London · Q2 2026
Double vs Triple Glazing
The third pane changes five things
Triple glazing isn't just "double, but more". The third pane changes the U-value, the weight, the rebate depth the frame has to accept, and the acoustic behaviour of the unit, though not always in the direction people expect. For most London replacement projects, modern double glazing already sits inside the Building Regulations limit of 1.4 W/m²K. Triple earns its place in specific cases: Passivhaus and deep retrofit, north-facing rooms, and very large glazed areas.
Spec comparison
What actually changes when you add the third pane. Where a figure depends on the specific glass make-up rather than on the number of panes, this table says so instead of inventing a number.
| Spec | Modern Double | Triple |
|---|---|---|
| Typical unit construction | 4 mm, 16 mm, 4 mm | 4 mm, 12 mm, 4 mm, 12 mm, 4 mm |
| Typical overall unit thickness | 24-28 mm | 36-44 mm |
| Cavity gas fill | Argon | Argon or krypton |
| Low-E coatings | 1 | 2 |
| Glass weight per m² (4mm panes) | About 20 kg | About 30 kg |
| Rebate depth the frame must take | Most systems | Only systems rated for it, check the published glazing range |
| Acoustic Rw | Depends on the make-up, not the pane count. Asymmetric and laminated units outperform symmetric ones. | |
| Solar gain (g-value) | Falls with each added pane and coating. Specify against the room's orientation. | |
| Part L limiting standard for a replacement window | 1.4 W/m²K, or Window Energy Rating Band B (Approved Document L, Table 4.2) | |
| Passive House reference glazing, cool temperate | Ug 0.70 W/m²K, with a component U-value of 0.80 W/m²K or better | |
What the third pane costs
Triple glazing costs more than double, in the glass, in the frame that has to carry it and in the hardware that has to move it. How much more depends on the system, the size, the make-up and the frame it goes into, so we price it as an option on a specific quote rather than publishing a percentage. If someone gives you a single uplift figure that applies to every window in the house, ask them how they worked it out.
When triple is worth it
If any of these apply to your project, the third pane earns its keep. Several apply to most Passivhaus and deep-retrofit projects in London.
| Scenario | Why triple wins |
|---|---|
| Passivhaus certification | The Passive House Institute's reference glazing for the cool, temperate climate is Ug 0.70 W/m²K, which needs three panes. |
| North-facing rooms with little solar gain | You lose little by reducing solar transmission, and gain the lower U-value. |
| Properties on major roads or under flight paths | Acoustic performance depends on the specific make-up, an asymmetric laminated unit is usually the answer rather than symmetric triple. |
| Large fixed and picture windows | The largest single glazed areas dominate the heat-loss calculation for the elevation. |
| Rooms with a persistent condensation problem | A warmer inner pane means internal humidity is less likely to condense on it. |
| Homes heated to a high temperature year-round | The indoor to outdoor temperature difference is larger, so the U-value matters more. |
When double is enough
For a much larger share of London projects, modern double glazing with argon fill, soft-coat low-E and warm-edge spacers does the job. Don't let upselling pressure you into spec you don't need.
| Scenario | Why double is enough |
|---|---|
| Standard semi-detached upgrade from older double glazing | Modern double glazing already sits inside the 1.4 W/m²K Part L limit, and older units rarely did. |
| South-facing or south-east rooms | Solar gain offsets some heat loss, and a third pane reduces the solar gain as well as the loss. |
| Quiet residential street, no major traffic | There is little noise to attenuate. Spend the money on glass area or on the frame instead. |
| Decent existing thermal envelope (cavity-insulated walls, loft insulation) | Windows are no longer the weakest link; spending elsewhere wins. |
| Budget-led renovation | Modern double + insulation top-up + draught-proofing beats triple-only. |
| Conservation area heritage frames | Slim heritage frames often cannot take the rebate depth a triple unit needs. |
| Buy-to-let or short-hold investment property | Building Regulations compliance is the requirement, and modern double meets it. |
The frame matters as much as the glass
A triple-glazed unit in a poor frame will underperform a double-glazed unit in a good one, because the published U-value belongs to the whole window, not to the glass. Which is why the useful comparison is between complete systems with published figures, not between glass make-ups in the abstract.
| System | Published whole-window U-value | Where that sits |
|---|---|---|
| Apex uPVC casement | From 1.2 W/m²K | Inside the Part L limit of 1.4 |
| Apex uPVC tilt & turn | From 0.85 W/m²K | Energy rating up to A++ |
| Cortizo COR 60 TB | From 1.0 W/m²K | Comfortably inside Part L |
| APX Casement aluminium | From 0.9 W/m²K | Well inside Part L |
| APX70 Tilt & Turn aluminium | From 0.84 W/m²K | At the Passivhaus component figure |
| Cortizo COR 70 Industrial TB | From 0.76 W/m²K | Below the Passivhaus component figure |
| Schüco AWS 90.SI+ | 0.79 W/m²K | Passive House Institute certified, cool temperate |
Sources: Apex product data sheets, Cortizo published system data, and the Passive House Institute component database (AWS 90.SI+, ID 0734wi03). All are "from" figures achieved with the manufacturer's reference glazing, not a guarantee for every size and configuration.
Frequently asked
Is triple glazing worth it in London?
For most standard London terraces and semis, no. Modern double glazing already sits inside the Approved Document L limiting standard of 1.4 W/m²K for a replacement window, and several of the systems we fit are published well below it: Apex uPVC casement from 1.2 W/m²K, APX Casement aluminium from 0.9 W/m²K, Cortizo COR 70 Industrial TB from 0.76 W/m²K. Triple glazing is worth it for Passivhaus and deep-retrofit projects, north-facing rooms with little solar gain, and very large glazed areas.
What is the U-value of double vs triple glazing?
The number that matters for compliance is the whole-window U-value (Uw), not the glass on its own. The Passive House Institute's reference glazing for the cool, temperate climate is Ug 0.70 W/m²K, which is a triple-glazed figure, and its criterion for a certified window component is Uw 0.80 W/m²K or better. For comparison, Approved Document L Table 4.2 limits a replacement window in an existing dwelling to 1.4 W/m²K, or Window Energy Rating Band B.
How much extra does triple glazing cost?
More than double, but how much more depends on the system, the size of the unit, the make-up and whether the frame can already take the rebate depth. We price it as an option on a specific quote rather than publishing a single uplift percentage, because a percentage that applied to every window in a house would not be a real number.
What's the acoustic difference, double vs triple?
Less than most people assume, and it depends on the make-up rather than the pane count. Two identical panes separated by a cavity resonate together, so a symmetric triple unit is not automatically quieter than a well-chosen double. For noise specifically, an asymmetric unit with a laminated pane, where the two glass thicknesses differ, usually outperforms symmetric triple. Tell us what you are trying to block out and we will spec the make-up against it rather than sell you a pane count.
Does triple glazing reduce condensation?
Yes. Condensation forms when the inner pane is cold enough for the room's humidity to reach dew point on it. A triple-glazed unit keeps the inner pane warmer, because it sits further from the cold outer pane behind an extra cavity, so condensation is less likely. The Passive House Institute makes the same point in reverse with its hygiene criterion, which sets a minimum surface temperature factor for a certified window rather than a U-value alone.
Is triple glazing heavier, does it need stronger frames?
Yes, roughly 50% heavier in the glass, since three 4mm panes weigh about 30kg per m² against about 20kg for two. The practical constraint is usually not weight but rebate depth: the frame has to be rated to take the unit. Check the published glazing range for the system. Apex uPVC casement takes up to 36mm, APX Casement up to 44mm, APX70 Tilt & Turn up to 40mm, and Schüco AWS 70.HI up to 62mm.
What is the payback time for triple glazing?
We don't publish one, because an honest payback figure depends on your existing windows, your heating fuel and price, how the house is used, and how much of the heat loss is actually through the glass rather than the walls and roof. Any installer who quotes you a specific number of years for your house without modelling it is guessing. The comfort, condensation and acoustic benefits, where they apply, arrive immediately and are usually the better reason to spend the money.
Can triple glazing fit existing window frames?
Usually no. A triple unit is typically 36 to 44mm thick against 24 to 28mm for a double, and most existing frames do not have the rebate depth for it. Every system publishes a glazing range, and if the unit you want is outside it, the answer is a new window rather than a glass swap.
Triple glazing or secondary glazing for noise?
For acoustic performance on a heritage window, secondary glazing is often the better route, because the wide air gap between the original window and the secondary pane does more for sound insulation than a narrow sealed-unit cavity does. It also preserves listed and conservation-area frontages where a new window would not get consent. Approved Document L specifically recognises this: where a window cannot meet the 1.4 W/m²K standard because of the character of the building, it allows single glazing to be supplemented with low-emissivity secondary glazing instead.
