Spectrally selective glazing: More daylight, less heat gain
"For a long time, double-silver-coated was thought to be the limit from a manufacturing standpoint," says James J. Finley, PhD, a Fellow at PPG Industries' Glass Business and Discovery Center in Harmar Township, Pa., who worked on the development of PPG's triple-silver-coated glass product, Solarban 70XL. (PPG and Cardinal Glass are the only domestic manufacturers to offer triple-silver-coated glass.) "Every time you add a layer, it becomes much more difficult and less cost-effective to manufacture. By building these stacks, from one, to two, to three layers, we can create a much better filter."
Finley says manufacturers are approaching what is considered to be the physical limit of thin-film-coated spectrally selective glazing—a light-to-solar-gain (LSG) ratio of 2.5—offering unprecedented levels of daylight with minimal solar heat gain. PPG's product, for instance, is rated at 2.37 LSG in a standard one-inch insulating glass unit, which equates to 64% visual light transmittance and a solar heat gain (SHG) coefficient of 0.27, meaning it blocks up to 73% of the sun's energy. "We're not going to get much better performance than that," says Finley.
In comparison, typical uncoated low-iron glass transmits about 84% of visible light, but has a 0.82 solar heat gain coefficient for an LSG factor of 1.02. Traditional blue/green reflective tinted glass has a low SHGC (0.31), but lets only 27% of visible light pass through, for an LSG of just 0.87 (see table below).
The benefits of implementing high-performance, spectrally selective glazing have been documented by the U.S. Department of Energy in a 47-page technical review of the technology.
"Because new spectrally selective glazings can have a virtually clear appearance, they admit more daylight and permit much brighter, more open views to the outside while still providing the solar control of the dark, reflective energy-efficient glass of the past," says the DOE study.
By blocking solar heat and making maximum use of daylight, spectrally selective glass can significantly reduce building energy consumption and peak demand related to heating, cooling, and electric lighting load. When properly specified and implemented in a building project, spectrally selective glazing also can enable Building Teams to downsize HVAC equipment, such as chillers, which reduces initial capital investment costs.
DOE estimates the payback for spectrally selective glazing at about 3-10 years for U.S. commercial buildings in cases where it replaces clear single-pane or tinted double-pane glass. A similar payback period would apply to most commercial buildings in the southern U.S. when spectrally selective glass is used instead of conventional high-transmission, low-e, double-pane windows.
Payback periods can be minimized even further when high-LSG spectrally selective glasses, such as the triple-silver-coated low-e technology, are specified.
Table compares the performance of glasses based on winter U-value, visual light transmittance, solar heat gain coefficient, and light-to-solar-gain ratio.