What glass spec means for the price
When you price a window, the frame style and material set most of the cost, but the glass specification is the lever that decides how well it performs and how much the unit adds on top. The jump that matters most is from single glazing, which most replacements are removing, to a modern A-rated double-glazed unit. That single step delivers the bulk of the comfort and the bill savings. Beyond it, triple glazing and the higher Window Energy Rating bands give steadily smaller returns for steadily more money. Our guide to U-values explains the numbers behind the labels in full.
| Glazing | Whole-window U-value | Notes |
|---|---|---|
| Single glazing | ~4.8-5.8 W/m²K | Old, uninsulated. Loses the most heat. |
| Old double glazing (pre-2002) | ~2.8-3.2 W/m²K | No low-E coating or gas fill. |
| Modern A-rated double glazing | 1.0-1.4 W/m²K | Low-E glass, argon fill, warm-edge spacer. Meets Part L. |
| Triple glazing | 0.6-1.0 W/m²K | Best performance, around 0.8 typical. Heavier and pricier. |
Source: web/lib/research/u-values-wer.md. Lower U-value is better. A-rated double glazing is the standard spec for replacements and the price benchmark.
A-rated double glazing: the benchmark
A modern A-rated double-glazed unit reaches a whole-window U-value of about 1.0 to 1.4 W/m²K, which meets Part L comfortably. It gets there through three components working together: a low-E coating that reflects heat back into the room, an argon gas fill between the panes, and a warm-edge spacer that cuts heat loss around the edge of the unit. This is the spec our headline prices assume, so when our master double glazing costs page quotes a uPVC casement at around £650 fitted, it is an A-rated double-glazed casement.
Building regulations set the floor here. Under Approved Document L, in force since 15 June 2023, replacement windows must reach a U-value of 1.4 W/m²K or lower, or a Window Energy Rating of band B or better, so any reputable installer will be quoting at least an A-rated unit as standard. If a quote is vague about the glass spec, treat that as a warning sign and read how to avoid being overcharged.
Triple glazing: how much more
Triple glazing adds a third pane and a second gas gap, taking the whole-window U-value down to around 0.8 W/m²K. It adds roughly 20 to 30% to the price of the equivalent double-glazed window, so on a uPVC casement at around £650 fitted you are looking at roughly £130 to £195 more per window. The units are also heavier, which can matter on large openings. The gain in bill savings over a good A-rated double unit is real but modest, so triple glazing earns its place mainly where noise is a problem, the site is very exposed, or you are deliberately building to a low-energy standard. To weigh it up directly, read double versus triple glazing and run the energy savings calculator.
The verdict on glass spec
Spend on the step that pays back. Single to A-rated double is the upgrade that transforms a cold room and trims the heating bill. Triple glazing and the very top WER bands are refinements, worth it for specific reasons rather than by default.
Best Buy A-rated double glazing: the sensible default spec for almost every replacement.
It depends Triple glazing: it depends. Worth the extra 20 to 30% for noise, exposure or a low-energy home.
Most people do not need triple glazing, they need good A-rated double glazing fitted properly. I would rather see a customer spend the money on a careful install with a proper warm-edge unit than chase a U-value they will never notice. Where triple earns its money is noise: on a busy road it makes a real difference.
Tom Bradley, FENSA-registered installer
Frequently asked questions
Triple glazing adds roughly 20 to 30% to the price of an A-rated double-glazed window in 2026. On a uPVC casement at around £650 fitted, that is roughly £130 to £195 more per window for the triple-glazed version.
Under Part L (Approved Document L, in force since 15 June 2023) replacement windows must reach a U-value of 1.4 W/m²K or lower, or a Window Energy Rating of band B or better. A modern A-rated double-glazed unit with low-E glass, argon fill and a warm-edge spacer comfortably exceeds this.
Not always. Moving from single glazing to A-rated double is the big win and pays back in comfort and bills. Stepping up to triple glazing or a top WER band gives smaller gains, so it is worth it mainly for noise, very exposed sites or a deliberately low-energy home.
Three things: a low-E coating that reflects heat back inside, an inert gas fill (argon, sometimes krypton) between the panes, and a warm-edge spacer that cuts heat loss at the edge. Together these lower the U-value and lift the Window Energy Rating.