Vacuum glazing vs slim double glazing
Both are sold as 'slimline' for heritage windows. They are not the same product, and the difference decides what fits your sashes and how it performs.
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Two ways to make a thin double-glazed unit
Slim gas-filled units
A conventional double-glazed unit is two panes with a 12–20 mm cavity of dry air or argon, sealed at the edge with a spacer bar. A slim unit shrinks the cavity to 4–6 mm and fills it with krypton or xenon, gases that conduct less heat than argon, to claw back some of the insulation lost by narrowing the gap. Overall thickness comes down to roughly 10–14 mm. Performance sits between single glazing and a standard modern unit: the manufacturer's comparison on this site puts an 11 mm slim unit at 1.9 W/m²K, against 1.2 for a standard 28 mm unit.
The narrow cavity brings a narrow edge seal, and slim units earned a mixed reputation in the 2000s when some products misted internally within a few years. Modern ones are better, but the physics has not changed: a 5 mm gap can never insulate like a 16 mm one.
Vacuum insulated glazing
A vacuum unit removes the air from the gap altogether. With nothing to conduct or convect heat across it, the gap can be a fraction of a millimetre — the two panes are kept apart by a grid of tiny pillars and sealed at the edge — and the unit ends up thinner than a single pane of old plate glass in some Victorian windows. Current units are under 7 mm overall (6.7 mm for the one in the comparison below) with centre-of-glass U-values around 0.7 W/m²K.
The technology dates from research at the University of Sydney in the late 1980s; the first commercial product was launched in 1997, and units without a visible evacuation port arrived in 2019. It is newer to UK houses than gas-filled slim glass, which is why the question of lifespan comes up — see below.
Side by side
| Single glazing | Slim gas-filled double | Standard double | Vacuum insulated glazing | |
|---|---|---|---|---|
| Overall thickness | 4 mm | ≈ 11 mm | 28 mm | 6.7 mm |
| U-value, centre of glass | 5.8 | 1.9 | 1.2 | 0.7 |
| Light transmission | 90% | 80% | 80% | 80% |
| Sound reduction Rw (C;Ctr) | 29 dB | 31 dB | 31 dB | 35 dB |
| Fits an original sash rebate | — | Usually, with a deeper rebate; some bars too thin | No | Yes, with a modestly deepened rebate |
| Weight per m² vs single glass | 1× | ≈ 2× | ≈ 2× | ≈ 2× |
| What holds the panes apart | — | Spacer bar at the edge | Spacer bar at the edge | Micro pillars across the pane |
| Visible at close range | — | Spacer bar in the rebate | Spacer bar | Pillar dots against bright sky |
Figures for thickness, U-value, light transmission and sound reduction are one manufacturer's published comparison (centre-of-glass values), reproduced in the chart on the slimline double glazing page. Whole-window performance depends on the frame and the seals as well.
What the table does not show
Rebate depth and glazing bars
The rebate is the step in the sash that the glass sits in. Georgian and early Victorian bars can be 16–20 mm wide with a rebate a few millimetres deep; deepening it for an 11 mm unit leaves little timber, and on the thinnest bars it is not possible at all. A 6.7 mm unit needs far less taken out. On plain Victorian sashes with wide rails either unit fits; on fine-barred Georgian work the thickness is often the deciding factor.
Condensation and cold glass
The inside face of a 1.9 unit on a cold night is several degrees below that of a 0.7 unit — warm enough to reduce condensation compared with single glass, but not to stop it in a damp room. The vacuum unit's inside face stays within about two degrees of the room.
Lifespan claims
Both products are warranted by their makers. For vacuum glass the long service-life figures published by manufacturers are their own claims, backed by warranty terms rather than by decades of independent field data in UK houses, because the product is comparatively new here. Ask for the warranty in writing and keep it; that is the honest answer for either product.
Toughened and laminated options
Slim gas-filled units can be made with toughened or laminated glass where safety glazing is required. Some vacuum manufacturers offer tempered units; the standard units fitted here are annealed glass. Where glass sits low or beside a door, the survey works out what is needed rather than assuming.
Cost
Vacuum units cost more per square metre than slim gas-filled ones. Because the joinery work is identical, the per-window difference is smaller than the per-unit one. What affects the cost covers the other factors.
FAQ
Is vacuum glazing the same as "slimline double glazing"?
Vacuum glazing is one kind of slimline double glazing — the thinnest kind. The phrase is also used for narrow gas-filled units of 10–14 mm, which are a different product with a different performance and a different history. Ask which one a quote is for.
Why did some slim double glazing fail early?
A narrow gas-filled unit has a narrow edge seal, and some early products used seals that could not keep the gas in or the moisture out for long, so units misted internally within a few years. Vacuum units have no gas to lose and a different edge construction; failure shows as a loss of vacuum, which manufacturers warrant against for defined periods.
Can I mix the two in one house?
Yes, though there is rarely a reason to. If a previous owner fitted slim gas-filled units in some windows, the rest can be done in vacuum glass; from outside the difference is not visible.
Which is cheaper?
Per unit, vacuum glass generally costs more than a slim gas-filled unit of the same size. Per window, the difference is smaller than it looks because the joinery work — rebating, seals, cords, putty and paint — is the same for both and is most of the labour. Quotes itemise the glass so you can see the gap for your own windows.
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Send us the number of windows, their style and your postcode and we will come back to you with the next step — usually a survey visit.