When you are comparing window quotes, the U-value keeps cropping up, and it is one of the most useful numbers on the page once you know how to read it. The average double glazing U-value has improved a lot over the years, so a figure that was impressive a decade ago is now fairly ordinary. Here is what the numbers mean and what to aim for.
A U-value measures how much heat escapes through the glazing. It is written in watts per square metre kelvin, but the practical takeaway is simple: the lower the number, the better the window holds its warmth. A high U-value means heat leaks out quickly, while a low one keeps rooms cosy and cuts cold spots by the glass.
It is the opposite way round to some other ratings, so it catches people out. With U-values, low is good.
Modern double glazing has moved on a great deal. As a rough guide:
So a typical average double glazing U-value for a good modern unit today sits comfortably in the low-to-mid 1s. Treat these as general guidance, because the exact number depends on the product and is measured against standards that are reviewed over time.
Here is a detail worth knowing. You will sometimes see a U-value for the glass unit alone and sometimes for the whole window, which includes the frame and the spacer bar. The whole-window figure is usually a little higher, because frames and edges lose some heat too.
When you compare quotes, make sure you are comparing like with like. A glass-only figure and a whole-window figure are not the same thing, and the honest way to compare products is on the same basis.
Several parts pull together to bring the number down:
Our uPVC windows come with A-rated glazing as standard, so the U-values sit firmly in that good modern range, and our casement windows pair that glazing with well-insulated frames.
Not quite. A great U-value on paper still relies on a proper fit. Gaps around a badly installed frame let heat out no matter how good the glass, and a failed sealed unit that has misted up has lost its gas and its performance. If your glazing has gone cloudy between the panes, that is a sign the unit needs attention, and our repairs team can advise whether a new unit or a whole window makes sense.
Building Regulations set the minimum standard that replacement windows have to meet under Part L, and those requirements are updated from time to time. Because the figures change, check the current rules with your installer or the local authority rather than relying on an older number. What counts as a compliant U-value in 2026 may differ from a few years back.
For a modern A-rated unit, a glazing U-value in the low-to-mid 1s is typical and counts as good. Lower is better, so figures around 1.2 to 1.4 are strong. Treat these as general guidance, as they are measured against evolving standards.
Older units often have U-values around 2.8 to 3.0, well above modern glazing, and may have lost their gas fill or misted up. That combination means far more heat escapes than through a current A-rated window.
It can be either, so always check. A glass-only figure is usually lower than a whole-window figure, which includes the frame and spacer. Compare quotes on the same basis to be fair.
It reduces heat loss through the glazing, but savings depend on your whole home, how you heat it and what you are replacing. Treat any figures as estimates rather than promises, and remember a good fit matters as much as the number.
Want to see our full range of energy-efficient windows? Download our latest brochure, or request a free quote and we will pop out for a look.