Guide
U-values and Window Energy Ratings explained
What Ug, Uw, g-value and the A++ to E energy ratings mean, and which figure to compare.
A U-value measures how much heat passes through a window, in watts per square metre per degree (W/m²K): lower is better. Ug is the glass alone; Uw is the whole window. A Window Energy Rating (A++ to E) combines heat loss, solar gain and air leakage into one figure, so a window with more solar gain can rate well despite a higher U-value.
The figures on a quote
- Ug (centre-pane U-value): heat loss through the middle of the glass. Good double about 1.1; good triple about 0.6.
- Uw (whole-window U-value): glass, spacers and frame together, at a standard size. This is the one the Building Regulations limits use.
- g-value: the share of the sun’s heat that gets through, from 0 to 1. Triple glazing is typically about 0.46 to 0.6.
- L-value: air leakage through the frame seals.
- Psi-value: heat loss along the glass edge, cut by warm-edge spacers.
Window Energy Ratings
The BFRC rating works out a single figure in kWh per m² per year from the U-value, g-value and air leakage (WER = 218.6 × g − 68.5 × (U + L)). Positive figures mean the window gains more energy over a year than it loses.
| Band | Energy index (kWh/m²/yr) |
|---|---|
| A++ | 20 or more |
| A+ | 10 to under 20 |
| A | 0 to under 10 |
| B | −10 to under 0 |
| C | −20 to under −10 |
Which figure should you compare?
For heat loss, compare Uw. For overall energy, the rating helps, but remember it rewards solar gain, which only helps windows that get sun. In England and Wales a replacement window must reach Uw 1.4 or band B; in Scotland 1.6; in Northern Ireland 1.6 or band C.
Warm-edge spacers
The spacer bar holding the panes apart used to be aluminium, which conducts heat. Warm-edge spacers cut edge heat loss by about a third and keep the glass edge about 2°C warmer, which reduces condensation along the bottom of the pane.
Questions
What is a good U-value for a window?
Building Regulations allow up to 1.4 in England and Wales for replacements. Good triple glazed windows reach about 0.8 to 1.0.
What does A++ mean?
An energy index of 20 kWh/m²/yr or more: the window gains more energy over a year than it loses, usually helped by solar gain.
Sources
More guides
- Is triple glazing worth it?
- Triple vs double glazing: the real differences
- Triple glazing cost guide (October 2026)
- Building Regulations for replacement windows: England, Wales, Scotland and Northern Ireland
- Do you need planning permission for new windows?
- Triple glazing and condensation
- Does triple glazing reduce noise?
- Can you put triple glazing in existing frames?
- Triple glazing in period homes and conservation areas
- Trickle vents and ventilation when replacing windows
- Window security: locks, laminated glass and PAS 24
- Windows for Passivhaus and EnerPHit projects
- uPVC vs aluminium vs timber vs alu-clad: choosing a frame
- Triple glazing and solar gain: which way do your windows face?
- What happens when your triple glazing is installed
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