When you compare windows, you will keep seeing the term “U-value”. It sounds technical, but it boils down to one simple question: how well does this product keep the heat in?
The simple explanation
A U-value measures how much heat passes through a material, expressed in watts per square metre per degree of temperature difference (W/m²K). The lower the number, the better the insulation. A low U-value means less heat escapes, so rooms stay warmer for less energy.
What counts as good?
Older single-glazed windows have a U-value of around 5 or above. Modern double glazing is far better, with the best systems reaching around 1.2 W/m²K or lower, and triple-glazed units reaching lower still. Building Regulations set maximum U-values that replacement windows must meet.
Window Energy Ratings
Window Energy Ratings (WER) grade windows from A++ down to E, taking into account not just heat loss but also useful heat gained from the sun and air leakage. An A-rated or better window is a strong choice, and the best double-glazed systems reach A++.
Window versus whole product
There are different U-values: the centre of the glass, the frame and the whole window. The figure for the whole window is the one to compare. Ask for it when you receive a quote.
What affects it?
- The number of panes (double or triple glazing).
- Low-E coatings and gas fill between the panes.
- The design of the frame, including the number of internal chambers.
- The spacer bar used at the edge of the glass unit.