Hot in the city, paint your roof white

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Hot in the city, paint your roof white

Postby zetreque on July 9th, 2014, 4:40 pm 

This seems like an obvious, but people are just now starting to implement white rooftops. lol

As part of a project known as Cool Roofs, volunteers in New York City have been painting black roofs white and have so far covered more than 500,000 square meters of roof, though that's less than one percent of the possible area. The U.S. Department of Energy suggests such reflective rooftops can keep a given roof 30 degrees C cooler than surrounding traditional rooftops. Even better, according to new research from Lawrence Berkeley National Laboratory, white roofs are the cheapest roofing option based on a study of 22 commercial roofing projects. The lab’s research confirms the Arizona findings that white roofs reduce global warming, proving three times more effective at countering climate change than even green roofs, thanks to all that reflected sunlight.

But the new research published in PNAS suggests that such white roofs would have different impacts in different places. So, in New York City any energy savings on air-conditioning in the summer are counterbalanced by increased heating usage in winter (although this can be addressed with optimal roof design or roofs with adjustable reflectivity). And white roofs can reduce precipitation as well, by reducing the amount of warm, humid air rising and, thus, the number of clouds and eventual rainfall. "Adaptation to urban-induced climate change depends on specific geographic factors," Georgescu adds, noting that white, reflective cool roofs work well in California, but could reduce rainfall from Florida up the U.S. east coast, for example. "What works over one geographical area may not be optimal for another," he says.


I'd think it would change precipitation down wind from the city, and not directly over it. And if the city was built over a cool and or green area anyway, I don't really see a difference. Actually I think that argument is ridiculous because the white rooftops will offset all the black streets, parking-lots, and sidewalks.

Green roofs may be a better fit for New York City, for example, because they provide better insulation during winter, along with cooling benefits in the summer. Water evaporation from the plants lowers overall temperatures—and releases more humidity into the air. And, they offer ancillary benefits like green space for weary urban minds.


But although expensive, the $5-million planted roof offers benefits beyond combating climate change, compared with white roofs. In New York City the rain falls at a pH of around 4.2—very acidic—but after filtered through a green roof it is 6.2 or higher (the higher the pH, the less acidic). The evaporative cooling provided by the roof also reduces local temperatures by around 2 degrees C.


It still seems very controversial with a lot more said on both sides of the argument.
Full read here:
http://www.scientificamerican.com/article/cool-roofs-might-be-enough-to-save-cities-from-climate-overheating/
http://www.asce.org/cemagazine/Article.aspx?id=23622330040#.U72nGvldUXt
If you live in NYC: be part of the project.
http://www.nyc.gov/html/coolroofs/html/home/home.shtml


I find the thought of reflective changing materials interesting. For example, when the sun is lower in the winter (colder), the surface side of the material the light falls on would be black, where in the summer when the sun is higher, the side surface of the material the sun falls on is white. Similar to a hologram where depending on the angle you view, you get a different surface.
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Re: Hot in the city, paint your roof white

Postby doogles on July 9th, 2014, 8:25 pm 

Good day to you Zetrique. Just a comment and a question.

You can experience the difference in heat absorption/reflectivity of black and white paints if ever you are in an unshaded car park in which a black and white car are parked adjacently. By placing a hand on each car at the same time you can detect a gross difference in the heat.

a question - What is the technology for attaining 'changing reflectivity of paints on the one roof?
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Re: Hot in the city, paint your roof white

Postby zetreque on July 9th, 2014, 10:31 pm 

Hello Doogles,

That is a good experiment for people to try with the cars parked in the sun.

About the changing reflective materials. I don't know. I just thought of that and sort of made it up as I wrote that post. But that is what came to mind when I read that part of the article. Maybe something like that already exists, or maybe we could invent it. :)

So, in New York City any energy savings on air-conditioning in the summer are counterbalanced by increased heating usage in winter (although this can be addressed with optimal roof design or roofs with adjustable reflectivity).


I was also thinking that at the very least, and a far more simple way to approach this would be to find a neutral color. If saving money was the goal, depending on the placement of the building, I am thinking you could look to see if it is more expensive to run the AC in the summer, or the heater in the winter. Then you could determine your color that way. If it was far more expensive to run the AC in the summer, you would go white, and then just not worry about using extra heating in the winter.

(Of course if your roof is covered in white snow in the winter anyway color doesn't much matter......that's why every situation is different)

Or if you just want to balance everything out and chose a neutral (when I say neutral I mean 50% reflective material/color), then you would be a little warmer in the winter, and a little cooler in the summer, and not run the heating and AC as intensely.

On a personal note: where I am living now, I was just thinking that I would just paint the roof white (or an off-white is probably a good more neutral color that works and doesn't stand out as much). It is covered in snow most of the winter anyway. I was just in the attic the other day to get some steel signs I was storing and they were almost too hot to touch. I couldn't believe it. Off topic, but another cool thing I have seen done is whole house ventilation systems. In the evening when people get home from work they flip on these massive fans that run just for a couple minutes to suck all the hot air out of the house, through the attic and exit the building. Then they will turn on their AC units. (or I have seen little fans you install at the vents and run off a solar panel on the roof, you could even tie in a thermostat)

There is also a difference in thermal properties of material type, as well as color when getting into this topic, that maybe someone else can talk about.

Note: Just to be more specific for sticklers. AC is Air Conditioning which means heating and cooling, but for my purpose and most peoples' they are incorrectly just referring to cooling when using the term.
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Re: Hot in the city, paint your roof white

Postby CanadysPeak on July 10th, 2014, 6:24 am 

There is benefit in the use of paint to achieve lower temperatures. Lawrence Livermore did some tests a while back in which they found that a silver car got almost 0.5 mpg better mileage than a black car. Some of the problem with implementing a program like this is the idea in the general public's minds that optical color equals heat color. Thus, people might choose a nice white color with low reflectance, only to find later that it is hotter than the hinges of hell when parked in the sun. 3M has done extensive research on the question of color in shingles, with some interesting results, for example that they can take a high reflectivity shingle and put a very thin dark color on top, with the result that the sun "thinks" the shingle is white and the eye "thinks" the shingle is black.
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Re: Hot in the city, paint your roof white

Postby owleye on July 10th, 2014, 9:31 am 

I tend to think of this idea along the lines of "be careful what you wish for." Snow serves the purpose of keeping winter temperatures very low up here in the northland, despite, or rather because of, the significant amounts of sunlight. Indeed, it's so cold that places like Florida and Arizona become a haven for snowbirds. In engineering circles, there's an old saying usually attributed to Murphy, about jelly bread always falling jelly side down, among many equally predictable outcomes. I think it was during the Carter era that someone had in mind lowering the overall energy cost by developing heating and air-conditioning HVAC and related systems that would lower and raise temperature to accord with the need to maintain a constant temperature range, only to find that the energy cost rose. But who knows, maybe we've learned a thing or two about thing or two (as de Niro, in "This Boy's Life" in referring to his own "wisdom", speaks of himself).
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Re: Hot in the city, paint your roof white

Postby Hendrick Laursen on July 11th, 2014, 6:04 am 

Thousands of years ago, people started another type of roofs. Hemispheres upon the roofs called Domes. Well actually domes are a better choice, since on every degree, only half is lighted by sun. Except the places too near to Equator, It is really better choice. Also check Windcatchers
http://en.wikipedia.org/wiki/Windcatcher
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Re: Hot in the city, paint your roof white

Postby zetreque on July 15th, 2014, 2:49 pm 

Thanks everyone for the input to this thread.

Here is something else related:

http://phys.org/news/2014-07-urban-heatnot-myth-worst.html
researchers found that variation in how efficiently urban areas release heat back into the lower atmosphere—through the process of convection—is the dominant factor in the daytime UHI effect.

"There is a synergistic relationship between climate conditions and the urban heat island," Lee said. "This relationship suggests that the urban heat island will exacerbate heat wave stress on human health in wet climates where temperature effects are already compounded by high humidity.
"This is a huge concern from a public health perspective.

Their results reaffirmed the consensus view that, regardless of the local climate, the release of heat stored in human-built structures is the dominant contributor to UHI during the nighttime.

But during the daytime, researchers found, convection is the dominant factor—particularly in "wetter" cities of the southeastern United States. In those places, the smooth surfaces of buildings and other human-made features are far less conducive to heat diffusion than the densely vegetated areas that surround them. Overall, in wetter climates urbanization reduces convection efficiency by 58 percent.
"The 'rougher' surfaces of the vegetation triggers turbulence, and turbulence removes heat from the surface to the atmosphere," said Lei Zhao, a doctoral student at F&ES and lead author of the study. "But where there is a smoother surface, there is less convection and the heat will be trapped in the surface."
Convection plays a key role in drier cities, too—albeit with far different consequences. In those settings—including in urban areas of the U.S. Southwest where surrounding vegetation is typically shorter and scrubbier—the rural areas are less effective at dissipating heat. As a result, the urban landscapes are actually 20 percent more efficient in removing heat than their rural surroundings, triggering a 1.5-degree C cooling within the cities.


Hard to believe building smoothness reduces convection and there are no micro-climates in cities.
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Re: Thermal Masses for heating efficiency

Postby wolfhnd on February 17th, 2015, 2:40 am 

This has got to be the weirdest use of mirrors I have ever seen.

High tech cooling mirror beams heat into space

"In the test, the device cooled objects by up to five degrees Celsius by absorbing heat and sending it directly into outer space, and it even worked in direct sunlight. In a three-story commercial building with a 1600-square-meter roof, using the radiative cooler would save an estimated 118,500 kWh annually, the engineers calculate."

http://www.proudgreenhome.com/news/high ... nto-space/
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Re: Thermal Masses for heating efficiency

Postby zetreque on February 17th, 2015, 3:55 am 

wolfhnd » Mon Feb 16, 2015 11:40 pm wrote:This has got to be the weirdest use of mirrors I have ever seen.

High tech cooling mirror beams heat into space

"In the test, the device cooled objects by up to five degrees Celsius by absorbing heat and sending it directly into outer space, and it even worked in direct sunlight. In a three-story commercial building with a 1600-square-meter roof, using the radiative cooler would save an estimated 118,500 kWh annually, the engineers calculate."

http://www.proudgreenhome.com/news/high ... nto-space/



I wonder if the material is cheaper than white paint :p

PS: I moved your post to this thread which is more relevant to that.
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Re: Hot in the city, paint your roof white

Postby zetreque on July 1st, 2015, 12:37 pm 

Hendrick Laursen » Fri Jul 11, 2014 3:04 am wrote:Thousands of years ago, people started another type of roofs. Hemispheres upon the roofs called Domes. Well actually domes are a better choice, since on every degree, only half is lighted by sun. Except the places too near to Equator, It is really better choice. Also check Windcatchers
http://en.wikipedia.org/wiki/Windcatcher



That is an interesting idea. Somewhat recently I also realized one of the engineering ideas behind the yurt. Being a cylinder you can maximize area while minimizing wall surface where heat is transferred. One step even better than that is a sphere or dome.

It became so hot here this week that I am really considering doing something about my roof. Scientists here have been saying that it is the night time temperatures that are increasing with global warming moreso than the day time temperatures and I don't know of it's an artifact of them saying it but it sure seems so this week. Night time is usually when I can open my windows, run the fans and cool the house down for the day, but lately I can't cool the house down at night time. I am still wary of painting my roof white because I don't want to lose the heating affect in the winter time... Thinking of ways I could maybe just staple a white reflective tarp over the sunny areas of the roof that can be removed for winter.
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Re: Hot in the city, paint your roof white

Postby smfood on July 3rd, 2015, 2:50 am 

In fact, the most effective method is to plant in the roof. But it takes a certain technical means(To prevent the plants to die). It's more effective than paint the roof white but seems difficult to achieve.
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Re: Thermal Masses for heating efficiency

Postby zetreque on July 22nd, 2015, 5:04 pm 

wolfhnd » Mon Feb 16, 2015 11:40 pm wrote:This has got to be the weirdest use of mirrors I have ever seen.

High tech cooling mirror beams heat into space

"In the test, the device cooled objects by up to five degrees Celsius by absorbing heat and sending it directly into outer space, and it even worked in direct sunlight. In a three-story commercial building with a 1600-square-meter roof, using the radiative cooler would save an estimated 118,500 kWh annually, the engineers calculate."

http://www.proudgreenhome.com/news/high ... nto-space/


I think this is the key part of that article and something to wrap our head around.

However, radiative cooling doesn’t work if what you're trying to cool is already cooler than the ambient temperature outside, which is why there is no such thing as completely passive air conditioning. This system bypasses the atmospheric temperature and uses the lower temperature of space to pull heat from a building to the dark, frigid reaches of space.


It seems like it's also dependent on the insulation of the space being "cooled". The insulation will prevent heat from the surroundings to penetrate the space while the heat inside the space is being transferred out the heat sink aka mirror.
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Re: Hot in the city, paint your roof white

Postby zetreque on May 3rd, 2016, 2:29 pm 

Just noticed the top of a brown UPS delivery truck is painted white.
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