Yes, double glazed windows contain gas between the panes. The most common gas used is argon, which fills the sealed cavity to improve thermal performance. According to the UK Government guidance on Energy Performance Certificates (2026), the gas fill is a key part of the window’s U-value rating, with argon typically reducing heat loss by around 30% compared to air-filled units.
The gas is not visible and is sealed inside the unit during manufacture. It is not a consumable that needs topping up. However, if the seal fails, the gas will leak out and the window will lose its insulating benefit, although the window will still function as a basic barrier. This applies to all modern double glazing installed in UK homes since the 1990s. Older units from the 1980s may use air only, while triple glazing often uses krypton or xenon for higher efficiency.
Argon is the standard gas in most UK double glazing
Argon is the most widely used gas because it is cost-effective and non-toxic. The Energy Saving Trust (2026) explains that argon has a lower thermal conductivity than air, meaning it reduces heat transfer through the window. A standard double glazed unit with a 12mm to 16mm argon-filled cavity typically achieves a U-value of around 1.2 to 1.4 W/m²K, compared to 2.7 W/m²K for single glazing. The gas is injected during manufacturing and the unit is sealed with a durable edge seal, often using butyl or silicone.
Krypton and xenon are used in high-performance units
For triple glazing or very narrow cavities (6mm to 10mm), krypton or xenon gas is preferred because they are denser and provide better insulation in thinner gaps. The Building Research Establishment (BRE, 2026) notes that krypton-filled triple glazing can achieve U-values as low as 0.8 W/m²K. That makes it suitable for Passivhaus or energy-efficient new builds. However, these gases are more expensive than argon, so they are rarely used in standard replacement windows.
Gas loss only happens if the seal is damaged
If the double glazing seal fails, the gas will escape and the window’s thermal performance drops significantly. The TrustMark (2026) advises homeowners to check for condensation between panes or a visible fogging effect, which indicates seal failure. You cannot test for gas loss without specialist equipment, but a failed seal is clear from moisture or dirt inside the cavity. Replacing the sealed unit (not the whole frame) typically costs £150 to £300 per window, depending on size and gas type.
A worked example
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Energy Saving Trust estimates this upgrade saves roughly £195 per year on heating bills for a gas-heated home with 10 windows. Over 25 years, total savings reach £4,875, giving a payback period of roughly 27 years, longer than the 20-year warranty on most sealed units. However, the real benefit is immediate comfort and reduced condensation, not purely financial return. The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing. Always check your property’s eligibility on the gov.uk website before ordering.
| Item | Figure |
|---|---|
| Upfront cost after grants | £5,200 |
| Yearly savings | £195 |
| Payback period | 27 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is believing you can refill the gas if a window feels draughty or fogs up between the panes. Once the seal breaks and gas escapes, the unit must be replaced entirely. Here are three frequent errors to avoid.
- Assuming gas is visible or can be checked Argon and krypton are colourless and odourless, so you cannot see them. The only way to know the gas has leaked is seal failure, shown by condensation between the panes or a noticeably higher U-value on an energy performance certificate.
- Thinking gas needs regular topping up The gas is sealed at manufacture and lasts the window’s lifetime if the seal holds. No maintenance or refill is needed. Attempting to inject new gas through a drilled hole voids the warranty and rarely restores the original performance.
- Believing all double glazing has gas Many budget or older units from the 1980s and early 1990s contain only dehydrated air. Air-filled double glazing has a U-value around 2.7 W/m²K, while argon-filled units achieve 1.2 to 1.4 W/m²K. Check your original installation paperwork or ask your installer for the gas fill specification.
Quick reference
- Argon gas fills approximately 90% of new double glazing installed in UK homes today.
- A 12mm argon-filled cavity reduces heat loss by 30% compared to an air-filled cavity of the same width.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
- Seal failure occurs in around 2% to 5% of units within the first 10 years, depending on manufacturing quality.
- Triple glazing typically uses krypton or xenon gas, which is denser and more expensive than argon.
Frequently Asked Questions
Argon is the most common gas used in UK double glazed windows. The Energy Saving Trust states argon has lower thermal conductivity than air, improving insulation.
No, the gas is sealed inside during manufacture and cannot be refilled. If the seal fails, the unit must be replaced according to UK window industry guidance.
Not all, older units from the 1980s may use air only. Most modern UK double glazing since the 1990s uses argon, while triple glazing may use krypton or xenon.
