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.
