Double glazing windows work by trapping a layer of insulating gas between two panes of glass, which dramatically reduces heat loss. A typical double-glazed unit with a 16mm argon-filled gap achieves a U-value of around 1.2 W/m²K, compared to 5.7 W/m²K for a single-glazed window (HM Government, 2026).
- A 16mm argon gap achieves a U-value of 1.2 W/m²K.
- Low-E coating reflects heat back inside, improving efficiency.
- Listed buildings may need secondary glazing instead of double.
The key variable that decides performance is the width of the gap and the type of gas used. A 12mm air-filled gap gives a U-value of about 2.7 W/m²K, while a 20mm argon-filled gap can drop to 1.0 W/m²K (BRE, 2026). This technology works for most homes, but listed buildings or conservation areas may require secondary glazing instead.
The sealed unit traps insulating gas
Two panes of glass are separated by a spacer bar, typically made from aluminium or warm-edge plastic, and sealed airtight. The gap is filled with argon, krypton, or xenon gas, which is denser than air and conducts less heat. A 16mm argon-filled gap reduces heat transfer by about 70% compared to single glazing (Energy Saving Trust, 2026). The low-conductivity gas slows the movement of heat from the warm inner pane to the cold outer pane. Without this gas, the unit would perform like a poorly sealed window.
Low-emissivity coating reflects heat back inside
A microscopically thin metal oxide coating is applied to one or both inner surfaces of the glass. This low-emissivity (low-E) coating reflects long-wave infrared heat back into the room while letting short-wave solar heat in. A low-E coating can improve the U-value by 0.5 to 1.0 W/m²K (SAP 2012, 2026). Without this coating, the glass would radiate heat outward, reducing the window’s insulation performance.
The spacer bar reduces condensation and heat loss
The spacer bar that separates the panes is often made from a warm-edge material, such as plastic reinforced with stainless steel. Warm-edge spacers reduce heat loss at the edge of the unit by up to 30% compared to aluminium spacers (FENSA, 2026). They also keep the inner pane warmer, which lowers the risk of condensation forming on the glass. A colder edge would create a “thermal bridge,” making the window less efficient overall.
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 cuts heat loss through windows by over 50%, saving roughly £195 per year on heating bills. 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 price lands at about 33 years, but with a grant the period drops to around 5 years. Over 25 years, total savings reach £4,875 at current energy prices, and the home’s EPC rating typically jumps from band D to band C.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,000 |
| Yearly savings | £195 |
| Payback period | 5 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is assuming all double glazing performs the same, which leads to choosing the cheapest option and missing out on real energy savings. Here are three frequent errors people make when selecting new windows.
- Ignoring the U-value rating Many homeowners pick windows based on appearance or price rather than thermal performance. A double-glazed unit with a U-value above 1.6 W/m²K wastes roughly £40 per year compared to an A-rated window with a U-value of 1.2 W/m²K, according to the Energy Saving Trust.
- Choosing aluminium spacer bars over warm-edge Standard aluminium spacers conduct heat around the edge of the pane, creating a cold bridge that reduces overall efficiency by up to 5%. Warm-edge plastic or foam spacers eliminate this heat loss and prevent condensation forming on the frame interior.
- Forgetting about trickle vents for ventilation Sealed double glazing can trap moisture indoors, leading to mould growth and voiding your warranty if humidity exceeds 70%. Modern building regulations require trickle vents or mechanical ventilation to maintain healthy air quality and protect the window seal.
Quick reference
- A 16mm argon-filled double-glazed unit achieves a U-value of 1.2 W/m²K, cutting heat loss by 70% compared to single glazing (Energy Saving Trust).
- Low-E coating reflects up to 80% of indoor heat back into the room, improving winter performance without blocking natural light.
- The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing.
- Replacing single glazing with A-rated double glazing saves the average semi-detached home £195 per year on heating bills.
- Installing double glazing in a listed building without listed building consent is a criminal offence, so secondary glazing is often the only legal option.
Frequently Asked Questions
It traps insulating gas like argon between two panes, reducing heat transfer by about 70% compared to single glazing (Energy Saving Trust, 2026). The low-E coating also reflects heat back into the room.
A typical double-glazed unit with a 16mm argon gap has a U-value of around 1.2 W/m²K (HM Government, 2026). For best performance, look for a U-value of 1.0 W/m²K or lower.
No, listed buildings and conservation areas usually require secondary glazing instead of double glazing. Always check with your local planning authority before installing.
