The 2026 Building Regulations require a centre-pane U-value no higher than 1.6 W/m²K and a whole-window U-value no higher than 1.8 W/m²K for replacement windows
If you are replacing windows in your home, you need to know the minimum energy performance standard set by law. The 2026 Building Regulations for England, under Approved Document L (2021 edition, as amended 2025), require a whole-window U-value (Uw) no higher than 1.8 W/m²K and a centre-pane U-value (Ug) no higher than 1.6 W/m²K for replacement windows (GOV.UK Approved Document L, 2025 amendments).
These limits also apply in Wales under Part L1B (Welsh Government, 2022 edition, as updated 2025). Scotland sets a slightly tighter whole-window U-value limit of 1.6 W/m²K for new-build, but 1.8 W/m²K for replacement windows in existing dwellings under Section 6 (2025 edition) (Scottish Building Standards Section 6, 2025 edition). Northern Ireland matches the 1.8 W/m²K whole-window limit for replacements under Technical Booklet F1 (2022 edition) (Northern Ireland Technical Booklet F1, 2022 edition).
The U-value calculation combines three separate measurements the glass centre-pane, the glass edge, and the frame
The whole-window U-value (Uw) is not a single measurement but a calculation combining three components. The formula is Uw = (Ag × Ug + Af × Uf + Lg × Ψg) / (Ag + Af), where Ag is glass area, Af is frame area, Ug is glass centre-pane U-value, Uf is frame U-value, Lg is length of glass edge, and Ψg is the linear thermal transmittance of the glass edge (British Fenestration Rating Council (BFRC) Technical Manual).
Centre-pane Ug values typically range from 0.7 W/m²K for triple glazing with low-E coatings and argon fill to 1.4 W/m²K for standard double glazing with air fill. Frame Uf values vary significantly by material. uPVC frames average 1.4–1.8 W/m²K, timber frames 1.2–1.6 W/m²K, aluminium with thermal breaks 1.8–2.2 W/m²K, and composite frames 1.0–1.5 W/m²K (Glass and Glazing Federation (GGF) U-value Calculator Guide). The glass-edge Ψg value typically ranges from 0.04 to 0.08 W/mK depending on the spacer material. Warm-edge spacers produce lower values.
Quick numbers typical U-values for common glazing and frame combinations in 2026
The table below shows typical whole-window U-values for common glazing and frame combinations. These figures are based on manufacturer data and assume standard dimensions for a typical residential window.
| Glazing type | Frame material | Centre-pane Ug (W/m²K) | Frame Uf (W/m²K) | Whole-window Uw (W/m²K) | Meets 2026 regs? |
|---|---|---|---|---|---|
| Double glazing (4-16-4, low-E, argon) | uPVC | 1.2 | 1.6 | 1.5 | Yes |
| Triple glazing (4-12-4-12-4, low-E, argon) | Timber | 0.7 | 1.4 | 1.0 | Yes |
| Double glazing (4-12-4, air fill) | Aluminium (no thermal break) | 1.4 | 2.2 | 2.0 | No |
| Double glazing (4-16-4, low-E, argon) | Composite | 1.2 | 1.2 | 1.3 | Yes |
Source: Energy Saving Trust Window Energy Ratings; BFRC U-value database; GGF technical sheet TSO-001.
related article on window energy ratings explained
The whole-window U-value is the figure Building Control officers check for compliance
Building Control will request the certified whole-window U-value from the manufacturer’s BFRC or equivalent test certificate at installation stage. The whole-window U-value must be ≤ 1.8 W/m²K for replacement windows in England, Wales, and Northern Ireland, and ≤ 1.6 W/m²K for new-build in Scotland (GOV.UK Approved Document L, 2025 amendments; Scottish Building Standards Section 6, 2025 edition).
Centre-pane Ug values alone are insufficient for compliance because the frame and edge effects can add 0.2–0.5 W/m²K to the overall Uw. A window with a centre-pane Ug of 1.2 W/m²K might have a whole-window Uw of 1.6 W/m²K or higher depending on the frame and spacer. You must check the certified whole-window figure, not just the glass specification.
The correct calculation method is the EN ISO 10077-1 standard, which requires manufacturer-provided component data
EN ISO 10077-1 specifies the calculation procedure for whole-window U-values using Ug, Uf, and Ψg values that are either measured (EN 673 for glass, EN 12412-2 for frames) or declared by the manufacturer (British Standards Institution (BSI) EN ISO 10077-1:2018). The BFRC provides a free online U-value calculator that follows EN ISO 10077-1 and accepts manufacturer data (BFRC U-value calculator user guide).
DIY calculations using assumed values are not acceptable for Building Regulations compliance. The calculation must use certified component data from the window manufacturer. If you are buying replacement windows, ask your supplier for the BFRC certificate showing the whole-window U-value. Do not rely on verbal assurances or generic figures from websites.
MCS-certified installers must provide a certificate showing the whole-window U-value for every window installed
Under the Microgeneration Certification Scheme (MCS) for energy-efficient glazing, installers must supply a product certificate (BFRC or equivalent) for each window type installed (MCS Standard 017, Issue 6.0, 2025). The certificate must include the whole-window U-value (Uw), the centre-pane Ug, and the frame Uf, all measured to EN ISO standards.
To verify an installer’s credentials, check the MCS Installer Database at mcsinstaller.co.uk or TrustMark at trustmark.org.uk for registered window fitters (TrustMark Scheme Rules, 2025 edition). If an installer cannot provide a certificate for the specific window model they are fitting, they are not complying with MCS requirements and the installation may not meet Building Regulations.
related article on finding a certified window installer
The payback period for upgrading from a 2.0 W/m²K window to a 1.4 W/m²K window is typically 8–15 years depending on fuel prices and window area
A typical semi-detached house with 15 m² of window area upgrading from 2.0 W/m²K to 1.4 W/m²K saves approximately £80–£120 per year in heating costs at 2026 gas prices (6.5p/kWh, Ofgem price cap October 2025–March 2026) (Ofgem price cap data, October 2025–March 2026). The installation cost for a standard uPVC double-glazed window meeting 1.4 W/m²K is approximately £400–£700 per window including fitting in 2026, based on Energy Saving Trust cost data (EST cost and savings tables for glazing, 2025 update).
The payback calculation depends on which cost you compare. The full window replacement cost (£6,000–£10,500 for 15 m²) divided by annual saving (£80–£120) gives a payback of 50–130 years. However, if you are already replacing windows and choose a higher-performance glazing option (costing £200–£400 extra per window), the incremental payback is 8–15 years (DESNZ household energy efficiency statistics, 2025). The actual payback will vary with your home’s heating system, fuel type, and how the windows are used.
Frequently Asked Questions
The 2026 Building Regulations require a whole-window U-value (Uw) no higher than 1.8 W/m²K and a centre-pane U-value (Ug) no higher than 1.6 W/m²K. These limits are set out in GOV.UK Approved Document L (2021 edition, as amended 2025).
The whole-window U-value (Uw) is calculated using the formula Uw = (Ag × Ug + Af × Uf + Lg × u03a8g) / (Ag + Af), where Ag is glass area, Af is frame area, Ug is glass centre-pane U-value, Uf is frame U-value, Lg is length of glass edge, and u03a8g is the linear thermal transmittance of the glass edge. This method comes from the British Fenestration Rating Council (BFRC) Technical Manual.
Triple glazing with low-E coatings and argon gas fill typically achieves a centre-pane U-value (Ug) of around 0.7 W/m²K, well below the 1.6 W/m²K maximum. Standard double glazing with air fill gives a Ug of approximately 1.4 W/m²K, according to the Glass and Glazing Federation (GGF).
The centre-pane U-value (Ug) measures only the thermal performance of the glass at its centre, while the whole-window U-value (Uw) includes the glass, frame, and edge effects. The frame Uf value varies by material, for example uPVC averages 1.4–1.8 W/m²K and timber 1.2–1.6 W/m²K, as per the Glass and Glazing Federation (GGF).
Yes. Scotland sets a whole-window U-value limit of 1.8 W/m²K for replacement windows but 1.6 W/m²K for new-build under Section 6 (2025 edition). Northern Ireland matches the 1.8 W/m²K limit for replacements under Technical Booklet F1 (2022 edition), as stated by the Scottish Building Standards and Northern Ireland Technical Booklet F1.
