White roofs reduce urban heat island effect

Black roofs have been the norm for commercial buildings for decades since early roofs used a tar-coated paper material and tar based coatings to provide water proofing. Black roofs also add heat, which in the winter, is not a bad thing for the building.  Black roofs also contribute to the urban heat island effect. This effect, caused not only by black roofs, creates warmer temperatures in urban areas as compared to the surrounding suburbs. A study by Stuart Gaffin of Columbia University in New York looked at the effect of roof color on temperatures. On the hottest day of the New York City summer in 2011, a white roof covering was measured at 42 degrees Fahrenheit cooler than the traditional black roof it was being compared to, according to a study including NASA scientists that details the first scientific results from the city's unprecedented effort to brighten rooftops and reduce its "urban heat island" effect. The dark, sunlight-absorbing surfaces of some New York City roofs reached 170 degrees Fahrenheit on July 22, 2011, a day that set a city record for electricity usage during the peak of a heat wave. But in the largest discrepancy of that day, a white roofing material was measured at about 42 degrees cooler. The white roof being tested was a low-cost covering promoted as part of Mayor Michael Bloomberg's effort to reduce the city's greenhouse gas emissions 30 percent by 2030. On average through the summer of 2011, the pilot white roof surface reduced peak rooftop temperature compared to a typical black roof by 43 degrees Fahrenheit, according to the study, which was the first long-term effort in New York to test how specific white roof materials held up and performed over several years. Widespread installation of white roofs, like New York City is attempting through the NYC CoolRoofs program, could reduce city temperatures while cutting down on energy usage and resulting greenhouse gas emissions, said Stuart Gaffin, a research scientist at Columbia University, and lead author on a paper detailing the roof study. The paper published online Mar. 7, 2012, in Environmental Research Letters. The urban landscape of asphalt, metal, and dark buildings absorbs more energy from sunlight than forests, fields or snow- and ice-covered landscapes, which reflect more light. The absorption leads to what scientists call an "urban heat island," where a city experiences markedly warmer temperatures than surrounding regions. New York City's urban heat island has a more pronounced effect at night, typically raising nighttime temperatures between 5 and 7 degrees Fahrenheit relative to what they would be without the effect, according to Gaffin's previous research.