U-Value.
The rate of heat transfer through a building element, in W/m²K — lower is better.
U-value (thermal transmittance) measures how much heat passes through a building element per square metre, per degree of temperature difference between inside and out. It is expressed in watts per square metre per Kelvin (W/m²K). The lower the U-value, the better the insulation.
On a curtain wall, three U-values are commonly quoted: Ug (centre-pane glass only), Uf (frame only) and Uw or Ucw (the whole-element value, weighted across glass and frame). The whole-element figure is what Building Regulations measure against and is always higher (worse) than the centre-pane figure because the frame is a thermal weak point.
Part L 2025 (England) sets a notional whole-element U-value of 1.6 W/m²K for non-domestic curtain walling and around 1.4 W/m²K for dwellings. Specifiers should always request a thermally-modelled whole-element U-value for the actual project grid and glass build-up, not the brochure centre-pane number.