This Web Exclusive article is by Bruce Lang, vice president of marketing & business development at Southwall Technologies, Inc.
Initial revisions to the Department of Energy (DOE)’s Energy Star® window performance standards, which will be effective January 1, 2010, should make clear that generic low-e glass no longer represents a level of energy efficiency required to “transform the market,” a key charter of the agency’s Energy Star program.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 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 provides and what green building practices promise in saving energy and reducing carbon emissions.
Despite heavily insulated walls and ceilings and the popularity of low-e glass, 25% to 35% of the energy used in buildings is wasted due to inefficient glass. So, it should come as no surprise that glass is responsible for >10% of the total carbon emissions in the U.S. annually.
The truth is that low-e glass thermal performance has reached practical limits. A low-e coating reflects heat, reducing heat transfer between panes of glass, and thereby improving insulation performance. The “e” in low-e stands for emissivity—the ability of a surface to radiate energy. Low-e coatings are rated for the amount of heat they radiate—the lower the number, the less heat is 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 window performance. Clearly, further improvements in glass thermal performance will not come from improvements in low-e coatings. Low-e coated glass has become a minimum performance baseline and no longer represents a path to improved energy performance.
Generic low-e insulating glass, consisting of two pieces of coated glass separated by a sealed, gas filled air space (or cavity), achieves a maximum thermal insulation value of R-4. With further advances in glass coating technology expected to provide minimal performance improvement, the focus has now shifted from coatings to cavities. Just as the introduction of single cavity insulated glass provided a breakthrough in performance beyond monolithic glass, the introduction of multi-cavity constructions, consisting of two, or even three, insulating cavities, is providing the next performance breakthrough for insulating glass.
Two alternatives to generic low-e insulating glass are currently available that can meet Energy Star’s proposed Phase2 window performance standards scheduled to debut as early as 2013. One is triple pane glass, consisting of three panes of glass and two low-e coatings. By using a third pane of glass to create a second insulating cavity, triple pane low-e glass improves generic low-e insulating glass performance from R- 4 to R-9. The bad news is that triple pane glass is 50% heavier than standard insulating glass, requiring stronger window framing and increasing cost accordingly.
An alternative consists of suspending a low emissivity and solar reflective film inside of an insulating glass unit. Without the weight disadvantages of a third pane of glass, film can create two, three, or even four insulating cavities that maximize light transmission and provide conservation performance ranging from R-6 up to R-20.
Such internally mounted film does not replace low-e glass. It leverages the benefits of film based and glass based technologies to create a lightweight, multi-cavity insulating glass that offers a new level of performance. Most units fabricated today use low-e coated glass to minimize solar heat gain, while using film to maximize insulation performance, block UV radiation, reduce noise, and increase occupant comfort more effectively than low-e glass alone.
Film based, multi-cavity insulating glass has been saving energy in such landmark buildings as the Audubon Association headquarters in New York City, the Rotch Library at MIT, the Hoover Dam Visitor Center, and the Museum of Flight in Seattle.