U-Values and WER: What the Numbers Actually Tell You

Two numbers dominate window energy talk: the U-value and the Window Energy Rating (WER). They are related but answer different questions, and quoting one when you need the other causes real confusion.

U-value: pure heat loss

The U-value is the rate of heat flow through the assembly — watts per square metre per degree of temperature difference. Lower is better. Centre-pane values (calculated to EN 673) describe just the glass; whole-window values include frame and edge effects and are always worse. Building Regulations Part L limits are whole-window figures: currently 1.4 W/m²K for replacements in dwellings.

WER: the balance sheet

The A++ to E rating treats a window like an appliance. It subtracts heat lost through the window from useful solar heat gained through it over a heating season. A window with a middling U-value but generous solar gain can out-rate one with better insulation but heavy solar control. Band C or better satisfies Part L for replacements as an alternative to the U-value route.

Which should you use?

• North-facing, shaded, or overheating-prone rooms: prioritise U-value
• Typical mixed orientations: WER gives the fairer whole-season picture
• Compliance paperwork: either route works — pick whichever your spec meets more comfortably

Model your own build-ups with the U-value calculator, and if a project needs a specific rating band, tell us at quote stage.