Window quotes are full of numbers. Here's what they mean and how to compare them.
U-value
A U-value measures how much heat passes through, in W/m²K. Lower is better. Make sure you're comparing the whole window (Uw), not just the glass (Ug), which always looks better.
Window Energy Rating (WER)
A WER rates the whole window from A++ to E. It balances heat lost through the window against free heat gained from the sun and air leakage.
What the regulations ask for
In England, replacement windows need a U-value of 1.4 W/m²K or better, or a WER of B.
How our systems compare
| System | Figure |
|---|---|
| Cortizo COR 70 | Uw from 0.85 W/m²K |
| Kömmerling C70 (triple) | 0.88 W/m²K |
| Kömmerling C70 (standard) | 1.2 W/m²K |
| Liniar EnergyPlus | WER A+ (A++ achievable) |
| AluK 58BW Flush HI | 1.4 double · from 1.2 triple |
Don't forget g-value
A g-value shows how much solar heat gets through. A high g-value helps in winter but can cause overheating in big south-facing glass, which is where solar control glass comes in. See Is SKN II solar control glass worth it?
