New York City’s concerted effort to be cooler is showing results.
We’re talking temperatures, as the Big Apple certainly isn’t known to suffer from the notion that it’s not the most hip city here in the U.S.—or anywhere else for that matter.
Patrick Theiner, Creative Commons
|The study found that even a low-cost white roof coating reduced peak rooftop temperatures in summer by an average of 43 F.|
Getting cooler is the goal of the city’s campaign to turn dark-colored rooftops white with the use of light-colored, solar-reflective coatings. And according to a study by Columbia University and NASA, there’s no question it’s cool to be white.
“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,” NASA said in an announcement on the study.
NASA said the study provides 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 F 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.
Gaffin et al.
|This comparison of white and black roof temperatures at a test site on top of the Museum of Modern Art in Queens reveals the consistent discrepancy between the two during the June-August 2011 period. The white surface was produced with an acrylic paint coating promoted by the NYC CoolRoofs program.|
The white roof being tested was a low-cost covering promoted as part of Mayor Michael Bloomberg’s campaign to reduce the city's greenhouse gas emissions 30% by 2030. The city’s program has been the subject of news reports in D+D News; see Cool Roofs Season Two Opens in NYC and Cool Million: On NY Roofs, White is the New Green.
On average through the summer of 2011, the pilot white roof surface reduced peak rooftop temperature compared to a typical black roof by 43 F, 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, NASA said.
Stuart Gaffin, a research scientist at Columbia and lead author of a paper detailing the roof study, said the research offers evidence that 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.
|In 2010, New York Mayor Michael Bloomberg joined city officials and volunteers at an event marking the one-millionth square foot of white reflective coating applied to rooftops in the city.|
The study, however, also strongly recommends the adoption of maintenance and washing programs to help retain solar reflectivity of roof surfaces painted with white, acrylic coatings.
The paper was published online Mar. 7, 2012, in Environmental Research Letters; see Bright is the New Black.
Long-Term Reflectance Examined
In addition to measuring rooftop surface temperature, the study also looked at how the reflectivity and emissivity of the white surfaces held up over time. Reflectivity measures how much light a surface immediately reflects skyward. Emissivity measures how much infrared radiation a surface emits after absorbing solar radiation.
The study found similar temperature reduction when all the white surfaces were first installed, but that the professionally installed membranes maintained their reflectivity better over multiple years.
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, the study found. These surfaces continued to meet standards set by the EPA's Energy Star Reflective Roof program.
The study indicated that the effectiveness of the white coating (which only costs about 50 cents per square foot) was approximately cut in half after two years, ultimately falling below the Energy Star standard. Gaffin said, however, that the low-cost surface improved “albedo” markedly over typical black, asphalt roofs. The fraction of incoming solar radiation reflected skyward determines what is called a surface's albedo
“It's the lowest hanging fruit. It's very cheap to do; it's a retro-fit,” Gaffin said of application of white coating. “You don't need a skilled labor force. And you don't have to wait for a roof to be retired.
“So if you really talk about ways in which you brighten urban albedo, this is the fastest, cheapest way to do it.”
Study data also showed a marked spike in albedo following the May 2011 recoating of the white-painted roof of the Museum of Modern Art in Queens.
New York City’s urban heat island has a more pronounced effect at night, typically raising nighttime temperatures between 5 and 7 F relative to what they would be without the effect, according to previous research by Gaffin. The heating leads to everything from spikes in electricity usage and greenhouse gas emissions to poorer air quality and increased risk of death during heat waves.
In recent years, city planners worldwide have discussed cutting into this effect by converting dark roofs to either “living” roofs covered in plants or to white roofs, the far less expensive option.
“Cities have been progressively darkening the landscape for hundreds of years. This is the first effort in New York to reverse that,” Gaffin said. “It's an ambitious effort with real potential to lower city temperatures and energy bills.
“City roofs are traditionally black because asphalt and tar are waterproof, tough, ductile and were easiest to apply to complex rooftop geometries. But from a climate and urban heat island standpoint, it makes a lot of sense to install bright, white roofs. That's why we say, ‘Bright is the new black.’”
In the study’s concluding remarks, the authors suggest that additional research into key factors leading to loss of reflectance of acrylic-painted roof surfaces “will be important for improving the technique and should be undertaken.”
In addition, the authors say the roof-coating program in the city “would greatly benefit from improved maintenance guidelines, including best washing methods,” since frequent recoating is not likely to be an economical option for building owners
Climate Change Adds to Cool-Roof Urgency
Scientists say that with climate change, the urban heat-island problem will likely intensify in coming decades.
“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,” said Cynthia Rosenzweig, a scientist at NASA’s Goddard Institute for Space Studies in New York City and a co-author of the paper on the white-roof study.
NASA studies the urban heat island effect to better understand and model how urban surfaces and expanding urbanization might impact regional and global climate, said Marc Imhoff, a biospheric scientist at NASA Goddard Space Flight Center, Greenbelt, Md.
"We're trying to build a capability where we can expand our knowledge with data on more locations, and ultimately develop computer models that would allow us to predict urban heat islands and urban temperatures on a town level," Imhoff said. "Eventually, we could incorporate our findings into large-scale, global climate models."
More information on New York City’s CoolRoofs program: NYC °CoolRoofs.