What Is a U-Value?
Written by James Walker · Last reviewed 16 August 2026
James Walker has 2 degrees in construction (BSc (Hons) Building Surveying · MSc Environmental Design of Buildings) and an extensive background in issue resolution, resolving over £4 billion of consumer issues.
A U-value measures how much heat passes through a building element, a window, wall, roof or floor. It is expressed in W/m²K, and lower is better.
Where you will meet it
Replacement windows and doors must meet a required U-value under the Building Regulations, which is why the figure appears on quotes.
Extensions and loft conversions must meet required values for walls, roofs and floors.
Insulation upgrades are described by the U-value they achieve.
How to read a quote
Compare the actual figures, not the descriptions. "A-rated" and "energy efficient" are marketing terms; the U-value is the measurement.
A window quoted at a lower U-value performs better, and on a whole-house replacement the difference is real. Ask what figure is being achieved and whether it is the whole window or just the glass, the two differ, because frames conduct heat.
Diminishing returns
Improving from poor to good makes a substantial difference. Improving from good to excellent makes much less, at disproportionate cost.
Triple glazing is a common example: better U-values, but the payback depends heavily on what you are replacing and how the house is heated.
How it works
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