Environment

New York rooftops 40 degrees cooler painted white, reckons NASA/Columbia study

New York rooftops 40 degrees c...
Research by scientists from Columbia University and NASA suggests that painting rooftops white can result in temperature drop of over 40 degrees Fahrenheit during summer months (Photo: Erik Drost)
Research by scientists from Columbia University and NASA suggests that painting rooftops white can result in temperature drop of over 40 degrees Fahrenheit during summer months (Photo: Erik Drost)
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A comparsion of dark and white surface temperatures over the New York summer of 2011 shows a marked reduction in surface temperature (Image: Stuart Gaffin/Columbia University)
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A comparsion of dark and white surface temperatures over the New York summer of 2011 shows a marked reduction in surface temperature (Image: Stuart Gaffin/Columbia University)
The white acrylic paint surface next to a black asphaltic control membrane on a Long Island rooftop (Image: Stuart Gaffin/Columbia University)
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The white acrylic paint surface next to a black asphaltic control membrane on a Long Island rooftop (Image: Stuart Gaffin/Columbia University)
A comparsion shot showing albedo-loss of the acrylic painted surface over one year, before being relocated (Image: Stuart Gaffin/Columbia University)
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A comparsion shot showing albedo-loss of the acrylic painted surface over one year, before being relocated (Image: Stuart Gaffin/Columbia University)
Research by scientists from Columbia University and NASA suggests that painting rooftops white can result in temperature drop of over 40 degrees Fahrenheit during summer months (Photo: Erik Drost)
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Research by scientists from Columbia University and NASA suggests that painting rooftops white can result in temperature drop of over 40 degrees Fahrenheit during summer months (Photo: Erik Drost)

It's long been suggested that white rooftops could help reduce the heat bubble microclimates that surround our cities simply by reflecting solar radiation directly back into space, and in 2010 we reported on NCAR efforts to demonstrate the effect through computer modeling. A new study goes one better, putting the theory into practice and pitting three white materials against one another on three New York rooftops. The results of the study appear to be overwhelmingly positive, with white roof coatings reducing peak rooftop temperatures in summer "by an average of 43 degrees Fahrenheit (about 24 degrees C)."

It's hoped that the deployment of white roofs could help reduce the warming effect of urban microclimates that can see city temperatures several degrees higher than surrounding areas. The effect is thought in part due to the dark materials with which we build our cities, that reflect much less light than natural landscapes. New York's microclimate can result in nighttime temperatures 5 to 7 degrees Fahrenheit (3 to 4 deg. C) higher, according to preceding research by Stuart Gaffin of Columbia University, who conducted this study into roof surfaces with the assistance of NASA scientists.

The three materials tested were an ethylene-propylene-diene monomer (EPDM) rubber membrane, a thermoplastic polyolefin (TPO) membrane, and an asphaltic multi-ply built-up membrane coated with white elastomeric acrylic paint (white paint on a typical roof surface, in other words). The latter acrylic paint-coated membrane is a low-cost material being promoted by Mayor Michael Bloomberg in the CoolRoofs program - part of a drive to reduce New York's greenhouse gas emissions by 30 percent by 2030.

The surface temperatures of each surface were monitored against a black control surface using infrared radiometers. In addition, pyranometers were used to measure shortwave radiation - both arriving at the surface (the incident radiation) and that leaving the surface (reflected radiation), with which the surfaces' emissivity (its ability to radiate energy) could be determined.

A comparsion of dark and white surface temperatures over the New York summer of 2011 shows a marked reduction in surface temperature (Image: Stuart Gaffin/Columbia University)
A comparsion of dark and white surface temperatures over the New York summer of 2011 shows a marked reduction in surface temperature (Image: Stuart Gaffin/Columbia University)

Results indicated that temperature differentials between the white test surfaces and the black control surfaces were most marked on sunny days (well duh, you might say), but on average the test surfaces were, at daytime peak, found to be 42.5 degrees Fahrenheit (23.6 deg. C) cooler.

The report concludes that all three materials showed "very similar" performance, each with an albedo (a coefficient of reflectance) of about 0.65, in contrast to a typical albedo of 0.05 for near-black roof surfaces. A crucial caveat is that the acrylic paint surfaces tested were brand new, while the "professional" EPDM and TPO membrane installations had aged three and four years respectively. A significant downside of the acrylic paint is that its albedo halved over a two-year period.

The report claims that this proves that professional membranes are more effective in maintaining reflectance over time. That said, even accounting for the two-year performance drop of acrylic paint, the report finds that it still provides "a significant boost" over ordinary roof surfaces.

The paint boasts obvious economic advantages. No reroofing is required, and the paint can be applied by home-owners or at a cost of US$0.50/square foot ($5.38/sq m) through a volunteer organization.

At first glance the more expensive coatings fared less well in other metrics. The EPDM membrane had a lower emissivity than expected (and, indeed documented in the product specification). The report points out, however, that this may actually be an advantage in winter in respect of avoiding heat energy penalties.

The results were published in a report entitled Bright is the new black - multi-year performance of high-albedo roofs in an urban climate in Environmental Research Letters yesterday and can be downloaded free of charge. It's worth bearing in mind that a three-roof study in a single city is necessarily limited in scope, and it would be fascinating to see if a similar margin could be replicated in other cities. The report also highlights that other weather conditions, such as wind-speed, were not taken into account in this study.

Source: Environmental Research Letters via NASA

20 comments
Tommygun
What about in the cooler months? Then you might be actually wanting to take advantage of the higher roof temperatures from solar radiation. I use a system in my house (in Auckland, New Zealand) where warm air is extracted from the warm roof space, to help warm (and ventilate) the house (in the cooler months). It works well but the roof does get excessively hot (60+degrees C or 140+ degrees fahrenheit) in summer and I'm sure that contributes to the house temperature getting too hot in summer. It's a pity you can't have a white roof in summer and a dark one for winter. :-)
Todd Dunning
Global warming would have been believable if the Greenies had stuck to these sort of simple, inexpensive solutions that make clear and obvious sense. But nobody gets rich just painting rooftops. So instead we were told to send billions to African dictators and subsidize wind and other alternative energy boondoggles that enriched Al and half the UN...to keep half the earth's surface from flooding and preventing the deaths of billions by this time. Now that the Chicago Carbon Credits Exchange is closed and the windmills are shutting down, the enviros are left to ponder other 'sustainable' schemes to fund their retirements with.
Theo Viljoen
How about white roads... those take up a lot of space and most are black
The Hoff
As usual Todd has nothing to add but fear, hate and conspiracy theories that ignore science. If it's a hot day do you wear a dark shirt that makes you hotter, or a white one that keeps you cool? The white glaciers are disappearing and the world will get hotter, we're adults and we feel it is our duty to do what we can to slow this. If he knew how much money could be made in paint he would probably think this science is a conspiracy too.
Slowburn
re; Theo Viljoen And with catalytic cracking a waste of perfectly good oil.
Foxy1968
What a waste of energy. Why not use a solar capture system and utilise the power to help reduce the cost of running the buildings. Why would you want to throw away all that energy by directing it staight back into space???
fjbremner
Please take note, residents of Perth, Western Australia. When visiting in Nov 2011, I was told that it's trendy to have dark grey tiles on your roof. I saw lots of evidence of this. Presumably the air conditioning industry is faring well. Frank Bremner, Somerton Park SA.
joeblake
@Tommygun: I don't think the white roof in summer and a dark one for winter would be of much use since the dark roof would lose heat faster than a light one (via radiation) in cold weather. Polar bears have white fur for camouflage, but if it meant that they lost body heat faster then they would not have survived. With white tiles on your roof the heat generated within the house will be trapped in your ceiling space for you to harvest.
ukrauskopf
You are on the right track. now plant on all flatroofs and you really lower temperatur, reduce rain run off significantly, help reduce co2 and on and on use nature to bring your good idea to life!
SKipJ
People have been doing that for years on Airstream trailers. Factory does it now. SkipJ