Glass is usually the weakest point of an extension

A well-insulated cavity wall might come in around 0.18 W/m²K. Even a good double-glazed unit struggles to beat 1.2. In other words, glazing loses heat roughly six or seven times faster than the wall next to it — and in a typical extension, glass can account for a third or more of the total wall area. Get the glazing wrong and you will feel it every winter evening, sitting near the doors with a cool draught rolling off the glass.

The good news is that modern glazing is genuinely impressive. The trick is matching the specification to the orientation, the room and your budget, rather than simply buying the most expensive units on the quote.

Double versus triple glazing: where the money goes

A decent double-glazed unit with a low-emissivity coating, argon fill and a warm edge spacer will typically achieve a whole-window U-value of around 1.2 to 1.4 W/m²K. A good triple-glazed equivalent will land nearer 0.8 to 1.0. That is a meaningful improvement, but it comes at a price: expect roughly 20 to 40 per cent more than double glazing, and sometimes more if you want slim frames.

• Choose triple glazing where comfort matters most: north-facing walls, rooms you use in the evening, and any elevation exposed to traffic or aircraft noise. The extra pane and wider cavities also cut sound noticeably.
• Choose high-quality double glazing on south-facing elevations where winter solar gain is doing much of the heating work, or where the budget is better spent on insulation, airtightness or a better door.
• Watch the payback maths. Triple glazing rarely pays for itself in fuel savings alone within a reasonable timeframe. Buy it for comfort, quiet and condensation-free glass — those benefits arrive immediately.

One practical note: the Building Regulations set a limiting U-value for windows in extensions, commonly around 1.4 W/m²K in England, with separate figures in Scotland, Wales and Northern Ireland. A reasonable double-glazed unit clears that bar; a poorly specified one will not.

Frame materials and the sightlines you will live with

Frames can account for a surprising share of the total heat loss, because they are a thermal bridge between inside and out.

• uPVC is the cheapest and thermally decent, particularly multi-chambered profiles with insulated cores. The trade-off is chunky sightlines and a look that suits some houses better than others.
• Aluminium gives you slim, strong frames and lovely big panes, but it conducts heat fiercely unless it is a properly thermally broken system. Cheap aluminium is a false economy; good thermally broken aluminium is excellent.
• Timber has naturally low conductivity and looks superb on period properties. It needs repainting or re-oiling every few years, which many people happily accept.
• Aluminium-clad timber is the best of both: a warm timber core inside, a maintenance-free aluminium skin outside. It costs more, but it is a sensible long-term choice for a home you plan to keep.

Solar gain: free heat in winter, an oven in July

Orientation drives everything here. South-facing glass brings welcome warmth from October to March, then becomes a liability in summer when the sun is high and the extension overheats. East and west glazing is worse for summer discomfort, because low morning and evening sun floods straight in under any overhang.

Ask about the g-value (the fraction of solar energy that passes through the glass). A typical unit sits around 0.6 to 0.7. If overheating is likely, specify solar control glass with a g-value nearer 0.3 to 0.4. Otherwise, plan physical shading — a deep roof overhang, external blinds or a brise-soleil — because internal blinds trap heat inside the room once it has already passed through the glass.

Roof lanterns and large rooflights deserve special care. They are wonderful for light, but they lose more heat than vertical glazing and are hard to shade. Specifying them to the same standard as the windows, and thinking about summer shading at design stage, saves a lot of regret.

The details that quietly decide performance

Two units with identical U-values can perform very differently in practice. Look for:

• Warm edge spacers rather than old-fashioned aluminium, which creates a cold stripe around the pane and encourages condensation.
• Argon or krypton fill, and cavity widths around 16 mm for double glazing — wider is not automatically better, as the gas starts to circulate.
• A continuous low-emissivity coating positioned correctly on the appropriate face of the glass.
• Careful installation: fully insulated reveals, cavity closers, airtight sealing around the frame and no gaps behind trims. A brilliant window fitted badly will underperform a mediocre one fitted well.

Making the choice with confidence

Start with orientation and how each room will actually be used, then set a sensible U-value target and work back to the frame material and glazing build-up. Put the money where it changes your daily experience: triple glazing on the cold, noisy elevation, excellent double glazing and good shading where the sun does the work.

Finally, remember ventilation. Airtight, well-glazed extensions need background ventilation and, in kitchens and bathrooms, mechanical extraction to keep humidity under control. Insist on a written specification from your supplier and fitter, with U-values, g-values and frame details stated in plain numbers. It is the simplest way to make sure the extension you build is the comfortable, bright, affordable-to-heat room you were promised.