On the hottest day of the New York City summer of 2011, some traditional black roof surfaces reached 170 F, while an experimental “white roof” came in at a relatively cool 128. The details are in the first study to systematically measure the effects of the city’s fledgling effort to introduce more reflective rooftops in order to reduce cooling costs and the overall heat burden on the city. The roofs are being tested as part of Mayor Michael Bloomberg’s effort to reduce the city’s greenhouse gas emissions 30 percent by 2030. The new study, published today in the journal Environmental Research Letters, was done by scientists at the NASA Goddard Institute for Space Studies and Columbia University’s Center for Climate Systems Research.
July 22, 2011, set a city record for electricity usage during the peak of a heat wave. On average, through summer 2011, one pilot white roof surface reduced peak rooftop temperature by 43 degrees F.
The urban landscape of asphalt, metal and dark buildings absorbs more heat than forests, fields, or landscapes covered by snow or ice, which tend to reflect energy. This leads to a so-called “urban heat island,” where a city sees markedly warmer temperatures than surrounding regions, said Stuart Gaffin, the Columbia scientist who led the study. In recent years, city planners worldwide have discussed converting dark roofs to either “green” roofs covered in living plants or to white roofs, which are a far less expensive option. Those tested in this study included two synthetic membranes requiring professional installation and a do-it-yourself white-paint coating being promoted by the city’s white roof initiative.
“Cities have been progressively darkening the landscape for hundreds of years. This is the first effort in New York to reverse that. It’s an ambitious effort with real potential to lower city temperatures and energy bills,” said Gaffin. “City roofs are traditionally black because asphalt and tar are waterproof, tough, ductile,and were easiest to apply. … But from a climate and urban heat island standpoint, it makes a lot of sense to install bright, white roofs.”
With climate change, the urban heat island problem will likely intensify in coming decades, said Cynthia Rosenzweig, a scientist at Columbia and the Goddard Institute, and a coauthor on the paper. “Right now, we average about 14 days each summer above 90 degrees in New York. In a couple decades, we could be experiencing 30 days or more,” Rosenzweig said.
The study found similar temperature reduction when all the surfaces were first installed, but that the professionally installed membranes maintained their reflectivity—how much light a surface immediately reflects skyward—better over time.
The study also looked at how emissivity held up. (Emissivity measures how much infrared radiation a surface emits after absorbing solar radiation.) Both the reflectivity and emissivity of the professionally installed white membrane coverings (which cost about $15 to $28 per square foot) held up remarkably well after even four years in use. These surfaces continued to meet Energy Star standards, set by the EPA’s Energy Star Reflective Roof program. The effectiveness of the white coating (about 50 cents per square foot) was about cut in half after two years, ultimately falling below the Energy Star standard. However, Gaffin said, the low-cost surface still improved reflectivity markedly over black asphalt roofs.
“It’s the lowest hanging fruit. It’s very cheap to do; it’s a retro-fit. You don’t need a skilled labor force. And you don’t have to wait for a roof to be retired,” said Gaffin. referring to the DIY acrylic method. “So if you really talk about ways in which you brighten urban albedo, this is the fastest, cheapest way to do it.”