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How Vacuum Glass Works: A Technical Guide for Architects

A vacuum-insulating glass (VIG) unit looks like an ordinary double pane on the outside, but the gap between the two plies is evacuated to roughly 0.1 pascals — about one millionth of atmospheric pressure at sea level. Removing the gas eliminates convective heat transfer, which is the dominant loss mechanism in a conventional insulated glass unit (IGU). The result is a glazing unit about 8 mm thick with a centre-of-glass U-value in the 0.5 W/m²K range — performance that a conventional triple-pane IGU needs 36 to 44 mm of thickness and gas fills to reach.

For architects specifying high-performance glazing on weight- or depth-constrained projects, this profile unlocks design space that conventional IGUs cannot. The remainder of this guide walks through how the unit is built, how it delivers its thermal performance, the standards that qualify it, and where it sits against double- and triple-pane alternatives.

Source: Huanqiu Glass technical explainer.

The physics behind the evacuated cavity

Heat moves through a glazing cavity in three ways: conduction through the gas, convection of the gas, and radiation across the gap. In a standard double-pane IGU, conduction and convection together account for roughly 60 percent of the heat transfer across the cavity. A low-emissivity coating reduces radiation but does little for the gas pathways.

Evacuating the cavity to roughly 0.1 Pa removes both conduction-through-gas and convection as meaningful heat-transfer paths. The remaining losses are:

Radiative transfer across the gap, addressed by a low-e coating on one inner surface.
Conduction through the pillar grid, which is small because the pillars occupy less than one percent of the cavity area.
Conduction through the edge seal, which is the main thermal bridge and the determining factor for edge-of-glass U-values.

The relevant figure of merit is the centre-of-glass U-value (Ug), measured perpendicular to the glass with a 15 K temperature differential per ISO 10292 / EN 673. For a VIG unit with one low-e coating, manufacturer-reported values typically sit between 0.4 and 0.7 W/m²K depending on pillar spacing and glass thickness.

Anatomy of a vacuum glass unit

A VIG unit comprises three subsystems, each engineered to keep the vacuum stable for the design life of the product.

Outer plies

The outer walls are usually two sheets of soda-lime float glass, 3 to 4 mm each. A low-e coating is applied to one inner face — typically a sputtered silver stack on the inner surface of the outer ply, facing the cavity. Tempered or heat-strengthened plies are used where code requires safety glazing or the unit is sloped or overhead. Some products use a laminated inner ply (3 mm + 0.76 mm interlayer + 3 mm) for post-breakage retention.

Micro-pillar grid

The cavity is held open by a regular array of stainless steel or ceramic pillars. Manufacturer-reported dimensions vary:

Pillar height (cavity gap): typically 0.1 to 0.2 mm in production, occasionally up to 0.5 mm
Pillar diameter: typically 0.4 to 0.6 mm
Pillar spacing (pitch): typically 20 to 40 mm in a square grid
Pillar material: stainless steel, ceramic, or low-carbon steel with anti-reflective treatment
Cavity area occupied: typically 0.4 to 1.0 percent of the visible area

The pillars are bonded to the inner glass surface with a small dot of epoxy or fired frit. Their job is mechanical: they transfer the roughly 100 kPa of atmospheric pressure from the outer ply to the inner ply so the cavity does not collapse. Because the pillars occupy less than one percent of the visible area and sit at a distance, they are imperceptible in normal viewing conditions. On clear sky reflections at very short range they can be detected as a faint regular pattern.

Glass-frit edge seal

The two plies are joined around the perimeter by a glass-frit paste screen-printed onto one edge, then fired at roughly 450 to 500 °C. The fused frit becomes a rigid glass-to- glass bond, typically 8 to 15 mm wide. An outer secondary seal of organic polymer (polyisobutylene or silicone) protects the frit from moisture and mechanical damage but does not need to be gas-tight. The frit is the only thing keeping the cavity at sub-atmospheric pressure for the life of the unit.

Unlike the organic edge seals used in conventional IGUs, the frit cannot be re-worked in the field. A leaking VIG unit must be replaced.

How the unit is made

The manufacturing process differs sharply from a conventional IGU line. The key stages:

Cut and prepare. Plies are cut to size, edges seamed. Holes for evacuation are drilled in one perimeter of the outer ply — typically one port per 0.5 to 1.0 m².
Place pillars. Pillars are screen-printed onto one inner surface at the correct pitch via a stencil. Placement accuracy of ±0.05 mm is essential — misplaced pillars short-circuit the cavity or leave unsupported regions that bow.
Print the frit. The edge seal is screen-printed around the perimeter of one ply as an 8 to 15 mm wide strip of low-melting glass paste, matched in thermal-expansion coefficient to the substrate.
Assemble and fuse. The two plies are aligned in a furnace fixture and heated to roughly 450 °C to fuse the frit. Pillar height at this stage sets the cavity gap.
Evacuate and pinch off. While the frit is still plastic, the unit is transferred to a vacuum chamber for 30 to 120 minutes. This drives out dissolved gases and moisture from the glass surfaces and the frit. The evacuation port is then sealed by pinching a softened glass tube while the cavity is still under vacuum.
Seal and inspect. A polymeric over-seal protects the frit from moisture and UV. Finished units undergo leak and visual inspection before shipment.

Key properties and typical specifications

Numbers below are typical of current commercial products. They are not universal — confirm with the manufacturer's datasheet before specifying.

Total thickness: typically 8.0 to 10.3 mm (most products)
Centre-of-glass U-value (Ug): manufacturer-reported ranges typically 0.4 to 0.7 W/m²K with one low-e coating
Light transmittance (Tv): manufacturer-reported 0.65 to 0.78 depending on coating and glass thickness
Solar heat gain coefficient (g-value): manufacturer-reported 0.45 to 0.65
Weight: manufacturer-reported around 16 kg/m² for 8 mm total thickness — about 40 percent less than a typical triple-pane IGU
Maximum panel size: manufacturer-reported typically 1.5 × 2.0 m, with some manufacturers offering 1.8 × 2.4 m
Design service life: manufacturer-reported 25 years when specified, fabricated, and installed to manufacturer guidelines
Sound insulation (Rw): manufacturer-reported 35 to 39 dB for single VIG; up to 45 dB when combined with a laminated inner ply and a third glass sheet
Operating temperature range: manufacturer-reported −40 °C to +80 °C
Fire rating: depends on the plies; the cavity itself does not propagate flame

Where vacuum glass is used today

Heritage and retrofit glazing

Listed-building windows often have rebate depths of only 15 to 25 mm. A triple-pane IGU will not fit; a double-pane IGU underperforms. A 10 mm VIG unit delivers triple-pane thermal performance inside the existing rebate, allowing thermal upgrades without altering the visible frame — usually a planning requirement for listed buildings.

Slim-profile curtain walls and sloped glazing

High-end curtain walls (Schüco, Reynaers, Technal) push for minimal frame sightlines. Swapping a 36 mm triple-pane IGU for a 10 mm VIG unit cuts the aluminium or steel frame depth by 25 mm or more. Weight reduction is similarly significant — a VIG skylight weighs roughly 16 kg/m² against roughly 28 kg/m² for a triple-pane IGU — which slims support structure and curbs.

Refrigeration and cold-climate builds

Outside architecture, VIG appears in commercial refrigeration display doors, where condensation on the cold-side surface is a constant energy penalty. The vacuum cavity suppresses dew-point issues and frost formation. Thinness and low weight also suit off-grid cabins and expedition shelters where transport loads matter.

Pros and cons at a glance

Strengths

Triple-pane U-values in a single 8 to 10 mm unit — slimmer than any conventional IGU at this performance level.
No gas fill to leak — the cavity is a hard vacuum, so there is no equivalent of the argon-depletion failure mode that haunts conventional IGUs after 15 to 20 years.
Lighter than triple-pane alternatives — important for sloped glazing, moving façades, and weight-limited structures.
Condensation resistance — the inner surface temperature stays close to the room temperature, minimising interstitial condensation risk.
Slim sightlines — enables contemporary minimal-frame architecture.

Honest limitations

Higher cost per square metre than a double-pane IGU; usually 2 to 4× the price of a comparable thermally broken double-pane unit. Premium shrinks when compared with triple-pane-plus-low-e at the same U-value.
Maximum panel size is limited — most manufacturers cap at 1.5 × 2.0 m; oversize panels need custom quotes and longer lead times.
Pillar visibility at close range — imperceptible from 1 metre onwards, faintly visible against a clean sky reflection at very short distance.
No field repair — a breached frit seal means full unit replacement.
Not yet covered by all regional building codes — confirm local acceptance before specifying; IGCC/IGMA certification is still being extended to VIG.

How vacuum glass compares with double and triple glazing

The decision below maps the three products on the dimensions an architect typically weighs. Values are typical of current products and should be confirmed against the manufacturer's datasheet before specifying.

Property	Double-pane IGU (with low-e + argon)	Vacuum glass (with low-e)	Triple-pane IGU (with 2 low-e + argon)
Total thickness	24 to 28 mm	8 to 10.3 mm	36 to 44 mm
Centre-of-glass U-value (Ug)	1.0 to 1.3 W/m²K	0.4 to 0.7 W/m²K	0.5 to 0.8 W/m²K
Weight	~20 kg/m²	~16 kg/m²	~28 kg/m²
Panel size limits	Limited mainly by handling	Typically 1.5 × 2.0 m	Up to 3 × 5 m on most lines
Cost per m² (relative)	1× baseline	2 to 4× baseline	1.4 to 1.8× baseline
Field repair (failed seal)	Unit replacement	Unit replacement	Unit replacement

VIG is the right call when the binding constraint is thickness, weight, or slim sightlines, and the project can absorb the premium. A triple-pane IGU is the right call when maximum panel size matters more than profile depth — large curtain walls and full-height storefronts. Double-pane IGUs remain the workhorse for cost-driven residential work where the Ug gap does not pay back.

For boundary cases the deciding factor is often rebate depth: if the existing or designed frame cavity cannot accept a triple-pane IGU, VIG moves from premium to necessity. A full cost analysis that captures slim-frame aluminium savings, reduced structural support, and the elimination of argon re-fills will often tip the calculation further toward VIG than the table suggests.

Standards and qualification

Three standards frame VIG specification and durability testing today.

ASTM E2190 — Standard Specification for Insulating Glass Unit Performance and Evaluation. It provides accelerated weathering and seal-durability test methods used in North American insulating-glass qualification.
EN 1279-5 — Insulating glass units: product standard. Part 5 covers evaluation of conformity and the gas-fill retention and edge-seal durability regime. European technical approvals (EAD/ETAs) for VIG reference EN 1279-5 for the long-term performance test.
ISO 19916-1:2018 — Glass in building: vacuum insulating glass, Part 1. This VIG-specific standard defines basic product requirements and evaluation methods.

Confirm the proposed product's current certification and declared performance before specifying; a generic reference to an IGU standard does not certify a particular unit.