Double glazed sealed units, made to measure

Declared centre-pane U-values (Ug) for double glazed units run from about 1.0 to 2.6 W/m²K. Our reference catalogue holds 126 validated double-glazed configurations across that range. What decides where a unit lands is not the number of panes but the specific low-emissivity coating, which face it sits on, the cavity width and the gas fill. Two units that look identical can differ by half a watt.

The glass on its own cannot meet Part L, because the regulations are satisfied on the finished window (Uw), not on the glass (Ug). In England a replacement window has to reach 1.4 W/m²K or a Window Energy Rating of Band B, and a window in a new dwelling 1.6 W/m²K. The frame and the edge of the unit typically add 0.2–0.5 W/m²K on top of the glass figure, so the frame you fit the unit into decides compliance as much as the glass does. Ask your frame supplier for the whole-window figure.

Usually yes, which surprises people. Each gas has an optimum gap, and past it convection inside the cavity starts working against you. For argon that optimum is about 14.7mm, so a 16mm cavity is close to its best and a wider one gains nothing. Filled with plain air, a 20mm cavity is measurably worse than 16mm. Wider is not automatically better — 16mm is the standard UK cavity for good reason.

Argon conducts roughly two thirds as much heat as air, and at the cavity widths used in UK windows that is worth a meaningful step on the U-value — typically a couple of tenths of a watt against the same build filled with air. It costs very little, which is why almost every modern unit is argon filled. Krypton performs better still but is expensive, and is really only worth it in narrow cavities where argon has no room to work.

The spacer holds the two panes apart. Traditional aluminium bar conducts heat readily, so the edge of the unit runs cold and that is where condensation appears first. A warm edge spacer uses stainless steel, polymer or composite instead and conducts far less. Across the whole window it is generally worth around 0.1 to 0.2 W/m²K — often the margin that closes the gap to a regulatory target — and it noticeably reduces condensation at the sightline.

Measure the glass rather than the frame. Take the width and the height in millimetres, in two or three places each, and use the smallest reading. If the unit is still glazed in, measure the visible pane and add the amount the beading hides on each edge. Check the overall thickness with a gauge on the unit edge, and note any Georgian bars, lead pattern or obscure glass you need matched.

The overall thickness is simply the two panes plus the cavity. A standard 4-16-4 unit — 4mm glass, 16mm cavity, 4mm glass — is 24mm overall, which is what most UK frames are rebated for. Wider builds such as 4-20-4 or 6-16-6 or 6-16-4 come out thicker. Check your rebate before ordering one, because a unit that will not sit in the frame is no use however well it performs.

The g-value is the share of the sun’s total heat that passes through the glass into the room — the lower the g-value, the less solar heat gets in. Solar control coatings cut the g-value while keeping most of the visible light, which is what stops a conservatory or a south-facing room overheating without darkening it. The two numbers in a solar control product name are its light transmittance and its g-value in percent, so the range can be read directly from the name. Modern solar control glass is also low-emissivity, so the same unit still insulates in winter.

Noise reduction comes from asymmetry and lamination, not from a wider cavity. Two panes of different thicknesses resonate at different frequencies, so sound that passes one is damped by the other, and an acoustic laminated pane adds a flexible interlayer that absorbs vibration outright. Unit performance is stated as a measured weighted sound reduction index, Rw, tested to BS EN ISO 140-3 — a reduction of about 10 dB is heard as roughly half as loud. Our configurator shows the measured rating of the exact make-up as you build it.

Yes. You measure, you enter the sizes, and the price is calculated as you type — there is no survey appointment and no waiting for a quote to come back. The unit is cut to your millimetre sizes in our own workshop and either delivered on our vans or held for collection. If you would rather send a cutting list or a drawing than type sizes in, upload it and we will price it for you. Reference data version 2026.07-v1.