What are double glazed windows filled with?

Double glazed windows are filled with an inert gas, typically argon, sealed between the two panes of glass. According to the Energy Saving Trust, argon gas is the most common fill because it improves thermal efficiency by reducing heat transfer compared to air (Energy Saving Trust, 2026).

The gas fill is not the only factor in window performance, the gap width, glass coatings, and spacer bars all matter. Argon is used in the vast majority of new double glazing units in the UK. Krypton and xenon are alternatives for narrower gaps, but they are far less common and more expensive. The gas is odourless, colourless, and non-toxic, and it is injected during manufacturing. Leakage rates are very low, typically less than 1% per year.

Argon is the standard gas fill

Argon makes up about 80% of the air we breathe, so it is cheap and widely available. When sealed between panes, argon reduces convection currents inside the gap, cutting heat loss by around 30% compared to an air-filled unit (BRE, 2026). A typical argon-filled double glazed window achieves a U-value between 1.2 and 1.6 W/m²K, depending on the glass coating and spacer type. The gas is injected at the factory and held in place by a primary and secondary seal. Most UK window manufacturers use argon as standard, and it meets building regulations for new installations under Approved Document L.

Krypton and xenon fill alternatives

Krypton and xenon have higher thermal resistance than argon, meaning they can achieve similar U-values with a narrower gap, useful for slim-profile windows or heritage properties. Krypton has roughly double the insulating performance of argon, while xenon is even better but extremely costly (Energy Saving Trust, 2026). A krypton-filled unit with a 6mm gap can match a standard argon-filled unit with a 12mm gap. However, krypton costs around five times more per unit than argon, so it is rarely used in mainstream UK homes. Xenon is reserved for specialist applications like high-end passive houses.

Air or vacuum fills are very uncommon

Standard air is sometimes used in older double glazing, but it offers lower thermal performance, typically U-values above 2.0 W/m²K. Modern building regulations require a centre-pane U-value of 1.2 W/m²K or lower for replacement windows (GOV.UK, 2026), making air-fill inadequate. Vacuum glazing, which has no gas at all, uses a near-vacuum gap of less than 1mm to achieve very low U-values (around 0.7 W/m²K). But it is expensive, typically £600–£800 per square metre, and still niche in the UK market. For virtually all standard double glazing, argon remains the default and most cost-effective choice.

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). According to the Energy Saving Trust, upgrading from single glazing to argon-filled double glazing can save a home like this roughly £195 per year on heating bills. That gives a payback period of about 33 years, which is longer than the windows’ typical 25-year lifespan. However, the real value comes from the comfort boost, a U-value drop from around 4.8 W/m²K (single glazing) to 1.4 W/m²K (argon-filled double glazing), and a reduction in condensation and draughts. The BUS grant does not apply to windows, but ECO4 may fund replacements for low-income households with inefficient frames.

What homeowners often get wrong

The most common mistake is assuming all double glazed windows are filled with the same gas and perform identically. Here are three frequent misconceptions that cost UK homeowners money or comfort.

1. Believing the gas fill alone determines window performance Many people think argon is the only factor in energy efficiency. The reality is that the spacer bar material, low-emissivity glass coating, and gap width (typically 12 to 20mm) have a bigger impact on the final U-value than the gas type, a poorly sealed unit with argon can perform worse than a well-built air-filled unit.
2. Thinking argon never leaks out over time Some homeowners expect the gas fill to last forever. In truth, even high-quality units lose about 1% of their argon per year according to BRE research, meaning after 20 years a window may have lost 20% of its gas fill, leading to a 5-10% drop in thermal performance and potential condensation between panes.
3. Assuming krypton is always better than argon A common belief is that krypton gas is superior in every situation. The correct answer is that krypton only outperforms argon in very narrow gaps (6 to 10mm), where it costs roughly three times more, for standard 12 to 20mm gaps, argon offers the best balance of performance and value, and using krypton in a wide gap wastes money with no measurable benefit.

Quick reference

• Argon gas fills 80% of all new double glazing units installed in UK homes, according to industry data.
• An argon-filled double glazed window typically achieves a U-value between 1.2 and 1.6 W/m²K, compared to 2.7 W/m²K for air-filled units.
• 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).
• Annual gas leakage from a well-sealed double glazed unit is less than 1%, meaning the gas fill remains effective for at least 20 years.
• Never attempt to refill a failed double glazed unit yourself, the gas must be injected under pressure at the factory, and a misted or failed unit requires complete replacement.