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

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March 8, 2012

Research by scientists from Columbia University and NASA suggests that painting rooftops w...

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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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 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

About the Author
James Holloway James lives in East London where he punctuates endless tea drinking with freelance writing and meteorological angst. Unlocking Every Extend Extra Extreme’s “Master of Extreme” achievement was the fourth proudest moment of his life.   All articles by James Holloway
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19 Comments

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. :-)

Tommygun
8th March, 2012 @ 10:42 am PST

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.

Todd Dunning
8th March, 2012 @ 12:27 pm PST

How about white roads... those take up a lot of space and most are black

Theo Viljoen
8th March, 2012 @ 01:30 pm PST

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.

The Hoff
8th March, 2012 @ 07:05 pm PST

re; Theo Viljoen

And with catalytic cracking a waste of perfectly good oil.

Slowburn
8th March, 2012 @ 07:11 pm PST

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???

Foxy1968
8th March, 2012 @ 07:21 pm PST

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.

fjbremner
8th March, 2012 @ 08:15 pm PST

@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.

joeblake
8th March, 2012 @ 11:35 pm PST

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!

ukrauskopf
9th March, 2012 @ 08:23 am PST

People have been doing that for years on Airstream trailers. Factory does it now.

SkipJ

SKipJ
9th March, 2012 @ 08:30 am PST

It has long been known that white reflects solar heat. The down side is that the heat will rise and lift the clouds higher. In a mild wind the clouds will move away taking with them a chance for rain. Here in Florida there have been entire subdivisions with white roofs and the rain fall is much less in these areas. Large trees around each house can help add the chance for rain.

donwine
9th March, 2012 @ 09:12 am PST

Excellent for reducing heat and therefore cooling costs within the home, etc. However, it is misleading nonsense to say this heat reflection is being sent into "space" and therefore would reduce the heat envelope of heavy population areas, e.g., cities. The heat is being reflected back into the local atmosphere - not space. To sum it up. Yes, for reducing heat within structures. No, for reducing local atmosphere heat build up, in fact the opposite.

coolfire
9th March, 2012 @ 10:13 am PST

re; The Hoff

Speaking of ignoring science such as making graphs that have been proven to be fraudulent (the hockey stick) conspiring to hide evidence (hide the decline) Claiming to have destroyed records and there for unable to prove they ever existed. (East Anglia Climatic Research Unit) Extortion to keep other studies out of peer reviewed journals.

Slowburn
9th March, 2012 @ 11:58 am PST

"The heat is being reflected back into the local atmosphere - not space. ... in fact the opposite. "

So that's why it is so hot in the winter here, all that winter sun radiating back up into the atmosphere? How do you figure??

James M. Van Damme
9th March, 2012 @ 12:49 pm PST

Great, with one major consideration. The production of titanium dioxide, the white components in both acrylic reflective paint and white EPDM or TPO roofing products is TERRIBLE for the environment.

Great to see we are thinking about this, why not take it a step further and create a 'full circle green" solution?

misty_gonzalez
9th March, 2012 @ 02:14 pm PST

Check out surfacestations.org 1007 of 1221 of the USHCN temperature monitoring stations in the USA have been surveyed by this volunteer project.

The results are appalling. The condition of over 92% of them is way outside the specifications established to ensure accurate temperature measurement.

"Favorite" inaccurate locations for the instruments are next to or in the middle of parking lots and other paved areas, over gravel, next to masonry buildings, on top of buildings with black roofs, near wastewater treatment pools and close to the warm exhaust air from air conditioners.

All of those are things which cause the air nearby to be warmer through reflection, re-radiation and conduction of absorbed heat to the air. Reflective surfaces bounce heat directly at the sensors. Many have been found by the edges of parking lots where heat from vehicle engines can radiate directly at them.

A very small number have been found sited in locations that would give inaccurate readings less than the ambient temperature.

What started this project was when meteorologist Anthony Watts set up some Stephenson screens to test the effect of the change from whitewash to white latex paint on the temperatures inside them. That specification change was made in the late 1970's, right around when the "global warming" thing got going. Watts set up three screens in the same location, one whitewashed, one with white latex paint and a third bare wood to represent a poorly maintained screen with its coating weathered away.

To gather information on how some currently active sensors were sited, he first visited the sensor at the fire station close to his home. He found it had been upgraded to an automated system in a plastic enclosure on a pole. (MMTS) Not a problem, except that it was right next to the sidewalk and parking lot, between a very shiny flagpole and the two AC exhausts of a building housing computers and other electronic equipment. No way at all that sensor could be recording accurate ambient temperatures.

As he examined more sites and dug into the histories of them he found more poorly located ones and discontinuities where sites were relocated but continued in the records as if they were in their old locations.

That's when he started the Surface Stations project to survey all of them. NOAA wouldn't do it, claiming they didn't have the money or people to do such a survey.

The initial hope was that the bad sites would be few but reality is the good sites are very few in number.

What's resulted is that the United States Historical Climate Network is in a very bad state and has been for a long time. The data from the majority of the stations has been erroneously high for years to decades. On top of that the change from whitewash to white paint introduced its own slight upward error, though some poorly maintained Stephenson screens never were repainted before being replaced by MMTS units, without correcting any of the site's deficiencies.

Gregg Eshelman
9th March, 2012 @ 02:35 pm PST

@Donwine

Large trees around each house can help add the chance for rain.

It's not just to do with temperature. Trees also transpire moisture from the soil into the atmosphere, increasing the relative humidity, which also raises the possibility of precipitation.

joeblake
9th March, 2012 @ 04:46 pm PST

White roofs (the original subject) are generally a good idea, but not always. For example, in northern climes where the number of cool days (requiring heating) compared to warm days (requiring A/C) is higher, a white roof may not save much energy and actually create problems when there is a need for solar gain on the roof to help melt snow (been there, done that). But in the example given the ratio of building cube to roof square footage is high, and the amount of internal heat generation (lights, computers, people, etc.) is high, and so a white roof makes sense. Light colored roads would help reduce the heat island effect also, but concrete is much more expensive, although quite durable, compared to asphalt. Once the binding emulsion is worn off the surface of asphalt a light-colored aggregate gets the road to a pale to medium grey.

Bruce H. Anderson
12th March, 2012 @ 07:56 am PDT

Twenty-five years ago I painted my roof white in Sacramento where the summers are triple digit for months. The paint had ceramic particles to help insulate and fireproof. It was very expensive but worked well. My point is: This is a very old technique. I wonder how much money the study cost to re-invent the wheel?

voluntaryist
16th March, 2012 @ 04:39 am PDT
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