Reference
Insulated glass performance, by coating, gas and spacer
Three choices decide what a glazing unit does: which coating is on the glass, what fills the cavity, and how the edge is built. They are not interchangeable. The coating sets the solar figure; the gas and a room-side coating move the U-value and leave the solar figure where it was. The tables below are the manufacturers’ own published values, each one printed with the build it was calculated for.
Centre-of-glass figures, not window ratings. A window rating covers frame, spacer and glass together and belongs to one tested configuration — see certifications and test standards for how our own figures are reported.
The published values
Four tables, each on the build and the standard the manufacturer published it under. Read the note under each one before comparing across tables.
Coating, and what a room-side coating addsNFRC, centre of glass
| Coating | Light % | SHGC | U, clear inner pane | U, with coating on #4 |
|---|---|---|---|---|
| LoE-180 on #3 | 79 | 0.69 | 0.26 | — |
| LoE-180 | 79 | 0.65 | 0.26 | 0.21 |
| LoE-272 | 72 | 0.41 | 0.25 | 0.20 |
| LoE-270 | 70 | 0.37 | 0.25 | 0.20 |
| LoE-366 | 65 | 0.27 | 0.24 | 0.20 |
| LoE-452+ | 52 | 0.22 | 0.24 | 0.19 |
| LoE-240 | 40 | 0.25 | 0.25 | 0.20 |
| LoE-340 | 39 | 0.18 | 0.24 | 0.20 |
Cardinal centre-of-glass values for a 3/4 in two-pane unit with 3 mm lites and a 13 mm argon cavity. LoE coatings sit on surface 2 except where a surface is named. The final column adds LoE-i89 on surface 4, the room-side coating: it drops the U-value by about 0.04 and costs a little light.
Source: Cardinal Glass Industries, Insulating Glass brochure, v4.1, March 2024. View the published data
Gas fill, on one buildNFRC, centre of glass
| Coating | Light % | SHGC | U, air | U, argon |
|---|---|---|---|---|
| Solarban 60 | 70 | 0.39 | 0.29 | 0.24 |
| Solarban 70 | 64 | 0.27 | 0.28 | 0.24 |
| Solarban 60 with Sungate ThermL on #4 | 69 | 0.38 | 0.23 | 0.20 |
| Solarban 70 with Sungate ThermL on #4 | 63 | 0.27 | 0.23 | 0.19 |
Vitro published values for a 6 mm + 13 mm + 6 mm unit, coating on surface 2. Argon moves the U-value and leaves the solar heat gain coefficient where it was, which is the useful thing to know: gas is a winter decision, coating is a summer one.
Source: Vitro Architectural Glass, Sungate ThermL / Solarban published performance comparison. View the published data
Metric values, NFRC conditionsNFRC 2010
| Build | Light % | SHGC | Ug winter W/m²K | Ug summer W/m²K |
|---|---|---|---|---|
| ClimaGuard IS 20, 3 mm + 12.7 mm argon + 3 mm | 80 | 0.73 | 1.978 | 1.916 |
| ClimaGuard IS 20, triple: 3 mm + 12.7 mm argon + 3 mm + 12.7 mm argon + 3 mm | 73 | 0.67 | 1.349 | 1.418 |
Guardian published values calculated to NFRC 2010, 10% air / 90% argon. Winter-night and summer-day U-values are given separately because the boundary conditions differ; quoting one without saying which is the usual way a comparison goes wrong.
Source: Guardian Glass, ClimaGuard IS 20 and SunGuard SN 68 published glazing data, NFRC 2010. View the published data
Metric values, EN 673EN 673
| Product | Build | Ug W/m²K | Light % | Note |
|---|---|---|---|---|
| Pilkington Optitherm S3 | 4 mm + 16 mm argon (90%) + 4 mm | 1.1 | 82 | Light reflectance 11% |
| Pilkington Optitherm S1A | 4 mm + 16 mm argon (90%) + 4 mm | 1.0 | — | For a 1.0 W/m²K requirement |
| Pilkington Insulight Therm Triple | Triple unit with Optitherm S1A | 0.4 | — | Calculated to EN 673 |
Pilkington published values to EN 673. These are not interchangeable with the NFRC figures above: the two standards use different boundary conditions, so a 1.1 W/m²K EN 673 value and an NFRC U-factor are not the same measurement even after a unit conversion.
Source: Pilkington, Optitherm S3 / Optitherm S1A product data, EN 673. View the published data
The spacer, and what it does not do
Spacer type is recorded as a code in the NFRC certified product directory, and the full code list is on the NFRC code reference page. A warm-edge spacer does not move the centre-of-glass figure above: it reduces the thermal bridge at the edge of the unit, so it shows up in the whole-window value and in the condensation risk at the glass edge, not in the middle-of-glass number.
What buyers and specifiers ask
Are these the values your windows achieve?
No. These are centre-of-glass values published by the glass manufacturers for a stated build. A whole-window value covers the frame, the spacer and the glass together, is produced by a laboratory for one tested configuration, and appears on the report for that configuration. What this page lets you do is choose the build before you ask for a quotation.
Does argon change the solar heat gain coefficient?
No. Argon lowers the U-value and leaves SHGC where it was, which is why the gas and the coating are separate decisions: coating is a summer question, gas is a winter one. The table that shows air and argon side by side shows the SHGC column unchanged between them.
Why are there two metric tables?
Because NFRC and EN 673 use different boundary conditions, so their numbers are not the same measurement. A 1.1 W/m²K value to EN 673 is not an NFRC U-factor converted to metric, and treating them as interchangeable is the most common way a cross-market comparison goes wrong.
Where does the spacer come in?
A warm-edge spacer does not move the centre-of-glass figure. It reduces the thermal bridge at the edge of the unit, so it shows up in the whole-window value and in the condensation risk at the glass edge. The spacer is recorded as a code on the NFRC label — the full code list sits on the NFRC code reference page.
What should I send to get a figure for my project?
The build, layer by layer: pane thickness, coating, spacer and cavity fill, plus the frame system and the opening size. A quotation is built against a configuration, and the performance figure is read from the report that covers that configuration.