Typical sheet resistance of low-e coatings
| Coating family | Sheet resistance (Ω/sq) | Normal emissivity | Notes |
|---|---|---|---|
| Uncoated soda-lime glass | insulating | 0.84 (hemispherical) | The baseline every low-e coating improves on. |
| Pyrolytic hard-coat (SnO₂:F) | 12–20 | 0.13–0.20 | Applied on the float line; durable enough for exposed surfaces and monolithic glass. Higher solar gain. |
| Sputtered ITO | 5–30 | 0.06–0.30 | Transparent conductor; heated and anti-condensation glass, displays. |
| Single-silver soft-coat | 3–6 | 0.03–0.06 | Most common residential low-e; moderate to high solar gain. Sealed inside an insulating unit. |
| Double-silver | 1.8–3.0 | 0.02–0.03 | Spectrally selective; blocks most near-infrared solar heat. |
| Triple-silver | 0.9–1.6 | 0.010–0.017 | Highest selectivity (light-to-solar gain about 2 or more) for cooling-dominated buildings. |
Ranges are typical of published product data and the literature; individual products vary, and manufacturers usually specify emissivity rather than sheet resistance. Use the measured value of your own coating for production control.
Why sheet resistance predicts emissivity
Sheet resistance is the resistance of a square of film, independent of its size: \(R_s = \rho/t\) for resistivity \(\rho\) and thickness \(t\). In the far infrared, where a 300 K surface radiates, a thin metal film behaves like a sheet of conductance \(1/R_s\). A plane wave meeting such a sheet is partly reflected, partly transmitted, and partly absorbed. The absorbed fraction, which equals the normal emissivity, is
and for the low sheet resistances of low-e coatings this reduces to the widely used rule \(\varepsilon_n \approx 4R_s/Z_0 \approx 0.0106\,R_s\) (with \(R_s\) in Ω/sq). Halving the sheet resistance halves the emissivity. That is why coaters add silver layers: two or three thin silver layers reach lower resistance, and lower emissivity, while keeping visible transmission high and the color neutral. The model assumes a free-electron (Drude) metal at long wavelengths; real coatings deviate by roughly 10–20 %, so calibrate the relation against emissivity measurements for a given product.
The same sheet conductance attenuates radio waves. A coated lite reflects most of an incident microwave, with a shielding effectiveness of about \(20\log_{10}(1 + Z_0/2R_s)\) dB, or 30–40 dB for silver low-e. This is why low-e windows weaken cell and Wi-Fi signals, and why some projects use frequency-selective laser patterning of the coating to let signals through.
How sheet resistance is measured
- Four-point probe. A current passes through the outer two pins and the voltage is read across the inner two, which removes contact resistance. It suits pyrolytic coatings and exposed conductors. The silver in soft-coat low-e sits under insulating dielectric layers, so contact probes may read poorly or scratch the coating.
- Non-contact eddy current. A coil induces currents in the film and senses the loss. It reads through the top dielectric layers without touching the coating, which makes it the standard choice for soft-coat silver stacks, for handheld checks, and for inline gauges that map sheet resistance across the full width of a coater.
- Emissometers. Handheld emissometers measure emissivity directly from thermal radiation. They are useful for verifying which surface is coated and for calibrating the sheet-resistance-to-emissivity relation.
Sheet resistance and color tell you different things. Sheet resistance follows the silver, while color follows the dielectric layer thicknesses through thin-film interference. Mapping both across a lite catches most coater drifts. The color tool explains how to measure low-e color, and the glazing calculator turns emissivity into U-factor and SHGC for a full window.
References
- H. J. Gläser, Large Area Glass Coating, Von Ardenne Anlagentechnik (2000), chapters on low-e layer systems and the sheet-resistance–emissivity relation.
- C. G. Granqvist, “Transparent conductors as solar energy materials: A panoramic review,” Sol. Energy Mater. Sol. Cells 91, 1529–1598 (2007).
- F. M. Smits, “Measurement of sheet resistivities with the four-point probe,” Bell Syst. Tech. J. 37, 711–718 (1958).
- ISO 15099:2003 and NFRC 100 for the U-factor calculation.