# U-value

U-value measures how effectively a window resists heat transfer—the lower the U-value, the better the insulation. Expressed in W/m²K (watts per square metre per kelvin), it indicates how much heat escapes through the window in cold weather.

Single glazing: ~5.0 W/m²K

Standard double glazing: ~2.8–1.6 W/m²K

Slimline double glazing: ~1.4–1.2 W/m²K

Vacuum Insulated Glazing (VIG): as low as 0.7 W/m²K

Existing home window replacements must comply with UK Building Regulations Part L 2022 by reaching a U-value of 1.4 W/m²K or achieving a Window Energy Rating of C or higher. The VIG system meets all heritage upgrade requirements because it maintains the original look and function of historic windows.