What Window U-Value Do I Need?
Quick Answer
1.6 W/m²K new-build limiting (1.2 notional SAP target) • 1.4 W/m²K replacements
Based on: Approved Document L 2021 (England). Triple glazing can achieve 0.8 W/m²K.
Part L 2021 Requirements (England)
- New build windows (limiting): 1.6 W/m²K maximum per AD L Vol 1 2021 Table 4; notional design target 1.2 W/m²K
- Replacement windows: 1.4 W/m²K maximum per AD L 2021 Table 4.2
- Roof windows: 1.8 W/m²K (new build limiting), 1.6 W/m²K (replacement)
- Doors (glazed): 1.6 W/m²K (new build limiting), 1.4 W/m²K (replacement)
Typical U-Values by Glazing Type
- Single glazing: 5.0–5.8 W/m²K
- Old double glazing (pre-2002): 2.8–3.0 W/m²K
- Modern double glazing (argon-filled, low-e): 1.2–1.4 W/m²K
- Triple glazing (argon, double low-e): 0.7–0.9 W/m²K
What Affects U-Value?
- Glass type: Low-emissivity (low-e) coatings reduce heat loss
- Gas fill: Argon or krypton between panes improves insulation
- Spacer bar: Warm-edge spacers reduce thermal bridging
- Frame material: Timber and uPVC outperform aluminium (unless thermally broken)
- Number of panes: Triple beats double beats single
Is Triple Glazing Worth It?
For most UK homes, high-performance double glazing (1.2 W/m²K) offers the best balance of cost and performance. Triple glazing (0.8 W/m²K) makes more sense for exposed/north-facing elevations, Passivhaus builds, or noise-sensitive locations. The payback period is typically 15–25 years compared to quality double glazing.
Last updated: April 2026. U-value requirements refer to Approved Document L (England). Scotland, Wales, and Northern Ireland have their own building regulations with different targets.