Argon gas is used in double glazed windows because it has a lower thermal conductivity than air, improving the window’s insulating performance by around 30% compared to standard air-filled units (Energy Saving Trust, 2026). This makes your home warmer and cuts heating costs.
Double glazing works by trapping a layer of gas between two panes of glass. Ordinary air contains moisture and conducts heat relatively quickly. Argon is denser than air, so it slows the transfer of heat from your warm room to the cold outside. The gas is inert, non-toxic and colourless. It is the most common filling for new double glazed units in the UK, and most modern windows come factory-filled with argon as standard. The performance gain applies to all homes with double glazing, but the biggest savings are seen in properties with large windows or those in colder regions.
Argon lowers the U-value significantly
The U-value measures how quickly heat passes through a window. A standard double glazed unit filled with air typically has a U-value of around 2.7 W/m²K. Filling the same unit with argon drops that to roughly 1.6 W/m²K (BRE, 2026). That is a 40% improvement in thermal resistance. Lower U-values mean less heat escapes, so your boiler runs less often. For a typical semi-detached home, upgrading from air-filled to argon-filled double glazing can save around £75–£100 per year on energy bills (Energy Saving Trust, 2026).
Argon reduces condensation on windows
Condensation forms when warm, moist indoor air hits a cold glass surface. Because argon-filled windows stay warmer on the inner pane, they are less prone to internal condensation. The inner glass temperature can be 2–3°C higher than an equivalent air-filled unit (HM Government, 2026). This reduces mould risk and keeps your windows clearer on cold mornings. It also helps maintain a more stable indoor humidity level, which is better for your health and your home’s fabric.
Argon must be sealed properly to work
Argon only provides its insulating benefit if the gas cavity remains sealed. Over time, seals can fail. This allows argon to leak out and air to seep in. A typical argon-filled unit loses around 1% of its gas per year (MCS Certified, 2026). After 20 years, the performance may be close to that of an air-filled window. To maximise longevity, choose windows from a manufacturer certified by the British Fenestration Rating Council (BFRC) and ensure they carry a FENSA certificate for installation. Replacement windows with failed seals should be replaced entirely, not refilled, because the gas cannot be topped up effectively.
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 cuts annual heating bills by £90, based on a gas-heated home with cavity wall insulation. Over 25 years, the total saving reaches £2,250, meaning the investment pays for itself in just over 26 years. If you are eligible for ECO4 funding, the upfront cost could drop to zero. The BUS grant does not apply here as it covers heat pumps only. Argon-filled units also reduce condensation risk and improve your home’s EPC rating by one band, which adds value when you sell.
| Item | Figure |
|---|---|
| Upfront cost after grants | £2,400 |
| Yearly savings | £90 |
| Payback period | 27 years |
| 25-year lifetime savings | £2,250 |
What homeowners often get wrong
The most common mistake is thinking all double glazing performs the same, regardless of the gas fill. Here are three misunderstandings that cost homeowners money and comfort.
- Believing argon lasts only a few years Argon does leak very slowly, but modern units with warm-edge spacers and quality seals lose less than 1% per year. A well-made argon-filled window still performs well after 20 years, and replacing units purely because of gas loss is rarely necessary.
- Assuming argon is only for new windows You can retrofit argon into existing double glazed frames if the seal is intact. A glazier drills a tiny hole, extracts the air, and injects argon before resealing. This costs roughly £40 per window and cuts U-values by 0.3 to 0.5 W/m²K, saving around £30 per year on a typical home.
- Thinking argon removes the need for trickle vents Argon improves insulation but does not solve condensation from poor ventilation. Homes with argon-filled windows still need background ventilation to meet Building Regulations Part F, otherwise mould can develop and void your warranty.
Quick reference
- Argon has a thermal conductivity of 0.0177 W/mK compared to air at 0.024 W/mK. That makes it 30% more effective at stopping heat loss.
- A standard double glazed unit with argon typically achieves a U-value of 1.6 W/m²K, meeting Part L Building Regulations for replacement windows in England.
- Argon is approved under the British Fenestration Rating Council (BFRC) scheme, which gives windows an A+ or A rating for energy efficiency.
- Replacing air-filled double glazing with argon-filled units in a semi-detached home cuts carbon emissions by roughly 150 kg of CO₂ per year.
- Never mix argon with krypton gas in the same unit, krypton is more expensive and only benefits triple glazing, so mixing wastes money without improving performance.
Frequently Asked Questions
Yes. Argon improves thermal performance by around 30% over air-filled units, according to the Energy Saving Trust. It also reduces condensation risk on the inside pane.
Yes. For a typical semi-detached home, upgrading to argon-filled double glazing saves £75-£100 per year on heating bills (Energy Saving Trust, 2026). The gas is non-toxic and inert.
Argon is the most common and cost-effective gas for double glazing in the UK. Krypton performs slightly better but costs significantly more, argon offers the best balance of performance and price.
