Spectrally selective glazing: More daylight, less heat gain

St. Thomas More Hospital in Canon City, Colo., is  among a handful of projects in the U.S. that  feature the latest in spectrally selective low-e  glazing: triple-silver-coated glass. The glass  blocks up to 73% of heat-generating infrared  light, while allowing 64% of visible light to pass  through. Photo: PPG

       
         
The nanomaterials revolution has brought many innovations to the building sector in recent years—concrete that cleans itself, glass that switches from transparent to opaque with the push of a button, and solar panels as thin as paper.
 

One of the more notable nanomaterial successes in the building industry dates back to the late 1970s. That's when scientists discovered that by applying nanometer-thick films—for the record, a nanometer is a billionth of a meter—of metal and metal oxide to glass, they could block high levels of heat-generating infrared light, while allowing most visible light to pass through. The discovery led to the development of thin-film-coated spectrally selective low-e glasses that offered Building Teams a near-crystal-clear glazing alternative to traditional multi-pane insulating glass units that use tinted or mirrored glass to control solar heat gain.

Over the years, glass coating manufacturers refined the coating process, eventually adding a second layer and—quite recently—a third layer of metal to reflect a much larger percentage of UV and infrared light while still maintaining a high level of visible light transmittance.
         
        

Illustration depicts single-, double-, and triple-silver-coated glass technology. Credit: PPG  
          

"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.