Spectrally Selective Glazings

Spectrally selective glazing is window glass that permits some portions of the solar spectrum to enter a building while blocking others. This high-performance glazing admits as much daylight as possible while preventing transmission of as much solar heat as possible. By controlling solar heat gains in summer, preventing loss of interior heat in winter, and allowing occupants to reduce electric lighting use by making maximum use of daylight, spectrally selective glazing significantly reduces building energy consumption and peak demand. 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, as shown in the figures on the left.

Because of its solar heat transmission properties, spectrally selective glazing benefits both buildings in warm climates where solar heat gain can be a problem and buildings in colder climates where solar heat gains in summer and interior heat loss in winter are both of concern. In other words, these glazings are appropriate for residential and commercial buildings throughout the United States. The energy efficiency of spectrally selective glazing means that architects who use it can incorporate more glazing area than was possible in the past within the limitations of codes and standards specifying minimum energy performance. When spectrally selective glazing is used, the capacity of the building's cooling system can also be downsized because of reduced peak loads.

Spectrally selective glazings screen out or reflect heat-generating ultraviolet and infrared radiation arriving at a building's exterior surface while permitting most visible light to enter. Spectral selectivity is achieved by a microscopically thin, low-emissivity (low-E) coating on the glass or on a film applied to the glass. There are also carefully engineered types of blue- and green-tinted glass that can perform as well in a double-pane unit as some glass with a low-E coating. Conventional blue- and green-tinted glass can offer some of the same spectral properties as these special absorbers because impurities in tinted glass absorb portions of the solar spectrum. Absorption is less efficient than reflection, however, because heat absorbed by tinted glass continues to radiate to the building's interior.

This technology is most costeffective for residential and nonresidential facilities that have high cooling loads, high utility rates, poorly performing existing glazing (such as single-pane clear glass or dark tinted glass), or are located in the southern United States. In the northern United States, spectrally selective low-E windows can also be cost-effective for buildings with both heating and cooling requirements.

In general, the technology pays back in 3 to 10 years for U.S. commercial buildings where it replaces clear single-pane or tinted doublepane glass and for most commercial buildings in the southern United States where it replaces low-E, double-pane windows. Spectrally selective glazing is applicable in both new and retrofit construction.

This Federal Technology Alert provides detailed information and procedures for Federal energy managers to consider spectrally selective glazings. The principle of spectrally selective glazings is explained. Benefits related to energy efficiency and other architectural criteria are delineated. Guidelines are provided for appropriate application of spectrally selective glazing, and step-by-step instructions are given for estimating energy savings. Case studies are also presented to illustrate actual costs and energy savings. Current manufacturers, technology users, and references for further reading are included for users who have questions not fully addressed here.