Vacuum glazing U-values explained: Ug, Uw and what actually counts
Ask any glazing salesperson how well their product keeps heat in, and you will hear a single number: the U-value. It is the industry’s shorthand for thermal performance, and for vacuum insulated glazing (VIG) it is genuinely impressive. But that one number hides three different measurements, and manufacturers almost always quote the most flattering of the three. A vacuum glazing U-value of “0.4” and one of “1.1” can both be truthful, honest figures for real products — and knowing why is the difference between an informed decision and a disappointing one. This guide explains what a U-value is, the three versions you will meet, why VIG figures vary so widely, and what the UK rules actually demand.
What a U-value actually measures
A U-value describes how quickly heat escapes through a building element — a wall, a roof, or a pane of glass. Formally it is the thermal transmittance, measured in watts per square metre per kelvin, written W/m²K. It tells you how many watts of heat pass through one square metre of the element for every one-degree difference in temperature between inside and out.
The key thing to remember is that lower is better. A low U-value means slow heat loss and a warmer, cheaper-to-run home; a high U-value means the element leaks heat like a sieve. A single-glazed Victorian sash sits around 5.7 W/m²K. A well-insulated wall might be 0.18. Glazing has always been the weak point in the thermal envelope, which is exactly why any improvement to it matters so much.
Because the figure is a rate of loss, small-looking differences compound. Dropping from 1.6 to 0.8 W/m²K halves the heat lost through that glass for a given temperature gap. That is the promise VIG makes — and the reason the number is worth scrutinising rather than skimming.
The three U-values: Ug, effective, and Uw
Here is where most confusion begins. A window is not a uniform sheet of glass. It is a pane, an edge zone where the panes are sealed together, and a frame around the outside — and each conducts heat at a different rate. Standards therefore define more than one U-value, and they are not interchangeable.
- Ug — the centre-of-glass U-value. This measures heat flow through the middle of the pane, well away from the edges and the frame. It is the best-case figure, because the centre of a VIG unit is where the vacuum does its finest work. This is the number you will see splashed across brochures.
- Effective (or edge-adjusted) glass U-value. Move towards the perimeter and the picture worsens. The edge seal and, in VIG, the array of tiny support pillars create extra heat paths that drag performance down. The effective glass figure blends the good centre with the worse edge zone across the whole glazed area.
- Uw — the whole-window U-value. This is the honest, real-world number for the complete window: glass, edge and frame combined, weighted by area. It is what Building Regulations assess and what governs your actual heating bill. A slim frame with good glass keeps Uw close to the glass figure; a bulky or poorly insulated frame can push it well above.
A centre-of-glass Ug and a whole-window Uw are not the same measurement and should never be compared head to head. A product quoting Ug 0.4 and a window quoting Uw 1.0 may be describing very similar hardware.
Manufacturers quote Ug because it is the flattering figure — it isolates the glazing technology at its best and ignores the edge and frame that every real installation includes. That is not necessarily dishonest, but it is selective. As an illustration, the Guardian and VELUX heritage roof window announced for the UK and Ireland quotes a whole-window Uw of 1.0 (0.83 for its hybrid build). That is a genuinely strong number — but it is a Uw, not a Ug, and it cannot be lined up against a centre-of-glass 0.4 as if they measured the same thing. When you compare two products, always check you are comparing like with like.
The U-value ladder
To place vacuum glazing in context, it helps to see the whole progression of glazing technology as a ladder of centre-of-glass figures. Each rung represents a genuine step change in how buildings hold onto heat.
| Glazing type | Centre-of-glass Ug (W/m²K) | Notes |
|---|---|---|
| Single glazing (4–6 mm float) | 5.7 | The historic baseline; most heritage sashes |
| Old double glazing (air, no coating) | ~2.8–3.0 | Sealed units from the 1970s–80s |
| Modern double glazing (low-E + argon) | ~1.2–1.6 (best ~1.1) | Today’s standard replacement unit |
| Triple glazing | ~0.6–0.8 | Two cavities; heavy and thick (~40+ mm) |
| VIG — Pilkington Spacia (frit-sealed, annealed) | 1.1 | Double-glazing performance at 6.2 mm |
| VIG — Super Spacia | 0.7 | Wider pillar array / tempered |
| VIG — LandVac (fully tempered) | 0.4–0.48 | Manufacturer figures |
| Hybrid VIG-in-IGU (VIG + argon/krypton pane) | ~0.4–0.5 | Composite; thicker overall |
The pattern is clear: adding cavities and coatings has steadily driven the number down, from single glazing that barely resists heat at all to triple glazing that approaches the performance of a solid wall. What makes vacuum glazing remarkable is where it lands on this ladder relative to its thickness. A classic Spacia unit matches a modern double-glazed unit for warmth while being roughly the thickness of a single pane — around 6 mm against the 24–28 mm of a sealed double-glazed unit. The best tempered VIG reaches down past triple glazing, into the ~0.4 region, in that same slim profile. For a fuller side-by-side, see our guide on vacuum glazing versus double glazing.
Why vacuum glazing U-values vary so much
If you have read that vacuum glazing is “0.4” and also that it is “1.1”, both figures are correct — they simply describe different products. The spread is real and worth understanding, because it maps directly onto how each unit is built.
Accuracy point worth pinning down: the original frit-sealed Pilkington Spacia is about 1.1 W/m²K centre-of-glass — equal to a good modern double-glazed unit, in single-glazing thickness. It is not 0.4. Only tempered, low-temperature-sealed VIG (such as LandVac) and hybrid VIG-in-a-sealed-unit builds reach the 0.4–0.7 range. Keep the two firmly apart.
Three design choices account for most of the variation:
The low-E coating
Once the vacuum has removed heat loss by gas conduction and convection, infrared radiation across the gap becomes the dominant remaining path — and a low-emissivity coating is what suppresses it. A better coating means a lower U-value. Uncoated glass radiates freely (emissivity around 0.84); a high-performance soft-coat drops that to roughly 0.03. Products that use the most effective coatings, or two opposing coatings, start with a thermal advantage before anything else is considered.
The pillar array
Every VIG unit relies on a grid of microscopic pillars to stop atmospheric pressure — around 10 tonnes per square metre — from crushing the panes together. Each pillar is also a tiny solid bridge for heat to cross. A denser grid means more bridges and a higher U-value; a wider, sparser array means fewer contact points and a lower one. Fully tempered glass is strong enough to span a wider pillar spacing without bending too far, which is one reason tempered products such as LandVac and Super Spacia post lower figures than the closely-spaced original Spacia array.
Hybrid construction
The lowest numbers come from combining technologies. A hybrid unit bonds a VIG pane into a conventional sealed unit alongside a second low-E pane and an argon or krypton cavity, stacking a gas-filled gap on top of the vacuum. This pushes Ug down toward 0.4–0.5 — but it also gives up the slimness that makes VIG special in the first place, ending up nearer the thickness of ordinary double glazing.
The edge seal shapes this too. Rigid glass-frit seals (as in Spacia) are fired hot enough to anneal the glass, historically ruling out tempering; low-temperature metal or alloy seals (LandVac, Panasonic) keep the glass fully tempered, which in turn enables the wider pillar arrays that lower the U-value. The chemistry and the thermal result are linked — a theme we explore in the guide to support pillars and edge seals.
How the edges pull the whole-window number up
The centre-of-glass Ug is the number VIG earns its reputation on. But the same two features that make VIG possible — the pillars and the edge seal — are also what stop the whole-window Uw from matching it.
Pillars conduct a little heat at every one of their thousands of contact points across the pane. In the centre this is already baked into the Ug figure, so it is not a hidden penalty there. The edge is a bigger problem. In a rigid frit-sealed unit, the edge seal itself is a continuous bridge of solid glass joining the warm inner pane to the cold outer one, short-circuiting the vacuum around the entire perimeter. This “edge effect” was quantified decades ago by the University of Sydney group and remains a defining constraint of VIG design; the physics is set out in the papers we collect on our science page.
The practical consequence is a hierarchy: the effective glass U-value is always somewhat higher (worse) than the centre-of-glass Ug, and the whole-window Uw is higher still once the frame is included. How much higher depends on the size of the pane — a small pane has more edge relative to its area, so the edge penalty bites harder — and on the frame. A slim, well-insulated timber or composite frame keeps Uw close to the glass figure; a thermally poor frame can undo much of the glazing’s advantage. This is why a large picture window in a good frame will always show a better whole-window number than a small pane in a clunky one, even with identical glass.
None of this diminishes VIG. It simply means the eye-catching Ug on the brochure is the ceiling of performance, not the figure your window will average across its whole area. When comparing options, the whole-window Uw is the number that tells the truth.
What the UK rules actually require
For most homeowners the U-value stops being an abstraction the moment they replace a window, because Building Regulations set a legal minimum. In England, this is Approved Document L (Conservation of fuel and power, Volume 1: Dwellings — the 2021 edition with 2023 amendments).
For a replacement window in an existing home, the rule offers two alternative routes, and meeting either one passes:
- a whole-window U-value no worse than 1.4 W/m²K, or
- a Window Energy Rating (WER) of Band B or better.
There is one further condition — the replacement must not be worse than the window it replaces — and a tighter limit of 1.2 W/m²K applies to windows in new-build and extensions. Scotland and Wales set their own figures through their own regulations, so the 1.4 above is specifically the England standard.
The 1.4 W/m²K threshold is a whole-window Uw, not a centre-of-glass Ug — and a WER band is a whole-window energy rating. This is precisely why the Ug-versus-Uw distinction matters in practice: the number the law tests is the honest, all-in one, not the flattering centre-pane figure on the brochure.
The good news is that vacuum glazing clears this bar comfortably. Even the classic 1.1 Ug Spacia unit, once installed in a sensible frame, achieves a whole-window Uw well below 1.4; the tempered and hybrid products do so with room to spare. The harder question with VIG is rarely regulatory compliance — it is heritage consent, because the slim profile is most often chosen for period sashes and casements where planning rules apply. Approved Document L runs alongside, not instead of, Listed Building Consent; we cover that intersection in the guide to vacuum glazing for listed buildings, and you can read a fuller summary of the standards on our regulations overview.
Reading a spec sheet without being misled
Armed with the distinction between the three U-values, a manufacturer’s data sheet becomes far easier to read honestly. A few habits keep you on solid ground:
- Check which U-value is quoted. If a figure is not explicitly labelled Uw or “whole window”, assume it is a centre-of-glass Ug and treat it as best-case.
- Ask for a whole-window figure. For a real comparison — and for Building Regulations — you need the Uw for your specific frame and pane size, not the glass alone.
- Look for the WER band. The A-to-G Window Energy Rating already bundles the whole-window performance into one letter, which is often easier than juggling U-values.
- Match technology to need. A classic Spacia at 1.1 Ug may be the right choice for a listed sash where thickness and appearance are everything; a tempered 0.4 unit or a hybrid makes more sense where raw thermal performance and safety glass matter more than a wafer-thin profile.
- Do not read too much into a decimal. The gap between a 1.1 and a 0.7 unit is real, but frame quality, installation and draught-proofing can move your actual comfort more than a tenth of a point on the glass spec.
The bottom line
The vacuum glazing U-value is one of the technology’s genuine strengths — but it is not a single number. Ug flatters, measuring only the middle of the pane; Uw tells the truth, measuring the whole window as installed; and the effective glass figure sits between them. VIG products span a wide range for good engineering reasons, from the frit-sealed Spacia at about 1.1 W/m²K to fully tempered units around 0.4–0.48 and hybrids lower still — and every one of those figures is legitimate for the product it describes. For the UK homeowner, the practical test is Approved Document L, which asks for a whole-window 1.4 W/m²K or WER band B and which VIG passes with ease. Know which number you are being shown, insist on a whole-window figure before you commit, and the U-value becomes exactly what it should be: a clear, comparable measure of how warm your window will keep you.