Window Energy Ratings and U-values explained
What U-values and Window Energy Ratings mean, and which figure the regulations use.
What is a U-value?
A U-value measures how much heat passes through a window, in watts per square metre per degree of temperature difference (W/m²K). Lower is better. The whole-window U-value includes the frame and the glass, not just the glass.
What is a Window Energy Rating?
A Window Energy Rating (WER) is a band from A++ down to E that combines heat loss, solar gain and air leakage into one rating for a window. It is issued through rating schemes such as BFRC and Certass.
Which figure do the regulations use?
In England, Approved Document L sets a limiting U-value of 1.4 W/m²K for replacement windows, or a Window Energy Rating of band B. Wales, Scotland and Northern Ireland set their own values, so tell us where the job is.
Does triple glazing always win?
Triple glazing lowers the U-value of the glass, but the frame and spacer also matter. A well-specified double glazed window can meet the replacement window requirement; triple glazing is often chosen for new low-energy homes or noisy locations.
This guide is general information for trade customers. The person carrying out the work remains responsible for meeting the building regulations and any planning requirements for the specific job.