U-Values Explained Simply

U-value is the rate at which heat passes through a material or assembly, measured in watts per square metre per degree Celsius (W/m²K). Lower means less heat loss — so a U-value of 1.0 is better than 1.4. It's the fundamental metric for thermal performance in construction.

This is where misleading quotes live. A quoted 'U-value 1.0' glass unit might sit in a frame with U-value 2.0, giving a whole-window U-value around 1.4. Always ask for the whole-window U-value — that's what Building Regulations reference and what your heating bill will experience.

Building Regs (Approved Document L 2022) require replacement windows to achieve a whole-window U-value of 1.4 W/m²K or better. New builds target 1.2 or lower. Premium products reach 0.8 with triple glazing. Anything above 1.6 is out-of-date specification and you should push back on the installer.

Three things: the number of panes (double vs triple), the coating on the glass (low-E), and the gas fill between panes (Argon or Krypton). A modern double-glazed unit with low-E coating and Argon fill reaches around U 1.1 for glass alone; without those, you're back to 2.0+.

WER (A++ to G scale) combines U-value with solar gain and air leakage into a single letter. It's easier to compare but hides the U-value in the calculation. For technical clarity, ask for both — WER for the label and U-value for the underlying performance.