UK · 2026 Standards
What U-Value Should Windows Have in the UK 2026?
The direct answer
Short answer
Approved Document L, 2021 edition incorporating 2023 amendments, sets the limiting whole-window U-value at 1.4 W/m²K, or Window Energy Rating Band B, for a replacement window in an existing dwelling (Table 4.2). For a window in a new dwelling the limiting standard is 1.6 W/m²K (Table 4.1), while the notional dwelling used in the SAP calculation assumes 1.2 W/m²K. The best systems we supply are published from 0.71 W/m²K. Lower is better.
U-value is measured in watts per square metre per kelvin: how much heat passes through one square metre of window for each degree of temperature difference. The whole-window figure, Uw, is what matters for compliance, calculated to BS EN ISO 10077-1 and taking in the frame, the glass and the spacer bar. Watch for quotes that give you the centre-pane Ug instead, which always looks better than the window it is fitted in.
U-values by glazing type
The glazing build-up is the biggest single driver of the U-value, and the frame does the rest. Rather than publish a table of typical values, which vary widely by manufacturer and by make-up, here are the figures that are actually fixed, because they come from the regulations and the certification criteria.
| Benchmark | Figure | Source |
|---|---|---|
| Replacement window, existing dwelling | 1.4 W/m²K, or WER Band B | Approved Document L, Table 4.2 |
| Window in a new dwelling, limiting standard | 1.6 W/m²K | Approved Document L, Table 4.1 |
| Notional dwelling window, SAP calculation | 1.2 W/m²K | Approved Document L, notional dwelling specification |
| Rooflight, both new and existing | 2.2 W/m²K | Approved Document L, Tables 4.1 and 4.2 |
| Passivhaus component, cool temperate | Uw 0.80, installed 0.85 W/m²K | Passive House Institute certification criteria |
| Passivhaus reference glazing, cool temperate | Ug 0.70 W/m²K | Passive House Institute certification criteria |
| Where building character prevents 1.4 | Centre pane not above 1.2 W/m²K | Approved Document L, paragraph 4.10 |
U-values by system
Published whole-window Uw figures for the systems Apex Glazing supplies, taken from each manufacturer's own data. These are "from" values achieved with the manufacturer's reference glazing, not a guarantee for every size and configuration. Where a manufacturer publishes only a frame-only Uf, this table says so rather than converting it.
| System | Material | Published U-value, from |
|---|---|---|
| Cortizo COR 80 Industrial | Aluminium | Uw 0.71 W/m²K |
| Cortizo COR 70 Industrial TB | Aluminium | Uw 0.76 W/m²K |
| Schüco AWS 90.SI+ | Aluminium | Uw 0.79 W/m²K, PHI certified |
| Schüco AWS 75.SI+ optimized | Aluminium | Uw 0.80 W/m²K, PHI certified |
| APX70 Tilt & Turn | Aluminium | Uw 0.84 W/m²K |
| Apex uPVC Tilt & Turn | uPVC | Uw 0.85 W/m²K |
| APX Casement / APX Flush Casement | Aluminium | Uw 0.9 W/m²K |
| Cortizo COR 60 TB | Aluminium | Uw 1.0 W/m²K |
| Apex uPVC casement, flush and heritage | uPVC | Uw 1.2 W/m²K |
| Schüco AWS 70.HI | Aluminium | Uf 1.5 W/m²K, frame only |
Sources: Apex product data sheets, Cortizo published system data, Schüco AWS 70 system documentation, and the Passive House Institute component database. Schüco publishes a frame-only Uf for AWS 70.HI rather than a headline Uw, because the whole-window figure depends on the glass, the spacer and the size of the unit.
What U-value to specify
The limiting Part L figure is a floor, not a target. Here is what we would actually specify, and why.
| Project type | Target Uw | Reason |
|---|---|---|
| Replacement windows, existing dwelling | 1.4 W/m²K or better | The limiting standard in Approved Document L Table 4.2. Most modern systems beat it comfortably. |
| New dwelling, limiting standard | 1.6 W/m²K or better | Approved Document L Table 4.1. This is the floor, not the design target. |
| New dwelling, notional building | 1.2 W/m²K | The value the SAP notional dwelling uses, so it is what the compliance calculation is measured against. |
| Passivhaus certified component | 0.80 W/m²K or better | Passive House Institute criterion for the cool, temperate climate. 0.85 W/m²K in the installed state. |
| Heritage or conservation replacement | As low as the sightline allows | Slim frames limit the rebate depth, so the honest target is set at survey rather than in advance. |
Reading a FENSA certificate
Every Apex Glazing installation comes with a FENSA self-certification (Reg 40373). A few things worth checking on any quote, ours or anyone else's:
| The figure quoted should be Uw, the whole window, not Ug, the centre of the glass. Ug always looks better. |
| For a replacement window in an existing dwelling, Approved Document L Table 4.2 requires 1.4 W/m²K, or Window Energy Rating Band B as an alternative route. |
| For a new dwelling, Table 4.1 sets the limiting standard at 1.6 W/m²K. The 1.2 W/m²K figure you may have seen is the notional dwelling value used in the SAP calculation, not a limit each window has to meet. |
| Rooflights are limited separately at 2.2 W/m²K under both tables. |
| Argon fill, a soft-coat low-E and a warm-edge spacer should all be written on the specification, not assumed. |
| Approved Document F requires background ventilators when windows are replaced in an existing dwelling: a minimum equivalent area of 8000mm² in habitable rooms and kitchens, and 4000mm² in bathrooms. |
| If the building's character means the window genuinely cannot meet 1.4, Approved Document L paragraph 4.10 allows a centre-pane U-value not exceeding 1.2 W/m²K, or single glazing supplemented with low-emissivity secondary glazing. |
Frequently asked
What U-value should windows have in the UK?
For a replacement window in an existing dwelling, Approved Document L Table 4.2 sets the limiting standard at 1.4 W/m²K, or Window Energy Rating Band B. For a window in a new dwelling, Table 4.1 sets it at 1.6 W/m²K. Rooflights are limited separately at 2.2 W/m²K. The best systems we supply are published considerably lower than any of these, from 0.71 W/m²K on Cortizo COR 80 Industrial.
What is a good U-value for windows?
Anything at or below the 1.4 W/m²K limit is compliant, and most modern systems are well under it: our uPVC casement is published from 1.2 W/m²K, APX Casement aluminium from 0.9 W/m²K, and our uPVC tilt & turn from 0.85 W/m²K with an A++ energy rating. If you are targeting Passivhaus, the Passive House Institute criterion for the cool, temperate climate is a component Uw of 0.80 W/m²K or better, and 0.85 W/m²K in the installed state.
Is a lower U-value better for windows?
Yes. The U-value measures the rate at which heat passes through one square metre of window for each degree of temperature difference, so a lower number means heat escapes more slowly. Just make sure you are comparing like with like: Uw is the whole window, Ug is the centre of the glass alone, and Uf is the frame alone. Ug is always the flattering number, and it is the one that tends to appear in advertising.
What U-value is required by UK Building Regulations?
Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments. Table 4.2 sets the limiting U-value for a window in an existing dwelling at "1.4 or Window Energy Rating Band B minimum". Table 4.1 sets 1.6 W/m²K for a window in a new dwelling. Rooflights are 2.2 W/m²K under both. The figures are whole-window U-values calculated to BS EN ISO 10077, not centre-pane values.
What's the difference between Uw, Ug and Uf?
Ug is the centre-pane U-value of the glass alone. Uf is the U-value of the frame alone. Uw is the whole-window figure, combining both plus the effect of the spacer bar, calculated to BS EN ISO 10077-1. Building Regulations use Uw. This matters in practice: Schüco publishes a Uf from 1.5 W/m²K for the AWS 70.HI rather than a headline Uw, so a "1.5" on a Schüco quote is not directly comparable to a "1.2" on a uPVC one.
Do triple glazed windows have better U-values?
Generally yes, and the Passive House Institute's reference glazing for the cool, temperate climate is Ug 0.70 W/m²K, which is a triple-glazed figure. The trade-offs are cost, roughly 50% more glass weight, and a rebate depth the frame has to be rated for. Note that several double-glazed systems we fit are already published below 1.0 W/m²K, so triple is not the only route under the limit.
What U-value do the Schüco systems achieve?
Schüco AWS 90.SI+ is certified by the Passive House Institute for the cool, temperate climate at Uw 0.79 W/m²K, and AWS 75.SI+ optimized and AWS 90.SI+ optimized are both certified at Uw 0.80 W/m²K. For AWS 70.HI, Schüco publishes a frame-only Uf from 1.5 W/m²K rather than a whole-window figure; the Apex data sheet for the system quotes a U-value from 1.2 W/m²K for a typical glazed unit. AWS 70.HI is not a certified Passivhaus component.
Is a 1.0 U-value good for a window?
Yes, it is well inside the 1.4 W/m²K Building Regulations limit for a replacement window and inside the 1.2 W/m²K notional dwelling figure used in the new-build calculation. Cortizo publishes COR 60 TB from 1.0 W/m²K, and several systems we fit go lower still: APX Casement from 0.9, APX70 Tilt & Turn from 0.84, COR 70 Industrial TB from 0.76 and COR 80 Industrial from 0.71.
