Window U-Values Explained: What's a Good Score in 2026?

Ask three installers for a quote and you’ll get three different U-value figures, usually with no explanation of what they mean. Here’s the short version: a U-value measures how much heat escapes through your window, in watts per square metre per kelvin (W/m²K). Lower is better. For a UK home in 2026, a good uPVC double-glazed window scores around 1.2 to 1.3 W/m²K, comfortably ahead of the legal minimum of 1.4.

That number decides how much heat you lose through every window in the house, every winter, for the next 20-plus years. Worth five minutes to understand before you sign anything.

Key points

• A U-value of 1.4 W/m²K is the Building Regs minimum for replacement windows, not a benchmark to aim for
• 1.2 to 1.3 W/m²K is standard for decent modern uPVC double glazing
• Triple glazing usually lands between 0.8 and 1.0 W/m²K
• Always ask for the Uw (whole window) figure, not Ug (glass only)
• Your installer’s FENSA certificate should state the U-value they’ve actually fitted, not just quoted

Reading time: ~7 min

What a U-value actually measures

A U-value is a rate of heat loss. It tells you how many watts pass through one square metre of window for every degree of temperature difference between inside and outside. A U-value of 1.2 loses less heat than a U-value of 2.0. That’s the whole concept — everything else is detail.

Two numbers get quoted, and installers aren’t always careful about which one they mean:

• Ug — the centre-pane glass value only, ignoring the frame and spacer bar
• Uw — the whole-window value, including frame, glass and spacer, tested as one unit

Ug always looks better than Uw because glass insulates better than most frame materials. A salesperson quoting you “0.9” without specifying which figure is worth a follow-up question. If it’s Ug, the real whole-window number could be a third higher.

What counts as a good U-value for UK windows right now

Anyone quoting a double-glazed window above 1.4 is either using an old catalogue or fitting a budget spec that scrapes past building control on a technicality. Neither is a great sign. On the other end, triple glazing below 0.8 sounds impressive but rarely pays for itself on a standard semi within a sensible timeframe — more on that below.

Building Regulations, Part L, and what’s actually enforced

Under Approved Document L, replacement windows in England must achieve a U-value of 1.4 W/m²K or better (or meet an equivalent Window Energy Rating of Band B). Scotland and Wales run their own versions of the same principle, with broadly similar numbers. Installers self-certify compliance through FENSA or Certass rather than going through council building control directly — that’s what the FENSA certificate you get after the job actually confirms.

There’s a tightening under discussion for future Building Regs updates, following the direction set by the Future Homes Standard for new-build. It hasn’t changed the 1.4 figure for existing-home replacements as of September 2026, but it’s a reasonable bet that the minimum drops over the next few years. Fitting comfortably below 1.4 now is cheap insurance against your windows looking dated sooner than the frames actually wear out.

Does the U-value change my EPC rating?

A bit, but don’t expect miracles. Your Energy Performance Certificate is generated from a SAP calculation that treats windows as one input among many — insulation, heating system, hot water, ventilation all get weighed too. Swapping 2.8 U-value 1990s double glazing for 1.2 uPVC will nudge your score upward. It won’t usually carry a mid-terrace from a D to a B on its own.

Where it does matter more directly: if you’re close to a band boundary, or selling to a buyer using a mortgage product that requires a minimum EPC rating, new windows can be the difference that tips you over. Worth checking your current EPC’s recommendations report — it often states the specific U-value improvement the assessor assumed.

What actually changes a window’s U-value

Six things move the number, and they’re not all about the glass:

1. Gas fill in the cavity — argon-filled units outperform plain air by a meaningful margin; krypton does better still but costs more
2. Low-emissivity (low-E) coating — a near-invisible metallic layer on the glass that reflects heat back inside
3. Spacer bar material — warm-edge spacers (foam or composite) beat old aluminium spacers, which conduct heat straight through the edge of the unit
4. Number of panes — double vs triple, obviously, but diminishing returns kick in fast
5. Frame material and design — uPVC with internal chambers, aluminium with a thermal break, or timber, each perform differently
6. Cavity width — wider gaps between panes generally insulate better, up to a point

An installer who can’t explain which of these six they’re using to hit their quoted figure hasn’t thought hard about the spec, or is hoping you won’t ask.

How to compare U-values across quotes: the 3-number check

Every quote you get should hand you three numbers, unprompted. If they don’t, ask.

1. Uw value — the whole-window figure, not Ug
2. WER/BFRC letter rating — the A++ to E scale that translates U-value, solar gain and air leakage into one grade
3. Spacer bar type — warm-edge or aluminium, since it’s the detail most often left off a glossy brochure

Two quotes both claiming “1.2 double glazing” can use completely different glass packages, spacer bars and gas fills to get there — and age very differently over 15 years. Asking for these three numbers, in writing, before you compare price is the fastest way to find out whether you’re comparing like with like. This is also exactly the kind of detail worth checking against the FENSA or Certass certificate once the job’s done, since the certificate records what was actually installed, not just what was quoted.

At a glance

A U-value below 1.4 is mandatory, below 1.3 is decent, and below 1.0 is genuinely high-performance. The number on its own tells you less than you’d think — ask whether it’s Uw or Ug, and get the spacer bar and gas fill in writing before comparing prices between installers.

Finding an installer who’ll actually explain the numbers

Not every fitter enjoys being asked to justify a spec sheet. If you’re starting from scratch, browse our UK window and glazing installer directory to find FENSA and Certass registered companies near you by town and county, sourced from the Companies House register. When you’re ready to compare quotes, our free quote request form puts your job in front of registered local installers with no obligation to accept anything back.

Getting the U-value right is one part of a bigger spec conversation — frame colour, security standards and glazing type all interact with it. If you’re weighing up double glazing against the secondary glazing route for a period property, the U-value trade-offs are part of that decision too.