What the rating actually measures
The British Fenestration Rating Council (BFRC) energy rating combines three things into a single letter grade:
- U-value: heat lost through the window per square metre per degree of temperature difference (lower is better)
- Solar factor (g-value): solar heat gained through the window (higher is better, in heating-dominated climates like the UK)
- Air leakage (L-value): air infiltration through gaps and seals (lower is better)
The combined number — Energy Index — is then mapped onto the A++ to E letter scale. Importantly, this is a net rating: a window with mediocre U-value but excellent solar gain can still rate A. That's a deliberate quirk of the UK system.
The full scale
| Rating | Energy Index range | Typical U-value | Typical use today |
|---|---|---|---|
| A++ | ≥ +20 | ~1.0 W/m²K | Premium new-build, very low-energy retrofits |
| A+ | +10 to +19 | ~1.1 W/m²K | Premium replacements |
| A | 0 to +9 | ~1.2–1.4 W/m²K | Standard quality replacements |
| B | -10 to -1 | ~1.5–1.6 W/m²K | Acceptable, just meets Part L |
| C | -20 to -11 | ~1.7–1.8 W/m²K | Below current Part L for most installs |
| D / E | ≤ -21 | ≥ 1.9 W/m²K | Replacement-grade only — not for new installs |
Why A is the sweet spot
The marginal cost-benefit drops sharply once you move past A-rated. Going from A to A+ to A++ each step adds roughly 5–10% to the unit cost for a 5–10% reduction in heat loss. In absolute terms on a typical 3-bed semi, the upfront cost outpaces the heating saving by a wide margin. A-rated is where the marginal pound stops working hard for you.
What a quality A-rated unit actually contains
The technology that gets you to A is well-established and not exotic:
- 16mm cavity between panes (the optimum for argon-filled units)
- Argon gas fill in the cavity — argon conducts heat ~33% less than air
- Soft-coat low-E coating on the inner face of the outer pane — reflects long-wave heat back into the room
- Warm-edge spacer bar at the perimeter — replaces the old aluminium spacer with a low-conductivity polymer or stainless
- Quality gaskets and seals on the frame — air leakage matters as much as glass U-value
If a quote doesn't specify warm-edge spacers, soft-coat low-E and argon fill, ask why — those three together are most of what gets you from C to A.
Where ratings can be misleading
1. Window orientation isn't accounted for
The rating assumes a national average of solar gain. A window facing north captures little solar gain regardless of glass spec, so its real-world performance is closer to the U-value than the headline letter rating suggests. North-facing windows benefit most from low U-value; south-facing benefit from high g-value.
2. Frame matters as much as glass
A whole-window U-value reflects glass and frame combined. A great glass spec in a thermally-poor frame underperforms. Always confirm the rating is for the whole window, not the glass unit alone.
3. Triple glazing isn't always rated
The BFRC scale tops out at A++. Triple-glazed windows often achieve 0.8 W/m²K U-values that would warrant an "A+++" if the scale extended that far — but they show as A++ on labels. Don't assume all A++ windows are equivalent.
Building regulations vs ratings
Building Regulations Part L (England and Wales, 2022 update) requires replacement windows to achieve 1.4 W/m²K or better at the whole-window level, which corresponds to a B rating or A. Anything below B is not compliant for replacement installs. New-build standards under the Future Homes Standard are tighter still and trending toward 1.2 W/m²K.
What to ask your installer
- What's the whole-window U-value (not just the glass U-value)?
- Are units argon-filled with soft-coat low-E?
- Are warm-edge spacer bars used?
- What's the air-permeability classification of the frame?
- Is Certass notification included?
Our standard double glazing installation in Essex uses A-rated as default, with A+ and A++ available on request. The wider glazing category also covers triple-glazed and acoustic-laminated options where the spec genuinely justifies them.