What U-value do windows need for UK Building Regulations?

Under Part L of UK Building Regulations (2021 edition), replacement windows must achieve a whole-window U-value of 1.4 W/m²K or better. For new build dwellings, the notional dwelling specification is 1.2 W/m²K. Alternatively, windows can comply via window energy ratings (WER) of band B or above for replacements.

What is the difference between centre-of-pane and whole-window U-value?

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.

Does argon gas make a big difference in double glazing?

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.

What is a warm edge spacer bar?

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.

Which frame material gives the best U-value?

uPVC frames typically offer the best thermal performance for domestic windows with a frame U-value around 1.3 W/m²K. Timber frames are slightly worse at around 1.6 W/m²K. Thermally broken aluminium frames achieve about 3.2 W/m²K, while non-thermally broken aluminium is poorest at around 5.8 W/m²K. Composite frames combining materials can achieve excellent results.

TradeCalculator.co.uk

Window U-Value Calculator — Whole Window Thermal Performance

Calculate centre-of-pane and whole-window U-values from glass type, gas fill, spacer bar and frame material. Check compliance with Part L of UK Building Regulations.

Glass TypeDouble glazing is standard; triple for Passivhaus

Gas FillArgon is the cost-effective standard

Spacer BarWarm edge reduces thermal bridging

Frame MaterialuPVC is most common in UK domestic

Window Width

Overall frame width

Window Height

Overall frame height

How We Calculate This

This calculator uses the simplified method from BS EN ISO 10077-1 to determine whole-window U-values from the component parts.

The formula

Uw = (Ag × Ug + Af × Uf + lg × Ψ) ÷ (Ag + Af)

Uw = whole-window U-value (W/m²K)
Ag = glass area (m²), Ug = centre-of-pane U-value
Af = frame area (m²), Uf = frame U-value
lg = glass perimeter (m), Ψ = linear thermal transmittance of spacer (W/mK)

Part L compliance

Replacement windows: maximum 1.4 W/m²K (or WER band B+)
New build: notional specification 1.2 W/m²K

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.