Energy
Window energy ratings and U-values, explained
U-value, WER, Low-E, argon, warm edge. What the labels on a window mean for your heating bills, and which figures to compare.
Quotes for new windows usually give a U-value, an energy rating or both. This guide explains what each figure measures, what Building Regulations require, and how to compare one quote fairly with another.
- U-value is the heat lost through the window. Lower is better.
- WER is a letter band for the whole window, with A++ at the top. Higher is better.
- In England, replacement windows need 1.4 W/m²K or better, or WER band B or above.
- Compare whole-window figures, not glass-only figures.
U-value: how much heat escapes
A U-value measures how much heat passes through a window, in watts per square metre for each degree of temperature difference (W/m²K). Lower is better. As a rough guide, old single glazing is around 5 and early double glazing around 2.8. A replacement window fitted today has to reach 1.4 or better.
Check which U-value you are given
A centre-pane figure (Ug) covers the glass only and usually looks better. The whole-window figure (Uw) includes the frame and the edge of the glass. Uw is the one the regulations use, so ask for it.
Window Energy Rating (WER): the whole picture
A WER grades the complete window, frame and glass together, on a letter scale like the one on a fridge. It combines heat lost through the window, free heat gained from the sun and air leaking around the opening parts.
That is why two windows with the same U-value can carry different ratings. Glass that lets in less sunlight gains less free heat, so it can score lower. Ratings are worked out for a standard window size, which makes WER a fair way to compare one product with another.
What Building Regulations require
In England, Approved Document L sets the limits for replacement windows and doors:
- Windows: 1.4 W/m²K or better, or WER band B or above
- Doors more than 60% glazed: 1.4 W/m²K or better, or Door Set Energy Rating (DSER) band C or above
- Other doors: 1.4 W/m²K or better, or DSER band B or above
Wales, Scotland and Northern Ireland set their own figures. Our Building Regulations guide covers ventilation, safety glass, escape and sign-off.
What makes a modern window efficient
- Low-E glass. A microscopically thin coating on the inner pane reflects heat back into the room while letting light through.
- Argon gas. It is denser than air, so less heat moves across the gap between the panes.
- Warm-edge spacer bars. The strip that holds the panes apart is made of an insulating material instead of aluminium. That cuts heat loss and condensation around the edge of the glass.
- Multi-chamber frames. uPVC profiles with several internal chambers slow heat loss through the frame.
- Good installation. A well-rated window still loses heat if the gap around it is badly insulated or sealed.
Is triple glazing worth it?
Triple glazing cuts heat loss further and can help with noise. The units are heavier and cost more, and for most UK homes the saving over good double glazing is modest. It is worth considering for north-facing rooms, very exposed sites and homes beside busy roads. If noise is the main problem, acoustic laminated double glazing is often the better buy.
Questions to ask on any quote
- What is the whole-window U-value (Uw), and what is the WER band?
- Is the glass Low-E, with argon fill and a warm-edge spacer?
- How will the work be certified under Building Regulations, and when will I get the certificate?