Window U-Value Calculator — Whole Window Thermal Performance

Under Approved Document L Volume 1 (2021 edition, incorporating the 2023 amendments), the limiting (backstop) whole-window U-values are 1.6 W/m²K for windows in a new dwelling (Table 4.1) and 1.4 W/m²K for replacement windows in an existing dwelling (Table 4.2). Replacements can alternatively comply via a Window Energy Rating (WER) of Band B or above. Note these are absolute limits — for a new dwelling the practical target is the notional-dwelling specification of 1.2 W/m²K, which is what the SAP/TER calculation is built around. Since 15 June 2023 timber windows must meet the same 1.4 W/m²K replacement limit as other materials (the earlier 1.6 transitional allowance for timber has ended).

Centre-of-pane U-value measures the thermal performance of the glass alone at its centre, ignoring edge effects. Whole-window U-value accounts for the glass, the frame, and the thermal bridging at the glass edge (spacer bar). The whole-window value is always higher (worse) than centre-of-pane because frames and edges lose more heat.

Argon gas filling typically improves the centre-of-pane U-value by about 0.2-0.3 W/m²K compared to air-filled units. Argon is denser than air and conducts less heat. It is the most cost-effective gas fill and is standard in quality sealed units. Krypton offers further improvement but costs significantly more.

A warm edge spacer bar is made from materials with low thermal conductivity (such as reinforced plastic or stainless steel composites) rather than aluminium. It reduces thermal bridging at the edge of the glass, improving the whole-window U-value by approximately 0.1-0.2 W/m²K and reducing condensation risk at the glass edge.

Be careful to separate the frame-only U-value (Uf) from the whole-window U-value (Uw). Using the default tabulated Uf figures in BS EN ISO 10077-1 Annex D, a modern multi-chamber uPVC frame is around 2.0 W/m²K (2.2 for a two-chamber profile) and a softwood timber frame is also around 2.0 W/m²K at a typical 60-70mm thickness — the two are broadly comparable. Thermally broken aluminium is poorer at roughly 2.5-4.0 W/m²K (we use 3.2), and aluminium with no thermal break is worst at about 5.5-7.0 W/m²K (we use 5.8). These are conservative generic defaults; a real product will usually have a better measured Uf on its datasheet, so use the manufacturer figure for a compliance submission.