One hundred seventy degrees Fahrenheit is the approximate temperature to which chicken should be cooked. It’s also the temperature that was recorded on some asphalt roofs in New York City last July during a heat wave that set a record for electricity use.
Because they absorb sunlight, dark roofs, dark buildings and dark streets and sidewalks make cities especially sweaty, a phenomenon known as the urban heat island effect. New York City can often be up to 5 degrees hotter than surrounding areas. So one of the simplest ways to cool cities, lower electricity usage and reduce the city’s carbon footprint is to make rooftops white, ensuring that they reflect heat rather than absorb it.
In a paper published online this week in the journal Environmental Research Letters, a team of scientists from NASA and Columbia University’s Earth Institute present results from the first long-term study of the performance of white roofing material in New York City.
They monitored three rooftops in New York, each of which had a different kind of white roofing. A rooftop at the Con Edison Learning Center in Long Island City, Queens was clad in E.P.D.M., or ethylene propylene diene monomer, a type of membrane roofing; a rooftop at the Queens Botanical Garden in Flushing showcased TPO, or thermoplastic polyolefin, another membrane option.
Finally, an asphaltic membrane on a rooftop at MoMA P.S. 1 in Long Island City was coated with a white acrylic paint, a strategy promoted under the city’s NYC CoolRoofs initiative, part of Mayor Michael R. Bloomberg’s plan to reduce the city’s greenhouse gas emissions by 30 percent by 2030.
The white roofs performed similarly in reducing temperatures by as much as 43 degrees Fahrenheit. However, the TPO rooftop and E.P.D.M. rooftop held up better over time, meeting standards set by the Environmental Protection Agency’s Energy Star reflective roof programthroughout the study period. The painted roof’s reflectivity degraded more rapidly and failed to meet these standards after two years.
According to the lead author, Stuart Gaffin, an associate research scientist at the Center for Climate Systems Research at Columbia University, the acrylic paint option is an effective, inexpensive do-it-yourself option for buildings with newer roofs that don’t yet need to be replaced. The paint only costs about 50 cents per square foot, while the professional synthetic option can run anywhere from $15 to $28 per square foot.
If a roof needs to be replaced, these synthetic options won’t cost any more than traditional roofing, but some special white paint can deliver similar results in the short term, the researchers said.
“To combat climate change, there’s no question that we need to increase the earth’s albedo,” or reflectivity, Dr. Gaffin said. “Some scientists are contemplating tinkering with the earth’s atmosphere to do this, but we shouldstart with what’s easy and uncontroversial — white roofs.
“We can do it now,” he said, “and it will have an immediate effect.”