Window U-values explained

A window U-value measures how fast heat escapes through the window, in watts per square metre for every degree of difference between inside and outside. The lower the number, the better the window keeps heat in. In England, a replacement window must have a U-value of 1.4 W/m²K or lower, so any window you buy from a registered installer will meet that.

What the window U-value tells you

Think of it as a leak rate. A window with half the U-value of another loses half as much heat on the same cold night. Single glazing has a very high U-value, which is why the room feels cold next to it. Early double glazing roughly halved that. Modern double glazing, with low-emissivity coated glass, gas between the panes and a warm-edge spacer, cuts it a great deal further.

So when you compare quotes, a lower U-value is better, and the difference between two modern windows is usually small next to the difference between any of them and the window you have now.

Whole window or centre pane

Brochures sometimes quote the U-value of the middle of the glass only. That always looks better than the window as a whole. The frame and the edge of the glass lose more heat than the centre. Therefore, ask for the whole-window U-value, sometimes written Uw, because that is the figure Building Regulations use and the one that reflects your heating bill.

The U-value limit for new windows

Approved Document L sets the limit for replacement windows in England at 1.4 W/m²K, or a Window Energy Rating of band B. The same standard now applies to uPVC, aluminium and timber. Wales, Scotland and Northern Ireland set their own limits, which your installer should know. See double glazing regulations for the rest of the rules.

U-value or energy rating?

A Window Energy Rating, shown as a letter band like an appliance label, combines the heat lost through the window with the free heat the sun brings in and the air that leaks round the frame. As a result, it reflects a British winter rather than heat loss alone. Either figure is fine for comparing windows; just compare like with like. Our guide to energy efficient windows explains the ratings.

What lowers a window’s U-value

• Low-emissivity glass. An invisible coating that reflects heat back into the room. It makes the biggest single difference.
• Gas fill. Argon between the panes conducts heat less than air.
• Warm-edge spacers. The bar that separates the panes, made from plastic composite instead of aluminium, so the edge of the glass stays warmer.
• A third pane. Triple glazing lowers it again, at a higher price.
• A well-insulated frame. Multi-chambered uPVC, thermally broken aluminium or engineered timber.

Working out heat loss

To estimate the heat a window loses, multiply its area by its U-value and by the temperature difference between inside and outside. The answer is in watts. It’s a useful way to see why big, old windows in a cold room make such a difference to the bills, and why replacing them first makes sense.

Next steps

Ask every installer for the whole-window U-value or the energy rating of the exact window they are quoting, then put them side by side with our compare quotes tool. See uPVC windows for typical prices.