What This Requirement Covers
The Window Energy Rating (WER) scheme, operated by the British Fenestration Rating Council (BFRC), provides a simple A++ to E rating for window energy performance. WER can be used as an alternative to U-value compliance under Part L.
Key Requirements
The WER Scale
- A++: Best performance (net energy gain; the window generates more energy through solar gain than it loses)
- A+: Excellent performance
- A: Very good performance
- B: Good performance
- C: Minimum standard for replacement windows under Part L (0 kWh/m2/year energy balance)
- D to E: Below the minimum standard; not compliant for replacement windows
How WER Is Calculated
The WER considers three factors
- Solar heat gain (g-value): How much solar energy passes through the glass and heats the building
- U-value: The rate of heat loss through the window
- Air leakage (L-value): Heat lost through air leakage around the window frame and seals
WER vs U-Value Compliance
- Part L allows compliance via either WER or U-value
- WER method: Replacement windows must achieve Band C or better
- U-value method: Replacement windows must achieve 1.4 W/m2K or better (whole window)
- For new dwellings, the window performance is entered into the SAP calculation; either WER or U-value can be used
Practical Compliance Tips
- Check the BFRC label on new windows before purchase; it shows the WER band, U-value, and g-value
- For south-facing windows, a higher g-value (more solar gain) improves the WER and helps heat the building in winter
- For north-facing windows, a lower U-value is more important as there is less solar gain
- When comparing windows, the WER gives a better overall picture of energy performance than the U-value alone
- Retain the BFRC certificate and FENSA/CERTASS certificate for Building Control and property sales
- Triple-glazed windows typically achieve WER Band A or higher
- Consider the orientation and shading of each window when selecting the specification; a one-size-fits-all approach may not optimise energy performance