Buildings’ Weak Link: Energy Conservation Through Windows
- Jun 03, 2009
When it comes to saving energy, windows constitute the weak link for buildings. Despite heavily insulated walls and ceilings and the popularity of low-e glass, 25 to 35 percent of the energy used in buildings and homes is wasted due to inefficient glass. In fact, it should come as no surprise that glass is responsible for greater than 10 percent of the total carbon emissions in the United States annually and is a major contributor to global warming.New technologies, however, can improve this picture—and they are acknowledged in initial revisions to the Department of Energy’s Energy Star® window performance standards, which will be effective Jan. 1, 2010, making clear the fact that generic low-e glass no longer represents the level of energy efficiency required to “transform the market,” a key charter of the agency’s Energy Star program.Because generic low-e glass provides insulating performance of about R-4 in a world in which R-19 insulated walls are the norm, there is a dramatic performance gap between what low-e glass provides and what green building practices promise in saving energy and reducing carbon emissions. R values measure a window or a wall’s ability to resist heat transfer. The higher the R value, the better the insulating performance.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, which stands for “emissivity,” is the ability of a surface to radiate energy. Low-e coatings are rated for the amount of heat they radiate — the lower the U value, the rate of heat transfer through the glass., 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 to low-e coatings. Low-e coated glass has become a minimum performance baseline and no longer represents a path to improved energy performance–the incremental performance benefit of using low-e glass is zero, because it is already assumed as a required product. With further advances in glass coating technology expected to provide minimal performance improvement, the focus has now shifted from coatings to cavities. Generic low-e insulating glass, for instance, consists of two pieces of coated glass separated by a sealed, gas-filled air space, or cavity. But it achieves a maximum thermal insulation value of R-4. However, new alternatives are becoming available. 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. There are currently two alternatives to generic low-e insulating glass available that can meet Energy Star’s proposed Phase 2 window performance standards, which are scheduled to debut as early as 2013. 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 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 percent heavier than standard insulating glass, requiring stronger window framing and increasing cost accordingly. A superior 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 to an amazing R-20 to meet the unique requirements of both commercial and residential new construction and renovation projects. 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 utilize 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. Clearly, film-based, multi-cavity insulating glass is tomorrow’s state-of-the-art window glass available today. It has been saving energy in such notable structures as the One Park Center office building in San Francisco, the William Morris Building in Beverly Hills and Kuwait Tower in Kuwait City, Kuwait. Bruce Lang is vice president of marketing & business development at film and glass manufacturer Southwall Technologies Inc.