Triple, hybrid and laminated vacuum glazing explained

A monolithic vacuum unit is an elegant bargain: the thermal performance of double glazing in the thickness of a single pane. But it is still a bargain, and bargains have limits. One evacuated gap, its residual heat loss suppressed with a low-emissivity coating, bottoms out at a centre-of-glass U-value of around 0.4 W/m²K in the very best tempered products — and closer to 1.1 in the classic frit-sealed originals. To push further — to match the numbers gas-filled triple glazing posts, or to add the safety and acoustic performance a bare vacuum unit cannot — the industry stacks the vacuum unit with something else. Three approaches matter, and they are routinely muddled together. Two are on sale today. The third is, for now, largely a laboratory result.

Where a single vacuum gap runs out of road

It helps to know what you are trying to beat. A standard monolithic vacuum insulated glazing (VIG) unit is two panes of glass separated by a sub-millimetre vacuum, held apart by a grid of tiny support pillars and sealed hermetically at the edge. Remove the gas and you remove conduction and convection across the cavity — roughly 70% of the heat that a normal sealed unit loses. What is left is radiation across the gap, which a low-E coating knocks down, plus two stubborn residuals: conduction through the support pillars, and conduction around the edge seal.

Those residuals are why the U-value ladder does not fall to zero. Classic annealed Pilkington Spacia sits at about 1.1 W/m²K centre-of-glass — the same as a good modern double-glazed unit, but in single-glazing thickness. Tempered products with a wider pillar array and a better coating do far better: HaanGlas reaches 0.47–0.5, and LandVac quotes 0.4–0.48 W/m²K (manufacturer figures). Get the full U-value picture in our dedicated guide; the short version is that a single vacuum gap, however well engineered, struggles to get below roughly 0.4.

To go lower, or to add properties a thin vacuum unit lacks, there are three moves. Each answers a different question, and it is worth keeping them apart.

The one-line distinction. Hybrid adds a gas-filled cavity for a lower U-value. Laminated adds a bonded pane for safety, sound and UV. Triple vacuum adds a second vacuum gap for the lowest U-value of all — and is not yet a product you can buy.

Hybrid VIG-in-IGU: a vacuum unit inside a sealed double

The most productive move commercially is also the simplest to describe. Take a finished VIG and treat it as one lite of a conventional sealed insulated glass unit (IGU). Add a spacer bar, an argon or krypton cavity, and a further low-E pane. You now have a vacuum gap and a gas gap working in series, one behind the other. The vacuum does the heavy lifting; the gas cavity adds a second insulating layer on top.

The numbers this unlocks are the headline of the whole category. AGC Fineo Hybrid — a Fineo vacuum unit combined with a third, gas-gapped pane — is quoted at Ug 0.4–0.5 W/m²K in a total build of 22–34 mm. Pilkington offers the same idea with Spacia: bond a Spacia unit into a sealed IGU alongside an Optitherm pane and an argon or krypton cavity, and the centre-of-glass figure falls to roughly Ug 0.5–0.7 with argon and about 0.5 with krypton (Pilkington data). In both cases you have taken a vacuum unit that was around 0.7–1.1 on its own and driven it into genuine triple-glazing territory.

The most visible UK arrival uses the same principle in a roof window. The Guardian Glass and VELUX partnership, announced in 2024, puts a tempered VIG into the VELUX Heritage range launching in the UK and Ireland in 2026. The standard version is a whole-window Uw 1.0 W/m²K; the hybrid version — VIG plus a third gas-filled pane — reaches Uw 0.83 W/m²K with 38 dB of sound reduction.

Read that figure carefully. The VELUX 0.83 is a Uw — a whole-window value that includes the frame. The Fineo and Spacia hybrid numbers are Ug — centre-of-glass only. They are not directly comparable, and a Uw will always look worse than the Ug of the same glass because the frame drags it down. Any time you compare hybrid units, check which letter you are reading.

The catch is thickness. A hybrid is no longer slim. At 22–34 mm it is the depth of an ordinary double-glazed unit, which means it has surrendered the single advantage that made monolithic VIG special: the ability to drop into a historic single-glazed sash or casement with the frame untouched. A hybrid belongs in new build, in deep modern rebates, or in a roof window designed around it — anywhere you have the depth to spare and want the lowest U-value on offer. If your reason for choosing VIG was to keep an original frame, a hybrid defeats the purpose.

Laminated VIG: bonding the vacuum unit to a second pane

The second approach looks similar — another pane added to the vacuum unit — but the goal is completely different. Laminating is not primarily about the U-value. It is about safety, acoustics and ultraviolet protection.

A laminated VIG bonds the vacuum unit to a further sheet of glass with a polyvinyl butyral (PVB) interlayer, exactly as ordinary laminated safety glass is made. That interlayer does three useful things. It holds the assembly together if the glass breaks, giving safety-glass behaviour to a VIG built on annealed rather than tempered panes. It blocks close to 100% of UV, protecting furnishings and finishes from fading. And it adds mass and a damping layer, which improves sound insulation.

Pilkington’s Spacia Shizuka is the worked example: a laminated Spacia in a build of about 9.2 mm achieving a weighted sound reduction index of Rw 37 dB, up from the roughly 35 dB of the bare 6.2 mm unit. The thermal figure barely moves — you are adding glass and an interlayer, not another insulating cavity, so the vacuum is still doing all the thermal work. What you gain is robustness and quiet, not a lower U-value.

Crucially, the thickness penalty is modest. At around 9 mm a laminated VIG is thicker than a bare vacuum unit but far slimmer than a hybrid, so it stays viable in many heritage frames where a 24 mm hybrid would never fit. For a busy road, a ground-floor window with a safety-glass requirement, or a room full of light-sensitive contents, lamination is often the right refinement of monolithic VIG — and it is fully commercial today.

Triple vacuum glazing: two vacuum gaps, still mostly on the bench

The third concept is the most ambitious and the least available. Instead of pairing one vacuum gap with a gas cavity, triple vacuum glazing uses three panes of glass and two evacuated gaps in series — each cavity with its own pillar array and its own hermetic edge seal. In theory it is the lowest-U-value glazing physically achievable in a transparent unit, aiming at a centre-of-glass figure near the ~0.2 W/m²K thermodynamic floor for vacuum glass.

The theory is well established. Manz, Brunner and Wullschleger (2006) set out the heat-transfer analysis and the basic mechanical design constraints for a two-vacuum-gap unit, showing U-values well below anything a single gap can reach. The modelling toolset came from the Ulster and Loughborough groups: Fang, Eames and colleagues validated numerical heat-transfer models against measured vacuum glazing, then applied them to triple designs. Their 2015 study on low-emittance coatings is the clearest illustration of the prize — reducing coating emittance from 0.18 to 0.03 dropped a triple-VG centre-of-glass U-value from about 0.41 to roughly 0.22 W/m²K.

So why can you not buy it? The blocker is the edge seal. A rigid glass-frit seal is fired at 350–450 °C, and that heat destroys the delicate soft-coat low-E coatings that a low U-value depends on — and de-tempers the glass. Building a triple unit means solving that problem twice over. The key advance came from Memon and Eames (2015), who developed a sub-200 °C composite edge seal (a Cerasolzer-and-epoxy construction) so that soft coatings could survive fabrication; a prototype triple vacuum unit reached about 0.33 W/m²K centre-of-pane. Later UK-specific simulation work by the same authors modelled retrofitting such ultra-low-loss triple VG into a solid-wall dwelling, with a sharp cut in space-heating demand.

That is genuine progress — but it remains prototype and simulation, not a catalogue product. A triple vacuum unit doubles almost everything that makes VIG hard to manufacture: two pillar arrays to lay and keep in place, two cavities to evacuate to around 0.1 Pa, two getters, two seals to hold vacuum for decades, and roughly twice the edge conduction to design around. Commercial effort has instead flowed to tempered monolithic VIG and to hybrids, both of which already reach 0.4–0.5 W/m²K with proven production lines. The full set of papers behind triple vacuum glazing — Manz (2006), the Fang and Eames modelling series, and the Memon and Eames edge-seal work — is collected on our science page.

The three at a glance

The table sets the three concepts against the monolithic baseline. Read the U-value column with care: every entry is centre-of-glass (Ug) except the VELUX roof window, which is a whole-window (Uw) figure and is not directly comparable.

For a like-for-like look at the mainstream monolithic products that anchor the top row, see our Spacia, Fineo and LandVac comparison.

What you trade away

None of these three moves is free. Each buys its improvement with something a plain vacuum unit already had.

Thickness and weight

Monolithic VIG’s whole appeal is that it is as thin as single glazing — roughly 6–8 mm — and about as heavy as the two panes alone. A hybrid throws that away: at 22–34 mm it is a full double-glazed unit again, with the extra pane and gas cavity to match. Laminated VIG is gentler, adding a few millimetres and the weight of one more sheet of glass plus interlayer, but it is still heavier and thicker than the bare unit. Triple vacuum glazing, if it were productised, would add a third pane and a second pillared cavity, with the extra mass that implies.

Cost

Every added layer adds a cost. Bare VIG already runs a premium over standard double glazing — a rule-of-thumb multiple of roughly 2–4×, indicative only and narrowing against triple glazing. Laminating adds a pane and a lamination step. A hybrid adds a whole conventional IGU fabrication on top of the vacuum unit. And a triple vacuum unit would carry the cost of two of the most difficult processes in the field — which is a large part of why it has stayed in the lab.

The heritage advantage

This is the subtle one. For most UK buyers, the entire reason to specify VIG is that a ~6 mm unit slips into a historic single-glazed sash where a 24–28 mm double-glazed unit physically cannot, letting an original frame be retained rather than replaced. A hybrid forfeits that outright — you may as well fit ordinary triple glazing if you have the depth for it. Laminated VIG keeps most of the slim-profile benefit and is often the sensible choice where safety or acoustics matter in a conservation context. Triple vacuum glazing, by aiming to stay genuinely thin while delivering the lowest U-values, is precisely the research direction that would one day let heritage retrofit and ultra-low-energy performance coexist — which is why the academic interest persists.

When each earns its place

• Choose a hybrid for new build, deep modern frames, or roof windows where you have 22–34 mm to work with and want the lowest achievable U-value — and where slimness is not the point.
• Choose laminated VIG when you need safety glass, better acoustics or UV protection and can accept a slightly thicker unit while keeping most of VIG’s slim-profile advantage.
• Watch triple vacuum glazing as the research frontier — the concept that targets the ~0.2 W/m²K floor, not a product to specify today.

The bottom line

There is no single “better VIG” — there are three different upgrades answering three different needs. Hybrid VIG-in-IGU trades thickness for the lowest U-values you can buy, and is real and shipping (Fineo Hybrid at Ug 0.4–0.5; VELUX Heritage roof windows at Uw 0.83 from 2026). Laminated VIG trades a few millimetres for safety, quiet and UV protection, and is equally available. Triple vacuum glazing — two vacuum gaps chasing the ~0.2 W/m²K limit — is the exciting one and the one you cannot order: a decade of solid theory and promising prototypes, held back by the difficulty of sealing two vacuum cavities durably and affordably. If you are weighing any of these against a plain vacuum unit, the first question is not which has the best number, but which of thickness, safety or absolute U-value you actually need to optimise — and whether keeping an original frame slim still matters more than shaving another tenth off the glass.