Most people meet vacuum insulated glass through one surprising fact: a unit roughly as deep as two pencils can outperform a triple-glazed insulated glass unit four times its thickness. That is not marketing — it is the removal of the one heat-transfer mechanism every other window still fights. Here is how vacuum insulated glass works, what the real numbers look like, and where it belongs in your next specification.
What vacuum insulated glass actually is
A vacuum insulated glass unit is two panes of glass separated by a microscopic evacuated gap — around a tenth of a millimeter — and sealed hermetically around the perimeter. Because the gap is evacuated, there is almost no gas inside to conduct or convect heat between the panes. Rows of nearly invisible support pillars hold the panes apart against atmospheric pressure, and the edge is finished with a fused glass-frit seal.
The frit seal matters more than most specifiers realize. THERMGLASS units are portless: the edge seal is fused and flush with the glass faces, so the unit's overall depth equals the glass stack itself. There is no pump-out port, no raised edge ribbon, and no hidden depth to account for when you detail the rebate.
The physics: why vacuum beats gas fill
Heat moves through a glazing cavity three ways: conduction through the fill gas, convection currents within the gap, and radiation between the pane surfaces.
A conventional insulated glass unit attacks only the first two, and only partially — it fills the cavity with argon or krypton to slow conduction and spaces the panes to suppress convection. A vacuum gap eliminates both outright. With no gas molecules to carry heat across, the remaining transfer path is radiation, and that is handled the same way it is in any high-performance unit: low-emissivity coatings.
The result is a center-of-glass performance ceiling set by the coatings rather than by the cavity. Standard uncoated vacuum glass measures 0.46 W/m²·K (0.081 Btu/h·ft²·°F) center-of-glass — already better than a typical argon-filled low-e double unit. Add increasingly aggressive low-e stacks and the same 6.3 mm build drops to 0.176 W/m²·K (0.031 Btu/h·ft²·°F).
The U-values you can spec today
Center-of-glass values from the THERMGLASS configurator methodology, derived using the EN 673 and ISO 10292 calculation framework:
- Standard VIG, uncoated: 0.46 W/m²·K (0.081 Btu/h·ft²·°F)
- Single hard low-e: 0.42 W/m²·K (0.074 Btu/h·ft²·°F)
- Double soft low-e: 0.39 W/m²·K (0.0687 Btu/h·ft²·°F)
- Triple soft low-e: 0.176 W/m²·K (0.031 Btu/h·ft²·°F)
- Hybrid VIG (VIG + IGU cavity), uncoated: 0.22 W/m²·K (0.039 Btu/h·ft²·°F)
- Hybrid VIG with low-e coatings: 0.142 W/m²·K (0.025 Btu/h·ft²·°F)
For solar control, the standard build carries a solar heat gain coefficient around 0.62; a double soft low-e drops that to roughly 0.27 while keeping visible transmittance near 0.68. Solar-control coatings reach an SHGC near 0.25 where summer gain is the priority.
One caveat belongs in every spec: NFRC 100/500 and EN 673 type-approval testing is in progress for these configurations. The figures above are calculation-derived using the published methodology, intended for design-stage comparison. Certified test data supersedes them when it is available.
Thickness and weight: the spec that changes the project
THERMGLASS standalone vacuum units come in three build depths:
- 6.3 mm — two 3 mm lites
- 8.3 mm — two 4 mm lites
- 10.3 mm — two 5 mm lites
Because the frit edge seal is flush, the unit's overall depth is exactly the glass stack — there is no perimeter build-up to add. For comparison, achieving comparable center-of-glass performance with conventional glazing typically takes a 25 mm-plus triple IGU with two coated lites and two gas cavities.
Where maximum performance fits the opening, a hybrid unit pairs a vacuum pane with a conventional IGU cavity, landing at roughly 20.3 to 26 mm overall. That is the only hybrid range THERMGLASS publishes — anything quoting a slimmer hybrid build is describing something else.
A practical note when comparing vendors: some welded-edge VIG designs add measurable perimeter height beyond the stack depth. Always verify the build-up per opening you are retrofitting, because the flush frit seal is what lets a 6.3 mm unit drop into a rebate cut for a single pane.
Where VIG earns its keep
The thin build and low U-factor make vacuum insulated glass the right tool wherever either depth or performance is constrained:
- Architectural retrofit — replacing single-pane sashes without rebuilding frames
- Curtain wall and new construction where slim sightlines carry the design
- Marine glazing, where every millimeter and kilogram matters and cold-surface condensation is a constant battle
- RV and specialty vehicle glazing with shallow frame systems
- Cold storage and refrigeration, where warm interior surfaces prevent frost and condensation on display-side glass
For vertical glazing under winter design conditions, the CR index (AAMA 1503) is available to quantify condensation resistance — it is reported for that configuration only, because the index is meaningful for vertical winter exposure.
Specifying with confidence: COG versus whole-window
The center-of-glass number is not the number your energy model sees. Whole-window U-value blends the glass with the frame:
Uw ≈ 0.80 × Ucog + 0.20 × Uframe
That 20% frame fraction comes from ISO 10077-1 assumptions. With a thermally broken aluminum frame at 1.80 W/m²·K, a standard 6.3 mm VIG unit lands near 0.73 W/m²·K whole-window — and the coated variants go lower still. The frame contributes a third or more of the whole-window loss in almost every assembly, which is why glass upgrades and frame upgrades compound.
Every value in this article is live in the free THERMGLASS configurators — enter your makeup, coating, environment, and tilt, and the same physics engine used across this site returns center-of-glass U-factor, SHGC, and visible transmittance for your exact combination.
The bottom line
Vacuum insulated glass removes conduction and convection from the window equation entirely, leaving coatings to manage radiation. The payoff is triple-glazed thermal performance in a 6.3 to 10.3 mm build — or hybrid performance near 0.47 W/m²·K whole-window in a 20 to 26 mm unit. For retrofit work, slim sightlines, and weight-sensitive applications, that combination is simply unavailable from any conventional glazing path.
Model your own combination in the configurator, or contact the THERMGLASS team for project-specific engineering support.