazed windows with argon-filled units costs roughly £4,500 after the 0% VAT saving (in place until March 2027), assuming no additional grant support. The Energy Saving Trust estimates that upgrading from single glazing to A-rated double glazing with an argon fill saves a typical semi-detached home around £195 per year on heating bills. That gives a simple payback period of about 23 years, which is long but the windows themselves are expected to last 25 years or more with the sealed gas unit intact. Over that 25-year lifetime, the total energy savings reach approximately £4,875, meaning the windows effectively pay for themselves by the end of their service life. If the home is eligible for ECO4 funding or the Great British Insulation Scheme, the upfront cost could drop significantly, shortening the payback period to under 10 years.

What homeowners often get wrong

The most common mistake is believing that the gas fill can be topped up if the window performance drops, like topping up a car tyre. This misunderstanding leads homeowners to waste money on ineffective DIY kits or call out tradespeople for a service that does not exist.

1. Thinking gas can be refilled The misconception is that a window can be re-gassed in situ. The right answer is that the gas is sealed inside the unit during manufacture, and if the seal fails, the entire double glazed unit must be replaced, typically costing £150 to £300 per window.
2. Assuming all gas fills are equal The misconception is that argon, krypton, and xenon are interchangeable. The right answer is that krypton and xenon are used in thinner units (down to 6mm gaps) for slim-profile windows or heritage projects, and using the wrong gas can void the warranty and fail to meet building regulations.
3. Believing gas lasts the window’s lifetime The misconception is that the gas fill stays at full concentration for 30 years. The right answer is that all sealed units lose a small percentage of gas each year (around 1% to 3%), and if the unit drops below 90% fill, the U-value can rise by 0.2 W/m²K or more, meaning you lose up to £50 a year in unnecessary heat loss.

Quick reference

• Argon gas is used in over 90% of new UK double glazed units because it is 1.4 times denser than air and reduces heat transfer by up to 30% compared to air-filled windows.
• A double glazed window with argon fill achieves a centre-pane U-value of 1.2 W/m²K, which meets UK building regulations for replacement windows.
• Krypton and xenon fills are typically used in slim-profile units with gaps of 6mm to 8mm, achieving U-values as low as 0.7 W/m²K in triple glazing.
• The 0% VAT on energy-saving materials, including double glazing, runs until March 2027 and applies to labour and materials for installations in Great Britain.
• If a sealed unit fails and the gas leaks out, the window will show condensation between the panes, and the only fix is a complete unit replacement, not a refill.