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Bright Is The New Black: New York Roofs Go Cool

By Climate Guest Contributor on March 8, 2012 at 11:04 am

"Bright Is The New Black: New York Roofs Go Cool"

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Even the least expensive white roof coating reduced peak rooftop temperatures in summer by an average of 43°F

by Patrick Lynch, re-posted from NASA

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.

Midtown Manhattan skyline

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 degrees Fahrenheit. 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 degrees Fahrenheit 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 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.

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 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 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.”

Patrick Lynch is with NASA’s earth science news team. This piece was originally published at NASA.

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7 Responses to Bright Is The New Black: New York Roofs Go Cool

  1. Rabid Doomsayer says:

    The highest albedo whites usually contain titanium dioxide. For a new roof the cost is effectively zero.

    OK so the Asian market does not like whites, but do have to stick with black looks soooo good? Gold is fairly reflective and many like the look of that. How about great globs of chrome plated metal?

    Sorrry for my cynicism is is a very good, well worthwile, project.

  2. George D says:

    What’s the effect on energy use in winter?

    • Joe Romm says:

      About 1/10 that of summer because winter days are shorter, sun is lower, and it’s cloudier.

  3. nyc-tornado-10 says:

    An important consideration is that new york has cold winters, and the heating season is usually 7 months, cooling is just 3 months. Going completely cool, or white, may reduce heat gain in winter, when it will offset heating costs. Winter also has less sun, which limits the effective heating of the sun, but february and march are significant heating months and have decent sun.

    Most roof resurfacings today use materials that reflect 30% of sunlight, which for now may be a good compromise, and in the future, these roofs can easily be painted white – cool. Even the 30% makes a significant difference, compared to just 5% for blacktop.

    I also believe that a more reflective roof will increase the life of the roof, which reduces costs and resource consumption involved in resurfacing or replacing roofs.

    Another means of limiting solar heat gain on roofs is to install solar panels, which also have the advantage of producing electric!

  4. J4zonian says:

    One thing that would help is some of the materials Day Chahroudi and others were working on decades ago, in the appropriate technology movement. Substances or structures like louvres or photogray lenses that open, close or change color according to light intensity or temperature, are examples.

    Combining these, as a roof’s roof, (or sunshade) with solar collection, water collection, food production (gardens, orchards, bee hives, egg layers (quail, ducks, chickens) even meat production (rabbits, pigeons)) could not only increase albedo but reduce water shortages, food miles, agricultural energy use, etc. Combining these systems, so animals living in the summer shade and winter sun provide fertilizer for the garden and the animals live partly on weeds and food scraps, for example, would provide a multiplier effect.

    According to Permaculture Activist Magazine, NYC and Philadelphia alone have over 100,000 acres of rooftops that might be used for various mixes of these.

  5. fj says:

    NYC solar empowerment zones have been declared as part of the process to rapidly accelerate deployment of solar photovoltaic especially on large commercial roofs such those on warehouses.