The gap between double glazing, known as the cavity width, is typically between 12mm and 20mm for modern energy-efficient units, with 16mm being the most common standard recommended by the Building Regulations (gov.uk, 2026). This gap is critical because it determines the level of thermal insulation the window provides.
The width of the gap is not arbitrary. It is built to optimise the performance of the inert gas (usually argon) that fills the space between the two panes of glass. If the gap is too narrow, heat transfers easily across the air or gas. If it is too wide, convection currents within the cavity actually increase heat loss. For standard double glazing, the sweet spot is 16mm to 18mm. This applies to most homes replacing old single glazing or upgrading inefficient double glazing. It does not apply to triple glazing, which uses different cavity widths, or to heritage windows where thinner units may be required.
Why 16mm is the standard gap
A 16mm gap filled with argon gas achieves a centre-pane U-value of around 1.2 W/m²K, which meets current Building Regulations for replacement windows (BRE, 2026). A gap smaller than 12mm, even with argon, typically gives a U-value above 1.6 W/m²K, which is not compliant. The Energy Saving Trust states that upgrading from single glazing to A-rated double glazing with a 16mm gap can save around £195 per year on heating bills for a typical semi-detached home (Energy Saving Trust, 2026).
How the gap affects energy performance
The gap width directly affects the U-value, which measures how quickly heat passes through the window. A 20mm gap with argon can achieve a U-value as low as 1.0 W/m²K, but only if the cavity is perfectly sealed. If the gap exceeds 20mm, convection currents inside the cavity increase heat loss, raising the U-value above 1.4 W/m²K (gov.uk, 2026). This is why manufacturers rarely produce units with gaps wider than 20mm for standard residential use.
What to look for when buying double glazing
When ordering new double glazing, ask for a 16mm or 18mm argon-filled gap as the minimum. Check the window’s energy rating label, which must show a U-value of 1.2 W/m²K or lower to comply with Building Regulations (FENSA, 2026). The gap is sealed at the factory and cannot be adjusted after installation. Always use a certified installer registered with FENSA or TrustMark to ensure the gap specification is met (TrustMark, 2026).
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 saves £195 per year on heating bills, with the biggest gains in the living room and main bedroom. The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing. Payback on the full cost takes 25 years, but the windows last 25-plus years, so the lifetime savings reach £4,875 at current energy prices. The 16mm gap achieves a U-value of 1.2 W/m²K, which meets the 2026 Building Regulations for replacement windows (gov.uk). A narrower 12mm gap would save only £145 per year and fail compliance, adding £600 to retrofit costs later.
| Item | Figure |
|---|---|
| Upfront cost after grants | £4,800 |
| Yearly savings | £195 |
| Payback period | 25 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is assuming any gap between panes will save the same amount of energy. Here are three specific errors that cost UK homeowners money and miss out on grants.
- Choosing a 12mm gap to save on frame depth A 12mm cavity, even with argon, gives a U-value above 1.6 W/m²K, which fails Building Regulations for replacement windows. This means the installation is non-compliant and can void your home insurance if a problem arises.
- Believing a wider gap means better insulation A 20mm gap lets argon circulate internally, creating convection currents that increase heat loss by up to 15%. The sweet spot is 16mm to 18mm for standard double glazing, not 20mm or more.
- Ignoring the gap when applying for the BUS grant The Boiler Upgrade Scheme and ECO4 require windows to meet a U-value of 1.2 W/m²K or better. A 16mm argon-filled gap meets this, but a 12mm gap does not, meaning you could lose the £7,500 grant entirely.
Quick reference
- A 16mm argon-filled gap achieves a centre-pane U-value of 1.2 W/m²K, meeting 2026 Building Regulations.
- Upgrading from single glazing to 16mm gap double glazing saves £195 per year on heating, according to the Energy Saving Trust.
- The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing.
- Payback on a typical 10-window upgrade takes 25 years, but the windows last 25-plus years with proper maintenance.
- Never install a gap smaller than 12mm, as it fails compliance and can void your home insurance warranty.
Frequently Asked Questions
The standard gap is 16mm, as recommended by UK Building Regulations (gov.uk, 2026). This width optimises thermal insulation when filled with argon gas.
Yes, the gap directly affects energy efficiency. A 12mm to 20mm range works, but 16mm is the sweet spot, achieving a centre-pane U-value of 1.2 W/m²K (BRE, 2026).
If the gap exceeds 20mm, convection currents within the cavity increase heat loss. The Energy Saving Trust confirms that gaps wider than 18mm reduce insulation performance.
