A U-value measures how much heat passes through a window, in watts per square metre per kelvin (W/m²K). Lower is better. A Window Energy Rating (WER) goes further, combining U-value, solar gain and air leakage into a single A++ to G band. Under Part L, replacement windows must reach a whole-window U-value of 1.4 W/m²K or lower, or WER band B or better as an alternative route. You can see what a more efficient window does for your bills in the energy savings calculator.
What a U-value actually tells you
The U-value is the rate of heat loss through the window. A low number means the window holds heat in well. The figure that matters for Building Control is the whole-window U-value (Uw), which accounts for the glass, the spacer bar around the edge and the frame together, not just the centre of the glass (Ug). A salesperson quoting a flattering centre-pane figure is not quoting the number Part L is judged on, so always ask for the whole-window Uw.
| Glazing type | Typical U-value | Notes |
|---|---|---|
| Single glazing | ~4.8-5.8 W/m²K | Old, uninsulated. Loses the most heat. |
| Old double glazing (pre-2002) | ~2.8-3.2 W/m²K | No low-E coating or gas fill. |
| Modern A-rated double glazing | 1.0-1.4 W/m²K | Low-E glass, argon fill, warm-edge spacer. Meets Part L. |
| Triple glazing | 0.6-1.0 W/m²K | Best performance, around 0.8 typical. Heavier and pricier. |
Source: web/lib/research/u-values-wer.md, drawn from UK installer and manufacturer technical pages.
Always ask for the whole-window U-value, not the centre-pane glass figure. The glass alone might read 1.0, but with the frame and spacer the real number could be 1.4. That gap is where some firms massage their marketing.
Tom Bradley, FENSA-registered installer
What Part L requires in 2026
Approved Document L sets the floor for replacement windows in existing homes. Since 15 June 2023 the full standard applies: the whole-window U-value must be 1.4 W/m²K or lower, or, on the alternative WER route, the window must be band B or better. The two are alternative compliance routes, so a window meeting either one passes. We do not claim band A is required. New build and the Future Homes Standard aim lower still, typically 1.2 W/m²K or below, but that is a stricter target than the replacement floor.
How modern glazing hits the target
Three components do the work. Low-emissivity (Low-E) glass carries a microscopically thin metal-oxide coating that reflects heat back into the room, cutting the centre-pane U-value by around 40%. A warm-edge spacer replaces the old aluminium spacer bar, which was a thermal bridge, and reduces heat loss and condensation at the edge of the unit. Argon gas in the cavity conducts heat less readily than air and helps the unit meet Part L without going to triple glazing. Together these let a good-quality double-glazed unit clear the 1.4 W/m²K bar comfortably.
Low-E glass, argon and a warm-edge spacer are the standard kit now, not an upgrade. If a quote leaves any of them out to look cheaper, the unit will struggle to meet Part L, and you will feel the difference on a cold morning.
Tom Bradley, FENSA-registered installer
U-values, WER and your EPC
The Energy Performance Certificate rewards lower heat loss, so glazing feeds into it. Replacing single glazing, or tired pre-2002 double glazing, with A-rated units improves the glazing element of the assessment and can contribute to a better EPC band, alongside insulation and heating measures. It is rarely a single-band jump on windows alone, but it counts, and a stronger EPC can matter when you sell or let.
Double or triple glazing for the U-value?
Triple glazing pushes the U-value lower still, to roughly 0.6 to 1.0 W/m²K, but it is heavier and pricier, and good A-rated double glazing already clears Part L. Triple glazing earns its place mainly where you need a whole-window U-value below 1.0, such as a passive-house design or a very exposed plot. Our double versus triple glazing comparison works through when the extra pane pays off.
Frequently asked questions
Under Part L (Approved Document L), replacement windows in existing homes must reach a whole-window U-value of 1.4 W/m²K or lower. Compliance is assessed on the whole-window figure (Uw), which accounts for the glass, the spacer bar and the frame, not just the centre of the glass.
Part L accepts Window Energy Rating band B or better as an alternative compliance route for replacement windows (the other route being a U-value of 1.4 W/m²K or lower). This full standard has applied since 15 June 2023. Many modern windows are sold as A-rated, which clears band B comfortably.
Lower is better. Single glazing is around 4.8 to 5.8 W/m²K, modern A-rated double glazing is 1.0 to 1.4 W/m²K, and triple glazing reaches 0.6 to 1.0 W/m²K. Anything at or below the Part L limit of 1.4 W/m²K is fit for a replacement window.
Low-emissivity (Low-E) glass has a microscopically thin metal-oxide coating that reflects heat back into the room. Soft-coat Low-E cuts the centre-pane U-value by around 40%, which is why it is used as standard in Part L-compliant double glazing.
Yes, argon is a standard component that helps modern double glazing meet Part L without triple glazing. It conducts heat less readily than air. Sources name it as a contributor but do not isolate its effect, so treat it as part of the package alongside Low-E glass and a warm-edge spacer.
They can. The Energy Performance Certificate rewards lower heat loss, so replacing single or old double glazing with A-rated units can lift the glazing element of the assessment and contribute to a better EPC band, alongside insulation and heating measures.