Choosing Energy-Efficient Glazing for New Build Projects

Why glazing deserves an early place in the design

Windows and doors are the thinnest part of any thermal envelope, and in a new build you get to decide how thin. A well-built UK wall now sits somewhere around 0.18 W/m²K, while a respectable double-glazed window is typically 1.2 to 1.4 W/m²K and a good triple-glazed unit drops to 0.8 to 1.0 W/m²K. That is still several times the heat loss per square metre, so the glass is where the money quietly leaves the building — or doesn't.

The trick is to think about glazing at the same time as the wall build-up, not after the drawings are signed off. Frame depth, reveal depth, lintel sizing, curtain tracking and even the position of radiators all depend on whether you are fitting a 28 mm double-glazed unit or a 44 mm triple-glazed one. Deciding late means compromise; deciding early means you can specify properly and still keep the daylight you are designing for.

Double or triple? Matching the glass to the wall

Triple glazing is not automatically the right answer, but it is usually the right answer for a new build. The reason is simple: the cost premium — typically 20 to 30 per cent on the glazing package — is far easier to absorb during construction than during a retrofit. You are already buying frames, scaffolding and labour.

• Double glazing suits modest openings, garden rooms, outbuildings and budget-led projects where the window-to-wall ratio is low.
• Triple glazing earns its keep on large expanses of glass, north-facing elevations, and anywhere you want to sit close to the window without feeling a draught.
• Weight matters. Triple-glazed units are roughly 50 per cent heavier, so frames, hinges and structural openings need to be specified accordingly.

Whichever you choose, look at the whole-window U-value rather than the centre-pane figure. A brilliant glass unit in a poorly insulated frame, with a cold spacer bar at the edge, will underperform. Insist on warm edge spacer bars and ask for the Psi-value at the frame-to-glass junction; anything around 0.04 W/mK or better is a sign of a properly engineered product.

Frame materials that earn their keep

The frame is roughly a quarter of the window area, and it is where cheap specifications give themselves away.

• uPVC remains the value option. Modern profiles with multi-chambered cores perform well, though sightlines are chunkier and the material's thermal expansion needs careful detailing at long spans.
• Timber offers excellent thermal performance and the lowest embodied carbon of the mainstream options. It needs a durable species, a proper factory finish and a maintenance schedule you will actually follow.
• Aluminium gives the slimmest sightlines and impressive strength, which matters for large sliding doors. It must have a thermal break — an insulated barrier between inner and outer sections — or it will act as a radiator fin.
• Timber-aluminium composites pair a timber structural frame with an aluminium outer skin. You get warmth inside, weather resistance outside, and a price tag to match.

Do not overlook the installation itself. Compressible airtightness tapes at the jambs and head, insulated cavity closers and a continuous insulation return at the reveal will do more for real-world performance than an extra few millimetres of glass. Expanding foam fills a gap; it does not seal one.

Solar control: keeping the heat out without dimming the room

In the UK, the biggest glazing mistake is not heat loss — it is overheating. A well-insulated, airtight new build with a large south or west elevation will cook itself from May to September if the glass is specified purely for insulation.

This is where the g-value comes in. It measures the fraction of solar energy that passes through the glass. A standard double-glazed unit might sit around 0.7; a solar control coating can bring that down to 0.35 or 0.4. Ask for the visible light transmittance (often written as LT or VLT) alongside it. A good solar control coating should keep light transmittance above roughly 0.6 in living spaces so the room still feels bright rather than tinted grey.

Soft coat low-emissivity coatings handle the winter side of the equation by reflecting long-wave heat back into the room. Some high-performance units now carry two low-e coatings — one for retention, one for solar control — plus an argon or krypton fill. The result is a window that keeps warmth in during January and turns heat away in July.

Daylight, overheating and the balance in between

Glass should be your last lever, not your first. Orientation, overhangs, brise soleil, external blinds and the depth of the reveal all control solar gain before it reaches the pane, and they do it without costing you a single lumen.

• North-facing glazing wants maximum light transmittance and no solar control — there is little gain to reject.
• South-facing glazing benefits from a moderate g-value and a generous roof overhang, which blocks high summer sun while admitting low winter sun.
• East and west glazing is the hardest to manage, as low-angle sun defeats most shading. Here a lower g-value and external shading are worth the investment.

Overheating is now a formal consideration under Part O of the Building Regulations in England, so this is not just a comfort question — it is a compliance one. Roof lights and light wells can recover daylight lost to solar control on the sunniest elevations, spreading illumination more evenly and reducing the temptation to over-glaze a single wall.

A short specification checklist

Before you sign off the glazing package, confirm the whole-window U-value, the g-value and the light transmittance for each elevation separately — not one blanket figure for the whole house. Check the frame material and its thermal break, the spacer bar type, the Psi-value at junctions, and the airtightness detail at every reveal. Then ask how the units will be handled on site, because a triple-glazed unit dropped on a corner is a very expensive mistake.

Get those details right and glazing stops being the weak point of the envelope. It becomes the part of the house you notice for all the right reasons: quiet, comfortable, bright, and cheap to run.