Two figures worth understanding before you sign off on new windows.
A U-value measures the rate of heat loss through a material, expressed in watts per square metre per degree Kelvin. The lower the U-value, the better the material is at retaining heat. For windows, a whole-window U-value takes into account the glass, frame and spacer bar together, rather than just the glass, which is why two windows with the same glass but different frames can have noticeably different U-values.
A Window Energy Rating, or WER, goes further than U-value alone by combining thermal performance with solar heat gain and air leakage into a single overall rating, shown on a letter scale similar to household appliance labels, from A++ at the top down through lower letters. This gives a more rounded picture of real-world energy performance than U-value on its own.
Under the current UK scheme, ratings run from A++ at the most efficient end down to lower letters for less efficient products, with a specific numerical index also shown alongside the letter. Most quality modern replacement windows achieve a strong rating, and current building regulations set a minimum standard that all new installations must meet or exceed.
Because WER combines multiple factors, a window with excellent glass but a poorly sealed frame, weak spacer bar or higher air leakage can still rate lower than expected. This is why the whole-window rating matters more than any single component figure when comparing different suppliers' quotes.
When comparing quotes from different installers, ask for the whole-window U-value and, ideally, the WER rating for the exact product being quoted, rather than a generic figure for the profile range in general. Two quotes referencing the same profile system can still differ if the glass specification, gas fill or spacer bar included varies between them.
UK building regulations set minimum energy performance standards for replacement windows under Part L, and FENSA or CERTASS registered installers are responsible for ensuring installations meet or exceed these standards, with certification provided as evidence once work is complete.
Lower is better, and current building regulations set a minimum standard for replacement windows. Quality modern double or triple glazed windows with low-e glass typically achieve figures comfortably within or beating that minimum requirement.
It combines thermal performance, solar heat gain and air leakage into a single overall rating shown on a lettered scale, giving a broader picture of real-world performance than U-value alone.
Because WER accounts for the whole window, differences in frame sealing, spacer bar type or air leakage between installers can affect the overall rating even where the glass specification looks similar on paper.
Yes, FENSA and CERTASS registered installers are responsible for ensuring replacement windows meet current building regulations, including minimum energy performance standards, with certification issued once work is completed.
Understanding the invisible coating that turns an ordinary window into an energy-saving one.
The component around the edge of the glass that quietly affects comfort, condensation and durability.
A plain-English look at the frame material behind almost every replacement window in the UK.
Weighing up an extra pane of glass against the cost, weight and payback for a typical UK home.
The invisible gas sitting between your panes, and why it matters more than most homeowners realise.
Understanding the two safety glass types and where the rules say each one belongs.
Book a free survey and we will show you the whole-window U-value and energy rating for your proposed windows, set out clearly in a fixed written quote.
