Most large commercial buildings are dominated by cooling loads, so window selection for commercial buildings is usually an exercise in maximizing daylighting and keeping summer heat out. Today’s best windows block heat transfer more than five times better than single-pane glass, the standard windows of just two decades ago. High-performance windows are not only a wise investment for new construction, but sometimes can be cost-effectively retrofitted, especially when timed with planned replacement of HVAC equipment. Proper selection of glazing can actually enable the installation of smaller mechanical equipment.
To understand the principles of window selection, refer to Figure 1, which shows the spectral distribution of solar energy. About half of the sun’s energy is at wavelengths visible to the human eye, while the other half (longer-wavelength near-infrared or shorter-wavelength ultraviolet) is invisible, contributing only heat to the building interior. To reduce cooling load, the ideal window would transmit as much of the visible portion of the sun’s energy as is desired for the particular application, while rejecting the rest of the solar radiation. That is, ideal windows provide light, not heat.
Figure 1: The solar spectrum
The ideal window for cooling load reduction transmits visible light and blocks ultraviolet and near-infrared radiation.
In hot climates, one effective way to reduce heat gain while letting in some light is through the use of semitransparent metallic coatings, which can be applied to surfaces of clear or tinted glass. Some reflective coatings are available for single-pane applications, and others must be sealed inside double-glass units. In general, most reflective glazings block daylight more than solar heat. Typical products offer solar heat gain coefficients from 0.13 to 0.35, with typical daylight transmittances of 5 to 35 percent. Solar heat gain coefficient, which has replaced shading coefficient as the standard window metric, is defined as the fraction of the incident solar energy transmitted through the window.
Reflective glass products have achieved their greatest market penetration in hot-climate applications where a high level of solar control is critical. However, they reduce cooling loads at the expense of daylight transmittance, and that reduction is offset somewhat by the heat of the additional lighting required.
Spectrally selective glazings
In 1983, Southwall Technologies introduced the first glazing products designed to provide enhanced solar selectivity without significantly shifting the transmitted or reflected color. Now offered by a number of manufacturers under a variety of trade names, these spectrally selective coatings are a variation on earlier “low-emissivity” (“low-e”) glazing coatings, which were designed to improve the insulation performance of windows while maximizing solar gain. Now selective glazings are available that can maximize or minimize solar gain, or can produce anything in between.