Most UK homes built before 2002 still have either single glazing or an older generation of double glazing that lacks a Low-E coating and argon fill. Both fall well short of what modern units can achieve. If you want to understand the numbers before you commit to a replacement project, the energy savings calculator lets you model the saving for your own house. This guide covers all three options, from single-pane glass through to triple glazing, so you can see exactly where each sits on U-value, cost and annual saving, and then decide whether the step up to triple is worth paying for.
The three options at a glance
The table below puts all four glazing tiers side by side. The U-value is the whole-window figure (Uw), which accounts for the glass, the spacer bar and the frame together. That is the number Part L is judged on, and the number that determines how much heat your window lets through. For a deeper explanation of what U-values mean and how they are measured, see the U-values and WER guide.
| Glazing type | Typical 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, drawn from UK installer and manufacturer technical pages.
The jump from single to modern double glazing is large: around a fourfold improvement in U-value. The jump from modern double to triple is real but much smaller in proportional terms. That asymmetry explains why triple glazing is not the obvious next step for most homeowners.
Single glazing: why it has to go
A single pane of glass offers almost no resistance to heat loss. With a whole-window U-value of around 4.8 to 5.8 W/m²K, a single-glazed window loses heat roughly four times as fast as a modern A-rated replacement. On a cold January night you can feel the cold radiating off the glass from half a metre away; condensation on the inside surface is almost unavoidable, and the window acts as a cold-radiating surface that lowers comfort in the room even when the heating is on.
Single glazing also fails Part L by a very wide margin. Any replacement window installed today must reach a whole-window U-value of 1.4 W/m²K or lower, or meet WER band B or better as an alternative compliance route. Single glazing cannot come close to either threshold. If you are having any replacement work done under a FENSA certificate, you have no option but to move to at least modern double glazing.
The financial case is equally clear. The Energy Saving Trust puts the annual saving from replacing single glazing with A-rated double glazing at up to about £140 a year on a typical semi-detached house. Over the 20 to 25 year life of a uPVC window that is a meaningful contribution to the payback, quite apart from the comfort improvement.
Single glazing is not really a choice anymore. If a homeowner still has it, every day they wait is money going straight through the glass. I have seen semi-detached houses where the heating bill dropped noticeably within the first winter after we replaced ten single-glazed sashes with A-rated double. It is not subtle.
Tom Bradley, FENSA-registered installer
Double glazing: the modern standard
Modern A-rated double glazing is the standard the building industry has converged on for good reason. A sealed unit of two panes, separated by a gas-filled gap, delivers a whole-window U-value of 1.0 to 1.4 W/m²K when specified correctly, which means Low-E glass, argon fill and a warm-edge spacer. That combination cuts heat loss through the glass by around 50% compared with single glazing, and it clears the Part L floor of 1.4 W/m²K comfortably.
It is worth distinguishing modern A-rated double glazing from old pre-2002 units. Pre-2002 double glazing was typically fitted without a Low-E coating, using air rather than argon in the cavity and an aluminium spacer bar, which is a thermal bridge. The result is a whole-window U-value of around 2.8 to 3.2 W/m²K: much better than single glazing, but still twice as leaky as a modern unit and well outside the Part L limit. If your double glazing is more than twenty years old, replacing it is worth considering on thermal grounds alone.
The glass specification guide covers how Low-E coatings, argon fill and warm-edge spacers each contribute to the final U-value, and what to check when comparing quotes.
The phrase "A-rated double glazing" is not just marketing. It means Low-E glass, argon fill and a warm-edge spacer, working together. Drop any one of those three and the unit will struggle to hit 1.4 W/m²K. When I am reviewing a quote I look for all three to be specified, not just the WER band on the front of the brochure.
Tom Bradley, FENSA-registered installer
Triple glazing: when the third pane pays
Triple glazing adds a third pane and a second gas-filled gap to the sealed unit. The result is a whole-window U-value of around 0.6 to 1.0 W/m²K, with roughly 0.8 W/m²K typical. That is a genuine improvement over the 1.0 to 1.4 W/m²K of A-rated double glazing, representing a further reduction in heat loss of around 15 to 20%.
The question is whether that improvement is worth the premium. Triple glazing adds roughly 20 to 30% to the installed cost of a window compared with double. The heat-loss reduction from the extra pane is real but incremental once you are already starting from a well-specified double-glazed unit. For a detailed buying-decision comparison that works through the numbers for a typical house, the double versus triple glazing comparison is the right place to go rather than duplicating those calculations here.
Triple glazing makes the strongest case in three situations. First, a passive-house or low-energy build where the target whole-window U-value is below 1.0 W/m²K, which good double glazing cannot reliably achieve. Second, an exposed or north-facing elevation where the heat loss through the glass has an outsized effect on the room's comfort. Third, where acoustic performance matters as much as thermal performance, because the extra pane and wider gap reduce sound transmission noticeably.
Triple glazing also adds weight. A triple-glazed unit is heavier than a double-glazed one of the same size, which has implications for the hinges and frames. This is rarely a problem with modern hardware, but it is worth confirming with your installer, particularly on large openings or older frames.
Cost and savings
The cost comparison between the three options comes down to two variables: the upfront price of the unit and the ongoing saving on heating bills. The table below sets out the key thermal and financial figures for each tier. For a personalised estimate of what you would save on your own property, use the energy savings calculator.
| Glazing tier | Whole-window U-value | Heat-loss reduction vs single | EST saving vs single (semi-detached) | Cost premium vs modern double |
|---|---|---|---|---|
| Single glazing | 4.8–5.8 W/m²K | Baseline (worst) | N/A | N/A |
| Old double glazing (pre-2002) | 2.8–3.2 W/m²K | Moderate improvement | Partial saving | N/A |
| Modern A-rated double glazing | 1.0–1.4 W/m²K | Around 50% vs single | Up to about £140/yr | Baseline |
| Triple glazing | 0.6–1.0 W/m²K | A further 15 to 20% vs double | Incremental further saving | Around +20 to 30% |
U-values from web/lib/research/u-values-wer.md. EST saving from Energy Saving Trust. Cost premium for triple vs double from DGCC research.
The numbers tell a consistent story. The biggest gain by far comes from leaving single glazing behind. Modern A-rated double glazing closes most of the gap that physics permits at a reasonable cost. Triple glazing closes the remaining gap, but at a premium that requires a specific reason to justify: a passive-house target, a very exposed site, or a strong preference for the best possible acoustic and thermal performance.
If you are comparing quotes and want to understand exactly how Low-E glass, argon fill and warm-edge spacers interact to produce the final U-value, the U-values explained guide covers each component separately, and the WER ratings guide explains how the A++ to G band translates the combined performance into a single label.
Frequently asked questions
Going from single glazing (around 4.8 to 5.8 W/m²K) to modern A-rated double glazing (1.0 to 1.4 W/m²K) cuts heat loss through the glass by around 50%. The Energy Saving Trust puts the annual saving at up to about £140 on a typical semi-detached house.
Triple glazing adds roughly 20 to 30% to the window cost versus double, while reducing heat loss by only a further 15 to 20%. For most UK homes that maths does not pay back quickly. It earns its place in passive-house designs, very exposed locations or wherever you need a whole-window U-value below 1.0 W/m²K.
Approved Document L sets the floor at a whole-window U-value of 1.4 W/m²K or lower, or WER band B or better as an alternative compliance route. Modern A-rated double glazing clears this comfortably. Single glazing fails by a very wide margin; old pre-2002 double glazing also falls short.
Old pre-2002 double glazing has a whole-window U-value of around 2.8 to 3.2 W/m²K because it lacks a Low-E coating, argon fill or warm-edge spacer. It is significantly better than single glazing but fails the Part L standard of 1.4 W/m²K by a wide margin. Replacing it with A-rated units is worthwhile.
The Energy Saving Trust estimates that replacing single glazing with A-rated double glazing saves up to about £140 a year on a typical semi-detached house. Actual savings depend on your heating fuel, thermostat settings and the number of windows replaced.
Triple glazing reaches a whole-window U-value of roughly 0.6 to 1.0 W/m²K, with around 0.8 W/m²K typical. That is meaningfully better than the 1.0 to 1.4 W/m²K of A-rated double glazing, but triple units are heavier and add roughly 20 to 30% to the installed cost.