Low-emissivity glass, universally abbreviated as Low-E, has transformed building energy performance over the past four decades. A Low-E coating is an optically transparent, microscopically thin metallic layer applied to glass that reflects long-wavelength infrared heat while transmitting visible light. This selective reflectance allows a window to admit daylight and passive solar heat while blocking the escape of interior warmth, or alternatively, while blocking unwanted solar heat gain.
Sputtered Coatings: Soft-Coat Low-E
Sputtered Low-E coatings, also known as soft coats or off-line coatings, are applied inside a vacuum chamber after the glass has been manufactured. The process, magnetron sputtering, involves bombarding a metallic target with ions to eject atoms that deposit onto the glass surface in precise, nanometre-scale layers.
Modern sputtered coatings use multiple layers of silver sandwiched between anti-reflective metal oxide layers. The silver layers, each only about 10 nanometres thick, are responsible for the low emissivity, while the anti-reflective layers suppress visible reflections and protect the silver. Triple-silver coatings, introduced in the mid-2000s, offer the best combination of high visible light transmission, low solar heat gain, and ultra-low emissivity.
Sputtered coatings are delicate. They require protection from the environment and must be sealed inside an insulating glass unit (IGU), positioned on one of the interior-facing glass surfaces, typically the number 2 or number 3 surface counting from the exterior. Exposed to atmosphere, a soft-coat Low-E will oxidise and lose performance within weeks.
Pyrolytic Coatings: Hard-Coat Low-E
Pyrolytic Low-E coatings are deposited during the float glass manufacturing process while the glass is still hot, typically at around 600 degrees Celsius. A chemical vapour containing tin compounds is sprayed onto the hot glass ribbon, where it decomposes to form a thin, transparent layer of tin oxide, chemically doped with fluorine or antimony to enhance conductivity.
Because the coating is fused into the glass surface at high temperature, pyrolytic coatings are extremely durable. They can be used on single-glazed windows without protection, can be cleaned with standard glass cleaners, and can even be tempered or bent after coating. The trade-off is performance: pyrolytic coatings have higher emissivity, about 0.15 to 0.20, compared to 0.02 to 0.04 for the best sputtered coatings, and they lack the spectrally selective properties that allow a sputtered coating to maintain high visible transmission while cutting solar heat gain.
Key Performance Metrics: SHGC and U-Factor
The Solar Heat Gain Coefficient, abbreviated SHGC, measures the fraction of incident solar radiation admitted through a window, expressed as a number between 0 and 1. SHGC includes both directly transmitted solar energy and the portion of absorbed energy radiated inward. In hot climates, a low SHGC is desirable to reduce cooling load. In cold climates, a higher SHGC may be beneficial to capture free solar heating during winter.
The U-factor measures the rate of heat transfer through a window assembly, expressed in watts per square metre per kelvin, or BTU per hour per square foot per degree Fahrenheit in the United States. A lower U-factor indicates better insulation. A single pane of clear glass has a centre-of-glass U-factor of approximately 5.7 W/m2K. A double-glazed IGU with clear glass and air achieves roughly 2.8 W/m2K. Adding a soft-coat Low-E and argon fill brings the U-factor down to approximately 1.1 to 1.4 W/m2K. A triple-glazed unit with two Low-E coatings and argon or krypton can achieve a centre-of-glass U-factor below 0.6 W/m2K.
Major Low-E Product Families
Cardinal Glass Industries, a privately held American company, produces the LoE series of coatings. LoE-272 is a standard dual-silver coating introduced in the 1990s that became an industry benchmark. LoE-366, a triple-silver coating, offers visible light transmission around 65% with a SHGC as low as 0.27. LoE-i89 was designed specifically for interior surfaces to reflect room heat back into the building.
PPG, now part of Vitro Architectural Glass, markets its coatings under the Solarban brand. Solarban 70, a triple-silver coating, provides 64% visible light transmission with a SHGC of 0.27 and an LSG ratio of 2.37, one of the highest in the industry.
Guardian Industries produces the SunGuard line. SunGuard SNX 60 combines 60% visible light transmission with an SHGC of 0.27 and an extremely low external reflectance.
In Europe, the Saint-Gobain Glass Planitherm and SGG Cool-Lite ranges dominate, while AGC offers Thermobel and Stopray product lines.
European Thermal Standards: EN 673
The European standard EN 673 defines the calculation method for the thermal transmittance, or U-value, of glazing. The standard accounts for emissivity of coated surfaces, gas fill thermal conductivity, gap width, and temperature. The standard uses an iterative finite-difference method to solve the heat transfer equations for conduction, convection, and radiation within the IGU cavity. Under EN 673, a double-glazed IGU with a standard low-e coating and argon fill typically achieves a centre-pane U-value of 1.1 W/m2K, while the best triple-glazed units with krypton fill can reach below 0.5 W/m2K.