UK · 2026 Standards
What U-Value Should Windows Have in the UK 2026?
The direct answer
Short answer
UK Building Regulations Part L (2022) require new windows to achieve a whole-window U-value of 1.4 W/m²K or better. Best-in-class systems achieve 0.76-0.84 W/m²K. Lower number = better insulation.
U-value (Uw) is measured in watts per square metre per kelvin and represents how much heat passes through one square metre of window per degree of temperature difference. The whole-window figure is what matters for compliance, calculated to BS EN ISO 10077-1, including frame, glass, and spacer bar.
U-values by glazing type
The biggest driver of U-value is the glazing build-up. Frame matters too, but going from single to double, or double to triple, makes the largest single jump.
| Glazing type | Centre-pane Ug | Whole-window Uw range | Notes |
|---|---|---|---|
| Single glazing (4mm float) | 5.0-5.7 W/m²K | 5.0+ W/m²K | Non-compliant for any new install |
| Standard double glazing (4-16-4) | 2.7 W/m²K | 1.6-1.8 W/m²K | Air-filled, no soft coat |
| Modern double glazing (Argon + Low-E) | 1.0-1.2 W/m²K | 1.2-1.4 W/m²K | Industry standard 2026 |
| Triple glazing (4-12-4-12-4 Argon) | 0.6-0.7 W/m²K | 0.8-0.9 W/m²K | Passivhaus territory |
| Vacuum glazing (LandVac, Pilkington Spacia) | 0.4-0.7 W/m²K | 0.8-1.1 W/m²K | Slim profile for heritage retrofit |
U-values by system
Whole-window Uw figures for the systems Apex Glazing supplies, all measured to BS EN ISO 10077-1 with the manufacturer's reference glazing. Triple-glazed configurations carry the lowest figures.
| System | Material | Double-glazed Uw | Triple-glazed Uw |
|---|---|---|---|
| APX (standard aluminium casement) | Aluminium | 1.2-1.4 W/m²K | 0.9 W/m²K |
| Schüco AWS 70.HI | Aluminium | 1.0 W/m²K | 0.84 W/m²K |
| Schüco AWS 75.SI+ | Aluminium | 0.94 W/m²K | 0.76 W/m²K |
| Cortizo Cor 70 Industrial | Aluminium | 1.0-1.3 W/m²K | 0.85 W/m²K |
| uPVC casement (70mm 5-chamber) | uPVC | 1.2-1.4 W/m²K | 0.8 W/m²K |
| uPVC heritage flush sash | uPVC | 1.3 W/m²K | 0.85 W/m²K |
| Timber casement (engineered) | Timber | 1.3-1.5 W/m²K | 0.9 W/m²K |
What U-value to specify
The minimum Part L figure is a floor, not a target. Here's the practical specification to aim for in 2026, depending on the project.
| Project type | Target Uw | Reason |
|---|---|---|
| Typical London home, replacement only | 1.2 W/m²K | Comfortably exceeds Part L; achievable in double glazing |
| New build (notional building target) | 1.0-1.2 W/m²K | Future-proof against tightening Part L revisions |
| EnerPHit retrofit (Passivhaus existing) | 0.8 W/m²K | EnerPHit standard requires triple glazing |
| Passivhaus new build | 0.80 W/m²K or lower | Whole-building energy demand drives this |
| Listed building / vacuum slim retrofit | 1.1 W/m²K | Best achievable without changing sightlines |
| Conservation area heritage replacement | 1.3 W/m²K | Slim sightlines limit glass build-up |
Reading a FENSA certificate
Every Apex Glazing installation comes with a FENSA self-certification (Reg 40373) showing the Uw figure. A few things to check:
| Uw is whole-window, not centre-pane Ug |
| Figure should be 1.4 W/m²K or lower for Part L compliance in existing dwellings |
| Window Energy Rating (WER) A or A+ corresponds to Uw at or below 1.2 W/m²K |
| Argon fill and Low-E coating should be specified, both add to U-value performance |
| Warm-edge spacer bars (Swisspacer, Edgetech Super Spacer) add 0.1-0.2 W/m²K improvement |
| Trickle vents are a Document F requirement on all replacement windows since 2022 |
Frequently asked
What U-value should windows have in the UK?
UK Building Regulations Part L (2022) require new and replacement windows in existing dwellings to achieve a whole-window U-value of 1.4 W/m²K or better. New build dwellings must achieve 1.2 W/m²K. Best-in-class systems available in the UK in 2026 reach 0.76-0.84 W/m²K. Lower numbers indicate better insulation.
What is a good U-value for windows?
For a typical London home, a whole-window U-value of 1.2 W/m²K is the practical sweet spot, it comfortably exceeds Part L, qualifies for most lender energy criteria, and is achievable with double glazing on aluminium or uPVC. For Passivhaus or EnerPHit retrofits, target 0.8 W/m²K with triple glazing. For a future-proofed new build, specify 1.0 W/m²K.
Is lower U-value better for windows?
Yes. U-value measures the rate at which heat passes through one square metre of the window for each degree of temperature difference (W/m²K). The lower the number, the slower heat escapes, so a 0.8 W/m²K window loses heat at half the rate of a 1.6 W/m²K window. Single glazing sits around 5.0 W/m²K, double glazing 1.2-1.4, triple glazing 0.8-0.9, and vacuum glazing 0.4-0.7.
What U-value is required by UK Building Regulations?
Approved Document L (Conservation of Fuel and Power), 2022 edition, Volume 1 for dwellings, sets the limiting U-value for windows in existing buildings at 1.4 W/m²K (whole-window). For new dwellings, the notional building uses 1.2 W/m²K. Rooflights are limited at 2.2 W/m²K. The figures are whole-window U-values calculated to BS EN ISO 10077, not centre-pane U-values.
What's the difference between Uw, Ug, and Uf?
Ug is the centre-pane U-value of just the glass. Uf is the U-value of the frame alone. Uw is the whole-window U-value, which combines both plus the spacer bar effect, calculated to BS EN ISO 10077-1. UK Building Regulations use Uw. Always compare windows on the Uw figure: a window can have excellent Ug glass but a poor Uf frame and end up below Part L.
Do triple glazed windows have better U-values?
Yes. Triple glazing typically reaches 0.6-0.9 W/m²K centre-pane Ug, versus 1.0-1.3 for double glazing. Whole-window Uw on triple-glazed aluminium or uPVC sits at 0.8-1.0 W/m²K. The trade-off is roughly 30-40% more cost, 50% more glass weight, and slightly lower visible light transmission. For Passivhaus or EnerPHit standards, triple glazing is essentially required.
What U-value does Schüco AWS 75.SI+ achieve?
Schüco AWS 75.SI+ achieves a whole-window U-value of 0.76 W/m²K with triple glazing, among the best mass-produced aluminium window systems in the UK in 2026. AWS 70.HI hits 0.84 W/m²K with triple glazing, and 1.0 W/m²K double-glazed. Both systems use polyamide thermal breaks and foam-filled chambers.
Is a 1.0 U-value good for a window?
Yes, 1.0 W/m²K is excellent. It is 30% better than the UK Building Regulations minimum, comfortably qualifies for Window Energy Rating A++, and is a sensible target for new builds and major retrofits. Achievable with high-spec double glazing on Cortizo Cor 70 Industrial, Schüco AWS 70.HI, or warm-edge triple glazing on uPVC casements.
