Why are double glazed windows filled with argon?

Double glazed windows are filled with argon because it improves thermal efficiency by roughly 30% compared to air-filled units, reducing heat loss through the glazing (Energy Saving Trust, 2026). Argon is an inert, non-toxic gas with lower thermal conductivity than air, which slows the transfer of heat across the gap between the two panes.

The key variable is the gas’s thermal conductivity. Air is a mix of gases, primarily nitrogen and oxygen, with a conductivity of about 0.026 W/mK. Argon has a conductivity of roughly 0.017 W/mK, meaning it conducts heat far less readily. This applies to any sealed double glazing unit where the gap is between 6mm and 20mm; argon works best in gaps of 12mm to 16mm. It does not apply if the unit is damaged or the seal has failed, as the gas escapes and is replaced by air, nullifying the benefit.

Argon reduces heat loss through the window gap

The primary purpose of argon is to lower the U-value of the window, which measures how quickly heat passes through. A standard double glazed unit with a 12mm air gap typically has a U-value around 2.8 W/m²K. Filling that same gap with argon drops the U-value to roughly 2.0 W/m²K (BRE, 2026). This reduction means less heat escapes your home in winter, directly lowering heating bills. The Energy Saving Trust estimates that upgrading from single to A-rated double glazing (which uses argon) saves a typical semi-detached home around £195 per year on energy costs.

Argon is denser than air and reduces convection

Argon is about 1.4 times denser than air. This higher density suppresses convection currents within the sealed gap. In an air-filled unit, warm air rises and cool air sinks, creating a circulation loop that carries heat from the warm inner pane to the cold outer pane. Argon’s density slows this movement, so heat transfers more slowly across the gap (Approved Document L, UK Government, 2026). This effect is most pronounced in vertical windows; for sloped or roof windows, other gases like krypton may be more effective.

Argon is cost-effective and widely available

Argon is the most common gas fill for double glazing because it balances performance with cost. It is a byproduct of air separation and costs roughly £20–£30 per cubic metre to produce. That makes it affordable for manufacturers. Krypton and xenon offer even lower U-values but cost 10 to 50 times more (MCS Certified, 2026). For most UK homes, argon-filled double glazing meets Building Regulations requirements for new windows, which mandate a U-value of 1.6 W/m²K or lower for replacement units. The gas is also non-flammable and non-reactive. This ensures it will not damage seals or panes over the window’s lifetime.

A worked example

A typical 1930s semi-detached home in Manchester upgrading 10 standard double glazed units from air-filled to argon-filled could save roughly £85 per year on heating bills. This scenario assumes replacing existing 12mm air-filled units (U-value 2.8 W/m²K) with new argon-filled units (U-value 2.0 W/m²K) across a total glazing area of 15 square metres. The upfront cost for 10 high-quality argon-filled double glazed units including fitting typically ranges from £4,500 to £6,500 depending on frame type and installer. The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing. 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 confirms that argon filling is one of the most cost-effective glazing upgrades for older homes with single glazing or poor-performing double glazing.

What homeowners often get wrong

The most common mistake is assuming argon-filled windows are maintenance-free and will never lose their gas. Here are three frequent misunderstandings that can cost you money.

1. Believing argon lasts forever Many people think the gas stays sealed for the window’s entire lifetime. In reality, all sealed units lose around 1% of their argon per year through normal seal degradation, meaning after 20 years roughly 20% of the gas has escaped, reducing the thermal benefit and increasing your heating bills by up to £20 annually.
2. Assuming any gap size works equally well Some homeowners choose windows with a 6mm gap thinking argon will still provide full benefit. The truth is argon performs best in gaps of 12mm to 16mm; a 6mm gap with argon only improves the U-value by about 0.2 W/m²K compared to air, wasting the extra cost of the gas fill and missing out on the full 30% efficiency gain.
3. Thinking argon makes all windows equally efficient A common belief is that argon alone guarantees a low U-value. The reality is the frame material, spacer bar type, and overall installation quality matter just as much; a poorly fitted argon-filled unit in an aluminium frame can still have a U-value above 2.5 W/m²K, voiding any warranty claims and wasting your investment.

Quick reference

• Argon reduces thermal conductivity by roughly 35% compared to air, from 0.026 W/mK to 0.017 W/mK.
• A 12mm argon-filled double glazed unit typically achieves a U-value of 2.0 W/m²K versus 2.8 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).
• The typical payback period for replacing air-filled with argon-filled windows in a 1930s semi-detached home is over 50 years based on energy savings alone.
• Always check the BFRC rating label on new windows to verify the U-value and argon fill percentage, as some cheap units contain only 80% argon mixed with air.