The Window Energy Rating (WER) scale runs from G at the bottom to A++ at the top, and the jump from A to A+ sounds small on a brochure but usually means crossing from double glazing into triple glazing, with a real cost consequence. A-rated double glazing sits at a U-value of around 1.2 to 1.4 W/m²K; A+ and A++ push that down to 0.8 to 1.2 W/m²K, adding roughly £100 to £200 per window. Whether that extra spend is sensible depends on your home and your expectations. Use the energy savings calculator to see what the bill difference actually looks like for your house, and check our glass specification cost guide for fitted price ranges by glazing spec.
The headline comparison
Before the detail, here is the side-by-side picture. Figures cover the glazing specification itself; fitting costs depend on frame material, window style and installer.
| Dimension | A-Rated (double) | A+ / A++ (triple) |
|---|---|---|
| WER energy index | 0 or above | Above +10 (A+) / above +20 (A++) |
| Typical U-value | 1.2–1.4 W/m²K | 0.8–1.2 W/m²K |
| Glazing type | Double glazing | Triple glazing |
| Cost uplift per window | Baseline | +£100 to +£200 (roughly +20 to 30%) |
| Part L compliant? | Yes (comfortably) | Yes (exceeds floor) |
| Part L requires | 1.4 W/m²K or WER band B+ | 1.4 W/m²K or WER band B+ |
WER scale set by the British Fenestration Rating Council (BFRC). U-values are whole-window typical figures. Cost uplift is for the glass unit; full fitted window prices vary.
What the bands mean
The Window Energy Rating is calculated by the British Fenestration Rating Council (BFRC). Unlike a bare U-value, which measures heat flowing out, the WER energy index also accounts for solar gain (the free heat that comes through the glass on sunny days) and air leakage. The index runs from roughly -40 at the bottom to +20 and above at the top. A-rated windows sit at 0 or above on that index, meaning the combination of solar gain and low heat loss at least breaks even over the year. A+ requires an index above +10 and A++ above +20, both of which demand meaningfully lower U-values than standard double glazing can deliver.
In practice the bands translate neatly into glazing tiers. Most quality double glazing reaches band A without difficulty once it is fitted with Low-E glass, argon fill and a warm-edge spacer. Clearing A+ or A++ almost always means adding a third pane, which is why A+ is effectively shorthand for triple glazing in the UK market. The full physics of why the U-value matters so much are covered in our guide to U-values and WER ratings.
Best Buy A-rated for most homes: it combines genuine thermal performance with the lower weight and cost of double glazing.
Cost of the upgrade
Moving from A-rated double glazing to A+ or A++ triple glazing adds roughly £100 to £200 per window to the glass-unit cost, an increase of around 20 to 30%. Across a typical ten-window house that is an extra £1,000 to £2,000 on the glazing bill alone. Triple glazing is also heavier, which can mean slightly heavier hardware and, on older timber frames, structural checks before installation. The glass specification cost page breaks down fitted prices for each tier so you can model the numbers for your own project. Bear in mind that Part L asks only for 1.4 W/m²K or WER band B or better, meaning band A already gives you margin to spare; A+ is a voluntary upgrade beyond the regulatory floor.
It depends The cost of the upgrade is modest per window but adds up across a full house. Run the sums before you commit.
The real-world bill difference
The U-value gap between A-rated and A+ glazing is roughly 0.2 to 0.4 W/m²K in whole-window terms. That improvement is real and measurable in the energy savings calculator, but the honest answer for most homes is that bill savings from this incremental step are modest. Heating bills depend primarily on the biggest sources of heat loss: the roof, walls and floor. Windows matter, but even replacing every single-glazed window with A-rated double glazing typically saves £100 to £150 a year. Going from A to A+ is a fraction of that saving, applied to a house that already has good double glazing. Payback on the A+ premium runs to many years in ordinary UK conditions.
The picture changes in cold or exposed locations, where wind chill and long heating seasons make every tenth of a W/m²K count more. It also changes in very large glazed areas, where the window represents a bigger proportion of the heat-loss envelope.
It depends The bill saving is real but modest for average homes. Exposed plots and large glazed areas shift the calculation.
Comfort and condensation
There is a comfort argument for triple glazing that goes beyond the heating bill. A lower U-value means the inner pane is warmer in cold weather. With standard A-rated double glazing the inner pane can reach temperatures that cause convective draughts near the glass, particularly noticeable in large openings or rooms where people sit close to the windows. Triple-glazed A+ or A++ windows keep the inner surface noticeably warmer, eliminating that cold-glass chill and reducing the risk of condensation forming on the room side of the glass. For open-plan living rooms, music rooms, home studios or conservatories, that comfort gain is tangible even when the bill saving alone would not justify the cost.
Acoustic performance is a secondary benefit. Triple glazing adds mass and an extra air gap, which together reduce the transmission of mid-frequency noise. This is relevant near busy roads or flight paths. Our double versus triple glazing comparison covers the acoustic side in more detail.
Worth it Worth considering for comfort in large rooms, cold plots or noise-sensitive properties, even when the bill maths is marginal.
When A+ is worth it
A-rated double glazing is the sensible default for the overwhelming majority of UK replacements. It clears Part L comfortably, it performs well in practice, and the weight and cost of triple glazing are not justified unless one or more of the following applies.
You are in a cold or exposed location. Northern England, Scotland, coastal sites and elevated ground see more heating-degree days each year, meaning the improved U-value of A+ glazing delivers more cumulative benefit than in a sheltered Midlands suburb.
You have large glazed areas. Floor-to-ceiling glazing, large picture windows or a glazed extension change the maths. The window is a much bigger share of the building's heat-loss surface, so the per-square-metre improvement from A+ compounds into a meaningfully larger annual saving.
Comfort is the primary goal. If you sit next to large windows in a room that feels cold in winter, or if condensation on the inner pane is already a problem, the warmer inner surface of triple glazing is a quality-of-life upgrade that most people notice immediately.
You are building to a higher standard. New-build projects targeting EPC A, passive-house certification or the Future Homes Standard will in many cases need U-values well below 1.4 W/m²K, and A+ or A++ glazing is part of the package needed to get there.
Near a busy road or under a flight path. The extra mass and pane of triple glazing reduces mid-frequency noise transmission, so A+ can be justified on acoustic grounds even when the thermal argument alone is borderline.
Worth it A+ is worth it in cold plots, large glazed areas, noise-affected homes and high-specification new builds.
I fit a lot of A-rated double glazing and very rarely regret it. Where I do recommend triple glazing is exposed semis in the north, rooms with floor-to-ceiling glass, and anyone next to a main road who complains about traffic noise at night. Do not pay A+ money on a south-facing back bedroom that barely loses heat; save it for the rooms where you actually feel the cold.
Tom Bradley, FENSA-registered installer
The verdict
A-rated double glazing is the Best Buy for most homes. It clears Part L comfortably at a U-value of around 1.2 to 1.4 W/m²K, delivers genuine energy savings over older glass, and costs£100 to £200 less per window than stepping up to A+ or A++ triple glazing. For a typical house having all its windows replaced, A-rated is the right default: the premium for A+ takes many years to pay back through lower bills in average UK conditions.
A+ and A++ triple glazing are Worth it in specific circumstances: cold or exposed homes in the north or on elevated ground; large glazed areas where the window is a major part of the heat-loss envelope; properties near busy roads where acoustic comfort matters as much as warmth; and new-build or renovation projects targeting a high EPC band or passive-house performance. In those situations the extra £100 to £200 per window is genuinely earned, both in comfort and in longer-term running costs.
The key number to remember: Part L asks for 1.4 W/m²K or WER band B or better. A-rated is already above the regulatory floor, so anything beyond it is an investment in performance rather than a compliance requirement. Make that investment deliberately, in the windows where it counts.
Frequently asked questions
The Window Energy Rating (WER) scale, set by the BFRC, combines U-value, solar gain and air leakage into one band. A-rated windows score an energy index of 0 or above; A+ windows score above +10; A++ score above +20. In practice A+ and A++ generally require triple glazing rather than double glazing.
Stepping from A-rated double glazing to A+ or A++ triple glazing adds roughly £100 to £200 per window, about 20 to 30% on the unit cost. On a ten-window house that is £1,000 to £2,000 more. The energy bill savings are real but modest, so payback takes years rather than months.
No. Part L (since 15 June 2023) requires replacement windows to reach a U-value of 1.4 W/m²K or lower, or WER band B or better, as two alternative routes. Band A is not required by Building Regulations. A-rated windows clear the threshold comfortably, but you are not legally obliged to go that high.
A++ glazing typically achieves a U-value as low as 0.8 to 1.0 W/m²K, which is triple-glazing territory. Standard A-rated double glazing sits around 1.2 to 1.4 W/m²K. Both comfortably clear the Part L compliance floor of 1.4 W/m²K, but A++ goes considerably further, which is where the cost premium comes from.