Proposed revisions to the U.S. Department of Energy's ENERGY STAR® glass performance standards (scheduled to debut in 2009) should make it clear that generic low-E glass no longer represents the level of energy efficiency required to transform the market (a key charter of the agency's ENERGY STAR program). Because generic low-E glass provides maximum insulating performance of about R-4 in a world in which R-19 insulated walls are the norm, there's a dramatic performance gap between what low-E glass might provide and what green-building practices promise in saving energy and reducing carbon emissions. Anywhere from 25 to 35 percent of the energy used in buildings is wasted due to inefficient glass, resulting in more than 10 percent of the total carbon emissions in the United States annually - a major contributor to global warming.
Two alternatives to generic low-E insulating glass are currently available that can meet the new ENERGY STAR glass performance standards for windows, fixed glass, and glass curtainwall:
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One is triple-pane glass, consisting of three panes of glass and two low-E coatings. The good news is that, by using a third pane of glass to create a second insulating cavity, triple-pane insulating glass improves generic low-E dual-pane insulating glass performance by about 50 percent (from R-4 to R-6). Triple-pane glass, however, is 50-percent heavier than standard insulating glass, requiring stronger glass framing (and an increased cost). The additional coated glass may also reduce visible light transmission by 20 percent or more, reducing the comfort and productivity benefits of natural daylighting.
Low-E: A Glass Half Full
Low-E coatings are rated for the amount of heat they radiate: the lower the number, the less heat that's radiated, and the better the insulation performance of the glass. Coated glass is commonly available today with emissivity ratings below 0.03, and lowering emissivity from 0.03 to 0.00 will have a negligible incremental improvement on glass performance. Clearly, further improvements in glass thermal performance will not come from enhancements in low-E coatings. The incremental performance benefit of using low-E glass is zero, because it's already assumed as a required product.
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The other alternative consists of mounting a lightweight, low-emissivity, solar-reflective film inside of an insulating glass unit. Without the weight disadvantages of a third lite of glass, suspended film can create two, three, or even four insulating cavities that maximize light transmission and provide insulation performance ranging from R-6 to R-20 to meet the unique requirements of commercial new construction and renovation projects. Such internally mounted film does not replace low-E coated glass; rather, it leverages the benefits of film-based and glass-based technologies by utilizing coated glass to minimize solar heat gain while using lightweight film to maximize insulation performance, block UV radiation, reduce noise, and increase occupant comfort more effectively than low-E glass alone.
Bruce Lang is vice president of marketing and business development at Palo Alto, CA-based Southwall Technologies Inc. He can be reached at ([email protected]).
