Flagship guide · 7 min read
U-Values and Thermal Performance Explained
The single figure most quoted in glazing brochures, and the one most often misunderstood. Here is what a U-value actually measures, and what to do with it.
Window Energy Ratings (WER) Explained
A Window Energy Rating combines several performance figures into a single familiar A to G band, similar to appliance energy labels.
3 min read
The Window Energy Rating, or WER, gives windows a single band from A++ to E, similar in spirit to the energy labels used on domestic appliances. It is a useful shorthand, but it combines several underlying figures, and it is worth understanding what goes into the band rather than treating it as interchangeable with a U-value alone.
This guide explains how a WER is calculated, why Band B is the effective compliance route under Part L, and what the band does and does not tell you about a specific window.
How a WER band is calculated
A WER is calculated from an energy balance equation that combines the whole-window U-value, the solar heat gain (g-value) and the air leakage rate, weighted to reflect the net thermal effect of a window over a standard heating season. A window with a slightly higher U-value but strong solar gain can, in some cases, achieve a similar or better band than a window with a lower U-value but poor solar gain, because the calculation accounts for the free heat solar gain provides in winter.
| Band | Approximate energy index | Notes |
|---|---|---|
| A++ / A+ | Best performing | Typically triple glazed, high-spec double glazed with excellent g-value |
| A | Very good | High-performance double glazing, well-specified triple glazing |
| B | Good — Part L compliant | Meets the WER route to Part L compliance for replacement windows |
| C and below | Below current new-build/replacement standard | Typical of older double glazing from the 1990s–2000s |
WER as a Part L compliance route
Part L allows replacement windows to demonstrate compliance either via a whole-window U-value of 1.4 W/m²K or better, or via a WER of Band B or above. In practice, most reputable manufacturers provide both figures, and a FENSA or Certass registered installer will confirm which route the certificate uses.
- Part L compliance route 1
- Whole-window U-value ≤ 1.4 W/m²K
- Part L compliance route 2
- WER Band B or better
- Certification bodies
- BFRC and other UKAS-accredited schemes
What a WER band does not tell you
A WER band is calculated for a standard reference orientation and does not reflect your specific elevation, exposure or room use. It also does not capture acoustic performance, security or aesthetic sightline width, so it should be read alongside the individual figures — U-value, g-value and air permeability — rather than as a complete performance summary.
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