Thermal Transmittance & U-Values (Ug vs Uw)
Center-of-pane U-value (Ug) measures heat loss through the glass alone, whereas whole-window U-value (Uw) incorporates the thermal bridging of the frame (Uf) and the linear thermal transmittance of the perimeter spacer bar (ψ). Approved Document L compliance requires Uw calculation, not merely declaring center-of-pane Ug.
1. Centre-of-Pane (Ug) vs Whole Window (Uw) Formula
Under BS EN ISO 10077-1, Uw = (Ag·Ug + Af·Uf + lg·ψ) / (Ag + Af). In commercial curtain walling, the spacer bar psi value (ψ) contributes up to 15% of total facade heat loss. Standard aluminium spacers have a poor ψ of ~0.075 W/mK, whereas modern composite warm-edge spacers reduce this to ~0.035 W/mK.
2. Soft-Coat Low-E Surface Mechanics
Low-E (low-emissivity) coatings utilize microscopically thin silver layers to reflect long-wave infrared heat back into the interior. In standard double glazed units, the Low-E coating is located on Surface #3 (the outer face of the inner pane). Placing it on the wrong surface reduces thermal efficiency.
3. Gas Fills: Argon vs Krypton
90% Argon gas cavities reduce convective heat transfer by ~30% compared to dry air. For standard 16mm cavities, argon delivers Ug ~1.1 W/m²K. For ultra-slim heritage cavities (4mm–8mm), krypton gas is required to suppress convection in narrow gaps.
Critical Specification Pitfalls to Avoid
- •Assuming a 1.0 Ug unit makes the whole window compliant (a thermally uninsulated aluminium frame can push Uw above 2.2 W/m²K)
- •Failing to verify cavity seal breather tubes on high-altitude projects
- •Using clear silicone sealants that allow UV radiation to degrade secondary unit seals