Term
U-value: what the term means
A measure of how much heat passes through a part of a building, such as a wall or window; the lower the number, the better the insulation.
A measure of how much heat passes through a part of a building, such as a wall or window; the lower the number, the better the insulation.
U-value in practice
The U-value expresses the rate of heat loss through a building element per unit area for each degree of temperature difference between inside and outside. Approved Document L sets limiting U-values for new and replacement elements in England, including windows and doors.
A replacement window or external door is a controlled fitting, so it must meet the relevant U-value. When comparing quotes for windows, insulation or a new roof, ask for the U-value the product or build-up achieves.
Source: gov.uk · planningportal.co.uk