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Published on March 12th, 2012 | by Zachary Shahan

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White Roofs Cool the Heck out of NYC Apartments

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March 12th, 2012 by Zachary Shahan
 
 
white roof

The last time I wrote on white roofs, the finding was that they could actually cause a net cooling. But the study didn’t take into account how much the cooling of the buildings reduced electricity demand. Looks like those electricity savings can be huge from a new study on the matter by a Columbia University scientist and others.

First, from the former white roofs study led by Mark Z. Jacobson, a professor of civil and environmental engineering at Stanford University:

Jacobson’s computer modeling concluded that white roofs did indeed cool urban surfaces. However, they caused a net global warming, largely because they reduced cloudiness slightly by increasing the stability of the air, thereby reducing the vertical transport of moisture and energy to clouds. In Jacobson’s modeling, the reduction in cloudiness allowed more sunlight to reach the surface.

The increased sunlight reflected back into the atmosphere by white roofs in turn increased absorption of light by dark pollutants such as black carbon, which further increased heating of the atmosphere.

Now, the new study results: “A new study of how different white roofing materials performed “in the field” in New York City over multiple years found that even the least expensive white roof coating reduced peak rooftop temperatures in summer by an average of 43°F. If white roofs were implemented on a wide scale, as the city plans to do, this reduction could cut into the “urban heat island” effect that pumps up nighttime temperatures in the city by as much as 5 to 7°F in the summer, said the study’s lead scientist, Stuart Gaffin of Columbia University.”

Looks promising.

Here’s a full announcement on the new study from NASA’s Goddard Institute for Space Studies (GISS) Earth Science Division (the remainder of this post):

On the hottest day of the New York City summer in 2011, a white roof covering was measured at 42°F 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.

roofs effect temperature

A new study of how different white roofing materials performed "in the field" in New York City over multiple years found that even the least expensive white roof coating reduced peak rooftop temperatures in summer by an average of 43°F. If white roofs were implemented on a wide scale, as the city plans to do, this reduction could cut into the "urban heat island" effect that pumps up nighttime temperatures in the city by as much as 5 to 7°F in the summer, said the study's lead scientist, Stuart Gaffin of Columbia University. (Image credit: Patrick Theiner, Creative Commons)

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° 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°, 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°F relative to what they would be without the effect, according to Gaffin’s previous research.

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 surface temperature of the two during a period of June-August 2011. The white surface here was the acrylic paint coating promoted by the NYC CoolRoofs program. Credit: Gaffin et al.

The problem 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. The options tested in this study included two synthetic membranes requiring professional installation and a do-it-yourself (DIY), white-paint coating that is 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 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.’”

With climate change, the urban heat island problem will likely intensify in coming decades, said Cynthia Rosenzweig, a scientist at NASA’s Goddard Institute for Space Studies in New York City and a co-author on the paper.

“Right now, we average about 14 days each summer above 90° 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 better over multiple years.

The fraction of incoming solar radiation reflected skyward determines what is called a surface’s albedo. The citywide program is in effect an “albedo enhancement” program. 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.

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 (which only costs 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 improved albedo markedly over typical 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.”

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.”

Reference

Gaffin, S.R., M. Imhoff, C. Rosenzweig, R. Khanbilvardi, A. Pasqualini, A.Y.Y. Kong, D. Grillo, A. Freed, D. Hillel, and E. Hartung, 2012:Bright is the new black — multi-year performance of high-albedo roofs in an urban climateEnviron. Res. Lett.7, 014029, doi:10.1088/1748-9326/7/1/014029.

Top Photo:  White Roof by Walmart Stores (h/t Green Building Elements)

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About the Author

is the director of CleanTechnica, the most popular cleantech-focused website in the world, and Planetsave, a world-leading green and science news site. He has been covering green news of various sorts since 2008, and he has been especially focused on solar energy, electric vehicles, and wind energy since 2009. Aside from his work on CleanTechnica and Planetsave, he's the founder and director of Solar Love, EV Obsession, and Bikocity. To connect with Zach on some of your favorite social networks, go to ZacharyShahan.com and click on the relevant buttons.



  • Tom

    A reduction of 42 degrees on the roof is a reduction of 2/3′s of the load for cooling the building to a comfortable 70 degree. Do I have that part right?

  • Tom G.

    Maybe I missed it but didn’t Professor Jacobson’s study say white roofs result in global warming? Seems to me that either the original study was flawed or this study conflicts with Mr. Jacobson’s study.

    Which one is correct and what course of action should we be telling people to take?

    • Ross

      That also seems to contradict the cooling effect from the earth’s high albedo ice caps. Perhaps its something to do with the higher humidity and air pollution over New York.

    • Bob_Wallace

      “One “geoengineering” proposal for reducing the impact of urban heat islands is to paint roofs worldwide a reflective white. Jacobson’s computer modeling concluded that white roofs did indeed cool urban surfaces. However, they caused a net global warming, largely because they reduced cloudiness slightly by increasing the stability of the air, thereby reducing the vertical transport of moisture and energy to clouds. In Jacobson’s modeling, the reduction in cloudiness allowed more sunlight to reach the surface.

      The increased sunlight reflected back into the atmosphere by white roofs in turn increased absorption of light by dark pollutants such as black carbon, which further increased heating of the atmosphere.

      Jacobson’s study did not examine one potential benefit of white roofs – a reduced demand for electricity to run air conditioning in hot weather. But a recent study done at the National Center for Atmospheric Research showed that the decrease in air conditioning use, which occurs mostly in the summer, might be more than offset by increases in heating during winter months.

      “There does not seem to be a benefit from investing in white roofs,” said Jacobson. “The most important thing is to reduce emissions of the pollutants that contribute to global warming.”

      ….

      “Between 2 and 4 percent of the gross global warming since the Industrial Revolution may be due to urban heat islands,” said Mark Z. Jacobson , a professor of civil and environmental engineering who led the study. He and graduate student John Ten Hoeve compare this with the greenhouse gas contribution to gross warming of about 79 percent and the black carbon contribution of about 18 percent.”

      http://news.stanford.edu/news/2011/october/urban-heat-islands-101911.html

      Here’s the study if you wish to read it…

      http://www.stanford.edu/group/efmh/jacobson/Articles/Others/HeatIsland+WhiteRfs0911.pdf

  • Pg

    Is there a wintertime counter effect? Do the top floor apartments get colder in the winter, due to the lack of heat trapped by the roof?

    • James Van Damme

      Yes, because they’re covered with snow. So you might as well paint them white.

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