What Window U-Value Do I Need?

Part L 2021 Requirements (England)

• New build windows: 1.2 W/m²K maximum whole-window U-value
• Replacement windows: 1.4 W/m²K maximum
• Roof windows: 1.4 W/m²K (new build), 1.6 W/m²K (replacement)
• Doors (glazed): 1.4 W/m²K (new build), 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.