What is a Whole Window U-value vs a Centre Pane U-value?

This is the distinction that catches people out. The centre-pane U-value (often written Ug) measures only the glass, at its centre, away from the edges and frame — so it’s always the most flattering number. The whole-window U-value (Uw) measures the complete unit: glass, spacer bar edges, and frame combined. Because frames and glass edges lose more heat than the centre of the pane, the whole-window figure is always higher (worse) than the centre-pane figure. Building Regulations and the BFRC rating are based on the whole-window U-value, so that’s the one that counts for compliance. Watch for product literature quoting an impressive centre-pane figure (say 1.0) when the whole-window value that actually matters is more like 1.4 — always confirm which one you’re being quoted before you order.