April 16, 2013
White Roofs Not Always Green
White roofing systems have been the system of choice because it was believed that they reduce global warming
From LEED, the Cool Roof Rating Council, the ENERGY STAR program, all the way up to the U.S. Department of Energy, there is widespread belief that white reflective roofing systems, even on buildings in northern cities like New York and Chicago, are more efficient and cost-effective than dark roofing. Based on studies done at the Lawrence Berkeley Laboratories, for the past 15 years white roofing systems have been the system of choice because it was believed that they also reduce global warming and the “heat island effect,” in which dark-colored building materials raise the ambient temperature in urban areas by a few degrees in summer. But architects, engineers, building owners, and roof system designers, who were not consulted on the Lawrence Berkeley studies, are finding that reflective membranes are not always a panacea for energy savings. Moreover, a study done at Stanford University, which uses the latest advances in atmospheric computer modeling, shows that white roofs may actually increase, not decrease, the Earth’s temperature. White roof membranes have high reflectivity that directs heat upward into the atmosphere and then mixes with black and brown soot particles, which are thought to contribute to global warming. Other studies show that white roofs increase average space heating use more than they decrease average air conditioning use in northern climates. Since owners have to spend more money heating their buildings with a white roof, they must consume more natural resources, thus increasing global warming. White roofs in colder climates are more prone to developing condensation, which leads to lower wind resistance, lower insulation values and the formation of mold. This problem is especially likely for under-insulated cool roofs in cooler climates. A cooler surface also makes it more likely for stagnant water to accumulate on white roofs. When not drained properly, membrane soiling, loss of reflectivity, and algae growth are likely on reflective roofs. The Superdome in New Orleans, which features a reflective roof, requires regular cleanings at a price tag of nearly $100,000 in order to maintain a clean, reflective surface and inhibit mold and mildew accumulation. Snow and ice can also build up on white roofs, which can damage roof components and pose dangers to workers on the roof and people below. And white roofs don’t have anywhere near the long-term ultra-violet and natural weathering resistance of non-reinforced black EPDM and are not easily repaired at the end of their service life.
Click here for the interactive map and view the energy efficiency of white roofs.
The U.S. Green Building Council, Cool Roof Rating Council, legislative bodies and the U.S. Department of Energy are encouraged to reevaluate their stance on reflective roofing. Energy-efficient roofing is a complicated issue, and no one material holds all the answers. White reflective roofing certainly has its place, which is primarily in southern climates. But as a green solution in northern states, it’s just not that cool.
Samir Ibrahim is director of design services for Carlisle SynTec Systems, a Carlisle, Pa. manufacturer of both white reflective and black roofing systems.