Quick answer: A window’s U-value measures how much heat it loses (lower is better; UK Building Regs require 1.4 W/m²K for replacement windows). The WER (Window Energy Rating) is a A++-to-E sticker that combines heat loss with solar gain — useful for marketing, less useful for cold UK winters. For most UK homes, U-value matters more than WER, and a U-value of 1.0 W/m²K or lower is the realistic premium target without going to triple glazing.
This guide explains both ratings, what numbers actually matter, and the four hidden specs (warm-edge spacers, gas fill, low-E coating, frame thermal break) that affect performance more than most installers admit.
U-value, in plain English
U-value measures how quickly heat passes through one square metre of the window. The unit is W/m²K (watts per square metre per degree Kelvin difference). Lower U-value = better insulation = less heat lost = lower bills.
| U-value (W/m²K) | What you’ve got | Performance |
|---|---|---|
| 5.0+ | Single glazing | Poor — basically no insulation |
| 2.7–3.0 | Pre-2002 double glazing | Outdated |
| 1.6–2.0 | 2002–2010 double glazing | Adequate but below current minimum |
| 1.4 | UK Building Regs minimum (2022) for replacement windows | Required — the floor, not the ceiling |
| 1.2 | Building Regs minimum for new-build | Good |
| 1.0 | Premium A-rated double glazing | Excellent |
| 0.8 | Triple glazing (mid-range) | Best practice |
| 0.6 or lower | Passive house standard | Specialist install |
The number quoted by an installer should be the whole-window U-value (Uw) — frame and glass combined — not the centre-of-glass U-value (Ug) which always looks better. Always ask: “Is that 1.4 the Uw or the Ug?”
WER (Window Energy Rating) explained
WER is the A++ to E rating you see in showrooms. It’s calculated by the BFRC (British Fenestration Rating Council) using a formula:
- Heat lost through the window (U-value)
- Minus heat gained from sunlight passing through (solar gain)
- Adjusted for typical UK conditions
Result: a single letter grade A++ to E.
| WER band | What it means | Approx U-value range |
|---|---|---|
| A++ | Top of range — usually triple glazing | 0.7–0.9 |
| A+ | Premium double glazing | 1.0–1.2 |
| A | Standard for new-build | 1.2–1.4 |
| B | Just below current Building Regs | 1.4–1.6 |
| C | Old standard double glazing | 1.6–2.0 |
| D / E | Single or very old units | 2.0+ |
Why we recommend U-value over WER
WER includes solar gain. In sunny climates, solar gain through south-facing windows can offset heat loss substantially — so a window with a worse U-value can have a better WER if it lets in more sun. The UK is mostly cold and overcast. The solar-gain factor matters less here than the marketing suggests. For a UK home, the absolute U-value is the more reliable indicator of how warm your house will actually be in February.
The 4 hidden specs that affect performance
1. Warm-edge spacers
The “spacer” is the thin strip running around the perimeter of the sealed unit holding the two panes apart. Old aluminium spacers conduct cold from outside to inside (the “thermal bridge”), creating cold edges where condensation forms. Warm-edge spacers use plastic or composite materials that don’t conduct nearly as much. Difference: 20–30% reduction in edge condensation, plus a small but real U-value improvement.
Ask: “Are these warm-edge spacers?” If they say “yes” follow up with “Which brand?” — common names are Edgetech Super Spacer, Swisspacer, Thermix.
2. Gas fill — argon vs krypton vs air
The cavity between the panes is filled with gas to slow heat transfer:
- Air: cheapest, worst insulation. Avoid.
- Argon: standard for modern double glazing. ~30% better than air. Should be the default.
- Krypton: best insulation but expensive. Only really worth it for triple glazing where the panes are very close together.
- Xenon: theoretical premium; almost never used in residential.
If your installer doesn’t mention argon as standard in 2026, walk away. It’s not optional any more.
3. Low-E (low-emissivity) coating
A microscopic metallic coating on one face of the glass reflects heat back into the room while still letting visible light through. Without it, U-value would be ~30% worse. Standard on every modern A-rated unit. Worth confirming because some budget units skip it.
4. Frame thermal break (aluminium only)
Aluminium conducts cold extremely well. Modern aluminium frames have a “thermal break” — a strip of polyamide between the inner and outer faces — that stops the cold transferring. Cheap aluminium frames skip this and you can literally feel cold radiating off them in winter.
Ask: “Is this a polyamide-broken frame? What’s the thermal break depth?” Anything less than 20mm is mediocre.
Frame material vs U-value
- uPVC — naturally low thermal conductivity. Achieves Uw 1.0–1.4 with standard A-rated glass. Best £/U-value for most homes.
- Aluminium with proper thermal break — Uw 1.2–1.6. Worse than uPVC for insulation but stronger structurally for big openings.
- Timber (hardwood) — Uw 1.0–1.4. Good thermal performance, premium look. Needs maintenance.
- Composite — Uw 0.9–1.2. Best performer but rare for windows; common for doors.
Triple glazing vs double — does it matter?
Going from A-rated double (Uw 1.0) to mid-range triple (Uw 0.8) reduces heat loss by ~20%. Cost premium: 20–30% over double. Real savings on a 3-bed semi: roughly £40–£90/year. Payback time: 12–25 years.
Triple is worth the spend if:
- You’re staying long-term (15+ years)
- You have large north-facing windows where the cold is most felt
- You also want significant noise reduction (triple is naturally better acoustically)
- You’re going for Passivhaus-adjacent build standards anyway
Don’t bother with triple if:
- Your loft and wall insulation are still poor — fix those first; cheaper, bigger payback
- You’re moving within 5 years
- Your windows are mostly south-facing — solar gain matters more than U-value here
What to ask installers about U-values
- “Is the U-value you’re quoting the Uw (whole-window) or just Ug (centre-of-glass)?”
- “Are you using warm-edge spacers? Which brand?”
- “Is the cavity argon-filled?”
- “Is the glass low-E coated?”
- (Aluminium only) “What’s the thermal-break depth?”
- “Can I see the BFRC certificate or the manufacturer’s spec sheet?”
An installer who knows the answers off the top of their head is competent. One who has to “check with the office and get back to you” is selling on price, not specification. Walk away from anyone who says U-values “don’t matter, what matters is the brand.” They matter enormously.
Real-world energy savings (3-bed semi)
| Upgrading from | To | Annual saving (2026 prices) | Payback |
|---|---|---|---|
| Single glazing | A-rated double (Uw 1.4) | £250–£500 | 10–18 years |
| 1990s double glazing | A-rated double (Uw 1.4) | £100–£250 | 20–50 years |
| 2010 double glazing | A-rated double (Uw 1.0) | £40–£100 | 50+ years |
| A-rated double | Triple glazing (Uw 0.8) | £40–£90 | 20–40 years |
Replace because of comfort, draughts, condensation or noise — not only for energy savings. The maths usually doesn’t work on energy alone for already-modern windows.
Frequently asked questions
What’s the lowest U-value possible for double glazing?
About Uw 0.8 for the very best premium double-glazed windows with krypton fill, dual low-E coatings, and high-end warm-edge spacers. Below that you need triple glazing.
Does U-value matter more than WER for a north-facing window?
Yes — definitely. North-facing windows get almost no useful solar gain in winter, so the WER’s solar-gain credit is irrelevant. U-value is the only number that matters here. Specify the lowest U-value you can afford for north-facing rooms.
If my window has WER A but U-value 1.4, is it actually energy efficient?
It meets the legal minimum (Building Regs require 1.4 for replacement) and is “decent” by current standards. It’s not premium. A genuinely high-performance window in 2026 would be Uw 1.0 or below. WER A is a fine starting point but don’t pay extra for that letter alone — pay extra for the actual U-value drop.
Are A++ windows worth the extra money?
For most UK homes, no. A++ usually means triple glazing with all the premium specs — annual saving over A-rated double is typically £40–£90 on a 3-bed semi. The premium pays back in 20–40 years. Worth it for staying long-term in cold homes; not worth it for short-stay or already-warm homes.
Will my installer give me the U-value certificate?
Yes — they’re legally required to under Building Regs Part L compliance, and your FENSA certificate (issued after install) records the Uw of the units fitted. If they refuse to specify the U-value before the sale, that’s a major red flag. See our before-you-sign guide for the full checklist.
Get quotes for energy-efficient windows
If you want quotes specifying U-value 1.0 or better, our service connects you with up to 4 vetted FENSA-registered local installers in 24 hours. We’ll only match you with installers who quote U-values clearly upfront. Free, no obligation.
For a budget figure first: cost calculator — 30 seconds, no contact details required.
Sources: BFRC (British Fenestration Rating Council), Pilkington WER and U-value guidance, FENSA Building Regs Part L documentation, BWF U-value updates 2022, Energy Saving Trust glazing efficiency analysis (2026), Edgetech / Swisspacer warm-edge spacer technical sheets. Last reviewed: April 2026.
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