White roofs three times as effective as green roofs
From Lawrence Berkeley National Laboratory and some members of the BEST team comes this surprise.
“We conclude that the choice of white vs. extensive green roof should be based on the environmental and societal concerns of the decision-maker. If global warming is a major concern, white roofs, which are around three times as effective at cooling the globe as green roofs, will be the preferred choice. On the other hand if the local environment is a primary interest, green roofs will be preferred. Of course, stormwater management may be a decisive factor in favor of green roofs, particularly in the presence of strict local stormwater regulations.”
The paper:
Economic comparison of white, green, and black flat roofs in the United States Julian Sproul,Man Pun Wan, Benjamin H. Mandel, Arthur H. Rosenfeld
Highlights
• The life-cycle costs of white roofs are less than those of black roofs.
• Green roofs are more expensive over their life-cycle than white or black roofs.
• Green roofs’ high installation/replacement costs outweigh their long service lives.
• Per unit area, white roofs cool the globe 3× more effectively than green roofs.
• Dark roofs should be phased out in warm climates for public health purposes.
Abstract
White and “green” (vegetated) roofs have begun replacing conventional black (dark-colored) roofs to mitigate the adverse effects of dark impervious urban surfaces. This paper presents an economic perspective on roof color choice using a 50-year life-cycle cost analysis (LCCA). We find that relative to black roofs, white roofs provide a 50-year net savings (NS) of $25/m2 ($2.40/ft2) and green roofs have a negative NS of $71/m2 ($6.60/ft2). Despite lasting at least twice as long as white or black roofs, green roofs cannot compensate for their installation cost premium. However, while the 50-year NS of white roofs compared to green roofs is $96/m2 ($8.90/ft2), the annualized cost premium is just $3.20/m2-year ($0.30/ft2-year). This annual difference is sufficiently small that the choice between a white and green roof should be based on preferences of the building owner. Owners concerned with global warming should choose white roofs, which are three times more effective than green roofs at cooling the globe. Owners concerned with local environmental benefits should choose green roofs, which offer built-in stormwater management and a “natural” urban landscape esthetic. We strongly recommend building code policies that phase out dark-colored roofs in warm climates to protect against their adverse public health externalities.
The paper is open access, and can be read here: http://www.sciencedirect.com/science/article/pii/S0378778813007652
h/t Steve Mosher
That’s what a lot of buildings look like in Detroit (except from the bottom up)
In generally warm climates like Australia, white roofs reflect heat in hot weather and fail to emit heat in cooler weather. This improves internal conditions generally. Dark roofs emit and absorb heat more readily and in cool climates like northern Europe where incoming radiation is low even in summer, they produce worse internal conditions. The dark roofs also deteriorate faster under UV radiation and need replacement sooner. Snow cover, highly insulated roofs and vegetated roofs improve insulation and reduce heat transfer and snow reduces radiation. This improves internal conditions. The effect on the outside environment is minimal, but the effect on the conditions for inhabitants is very large. The inhabitants can easily make their own decisions.
Just replace traditional roofing materials with LCD screens. Each panel can be a solid color, and they can make flexible LCD sheets so they could come as cut-to-fit roll roofing, so the roofs can be any color they want and can change as needed.
Mandate them for new construction and roof replacements on non-historic buildings. It’ll be pricey, but as it’s for both energy savings and fighting global warming, I’m sure there’ll be a government grant and/or tax rebate program that’ll cover it. Just hand Barry O his pen so he can Executive Order it all into existence, mission accomplished.
@kadaka: obviously you are correct, I inadvertently assume warmer weather situations, such as we have here 11 months of 12.
For white roofs snow is the perfect material. It reflects sunlight, it is an efficient radiator in thermal IR and it is a great insulator. With global warming, as everyone knows, we shall never run out of it, because warming increases evaporation, which brings ever more precipitation, snow included, presumably. At least that’s how increasing snow extent is explained away.
MarkW says:
January 31, 2014 at 1:18 pm
“The convection is so small that it isn’t worth considering. Most of the heat radiated by heat sinks is in the form of radiation anyway.”
Go tell that to the water radiators in my house. They work vertically and by convection as the major source of heat transfer.
bill_c says:
January 31, 2014 at 1:22 pm
” understand what you’re saying but remember we’re not talking about energy loss or gain from buildings (at least for UHI and roofs), we’re talking primarily about an albedo effect.”
I know. Think about this then. If you were to measure the temperature of a south facing black wall and a black drive against it, which do you think will get hotter and why? Assuming that we are talking about throughout the day and somewhere, say, at Lat 45 degrees N or S. Now over the whole year? See what I am talking about?
Roughly more energy will fall on the wall than the roof, over most days. over most of the year. And it will still be cooler because the energy leaks away faster into the air from the wall. All that lovely vertical space moving the air faster.
If you want to improve solid to air energy movement, you make as rough a surface as you can because the energy transfer is faster. The max is probably around where the shadowing of the ‘next’ vertical surface starts to become a significant influence on the input.
IMHO anyway.
While I agree that this study seems to be a verification of common sense, I think we should back off on ridiculing the academics. It is the moronic politicians who need studies like this. If they don’t get reports that say “Hey, tardo! Don’t do that! It’s a waste of money, and plus it is a really, really dumb idea!” Then, they go ahead and write laws that force us all to comply with the really, really dumb idea.
I think we should be thanking these researchers for making this common sense part of the literature BEFORE we all either had to foot the bill for our own green roofs, or foot the bill for the subsidies for everyone else’s green roof.
bill_c says:
January 31, 2014 at 1:27 pm
“. . . how far down wind . . .”
A study for the Chicago region (I think) was done in the late 1940s or early ’50s but the emphasis was on the down wind rainfall, not temperature. Dust and chemicals from urban functions were greater years ago than now so there were many concerns. The better neighborhoods were often on higher ground away from the “Big City” (see Haggard, Merle) while the stock yards were down wind of the better neighbors.
Anyway, one doesn’t have to go far from the built up area to escape the UH but other things farther away can be impacted.
A couple of years ago the host of WUWT did a post on a car mounted data-logger that allowed you to do your own research on this topic for a city near you. Graphs and photos included:
http://wattsupwiththat.com/2010/04/04/measure-uhi-in-your-town-with-this-easy-to-use-temperature-datalogger-kit/
http://wattsupwiththat.com/2012/07/10/new-gadget-remote-wifi-temperature-humidity-data-logger/
Something to do with the price of lettuce?
http://fullcomment.nationalpost.com/2014/01/28/brian-hutchinson-vancouvers-salad-days-die-with-bankrupt-roof-top-garden/
Mosher’s linked article claims that a 1% increase in the surface albedo of 1.3% of the global land area (0.39% of the total global area) will result in a 0.07 C temperature drop. That amounts to an albedo change of 0.01*0.0039 = 39 ppm.
Using a sensitivity of d(albedo) = 0.011 per degree C, I estimate 0.0035 C, a factor of 20 smaller.
Also, a white roof requires much more maintenance to keep it white, compared with a dark colored roof. Anyone who lives in Florida near an ‘evergreen’ (ho ho!) live Oak tree understands this in spades.
Ah well, when it comes to saving the world from too many carbon units, its the thought that counts…
Obama can come to the rescue with some of his talked about presidential initiative. He can let his hair (roof) continue to go fully white. Maybe Jay Carney can help out by stealing the black shoe polish out of the WH. My dad always accused my mom’s immigrant step dad of using that stuff to keep his hair dark well into his eighties.
“…Owners concerned with global warming should choose white roofs, which are three times more effective than green roofs at cooling the globe…”
WTF?
Are the authors suggesting that thin air taxes, wind turbines, solar cells and a lot of overpaid United Numpties bureaucrats are all unnecessary because if we all erect a white roof we can cool the globe?
Sounds like the problem of tackling climate change is solved, we just have to paint our roof white. BEWDY!
…do I need to add, /sarc
@bill_c
…dodgy geezer,
that doesn’t explain asphalt shingles….
Doesn’t it? I smell a grant coming on…
Bloomberg to the rescue!
31 Jan: Reuters: Michelle Nichols: United Nations appoints former NYC Mayor Bloomberg cities, climate change envoy
U.N. Secretary-General Ban Ki-moon on Friday appointed former New York City Mayor Michael Bloomberg as his special envoy for cities and climate change, in a bid to build momentum ahead of a planned U.N. conference in September.Ban said Bloomberg will assist him in “consultations with mayors and related key stakeholders, in order to raise political will and mobilize action among cities as part of his long-term strategy to advance efforts on climate change.”…
The U.N. role for Bloomberg – a billionaire philanthropist who left office last month – was reported by Reuters on Thursday.trueIn a statement, Bloomberg said cities had emerged as a leading force in the battle against climate change. His appointment as U.N. special envoy is for two years…
Samantha Power, U.S. ambassador to the United Nations, was quick to welcome Bloomberg’s appointment, posting on Twitter: “Mayor @MikeBloomberg knows how to get things done. We need more leaders like him here @UN.”…
Bloomberg has played a leading role in the C40 Cities Climate Leadership Group, an international group of mayors created in 2005 and dedicated to reducing greenhouse gas emissions. The C40 group, of which Bloomberg is president of the board, is to meet in Johannesburg next week…
http://www.reuters.com/article/2014/01/31/us-climate-un-bloomberg-idUSBREA0U02Q20140131
not a hint of the following in the above Reuters report:
23 Jan: NYT: Carol Davenport: Industry Awakens to Threat of Climate Change
The California hedge-fund billionaire Thomas F. Steyer, who has used millions from his own fortune to support political candidates who favor climate policy, is working with Michael R. Bloomberg, the former New York mayor, and Henry M. Paulson Jr., a former Treasury secretary in the George W. Bush administration, to commission an economic study on the financial risks associated with climate change. The study, titled “Risky Business,” aims to assess the potential impacts of climate change by region and by sector across the American economy.
“This study is about one thing, the economics,” Mr. Paulson said in an interview, adding that “business leaders are not adequately focused on the economic impact of climate change.”
Also consulting on the “Risky Business” report is Robert E. Rubin, a former Treasury secretary in the Clinton administration. “There are a lot of really significant, monumental issues facing the global economy, but this supersedes all else,” Mr. Rubin said in an interview. “To make meaningful headway in the economics community and the business community, you’ve got to make it concrete.”…
Although many Republicans oppose the idea of a price or tax on carbon pollution, some conservative economists endorse the idea. Among them are Arthur B. Laffer, senior economic adviser to President Ronald Reagan; the Harvard economist N. Gregory Mankiw, who was economic adviser to Mitt Romney’s presidential campaign; and Douglas Holtz-Eakin, the head of the American Action Forum, a conservative think tank, and an economic adviser to the 2008 presidential campaign of Senator John McCain, the Arizona Republican.
“There’s no question that if we get substantial changes in atmospheric temperatures, as all the evidence suggests, that it’s going to contribute to sea-level rise,” Mr. Holtz-Eakin said. “There will be agriculture and economic effects — it’s inescapable.” He added, “I’d be shocked if people supported anything other than a carbon tax — that’s how economists think about it.”
http://www.nytimes.com/2014/01/24/science/earth/threat-to-bottom-line-spurs-action-on-climate.html?_r=0
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“Owners concerned with global warming should choose white roofs, which are three times more effective than green roofs at cooling the globe.” So say the authors.
The only way the globe will warm or cool, if the experts are right, is by loss of radiation to space.
I’ll be interested to read if the authors have accurately traced the energy path from roof to space to show net gain or loss.
Seems to me, if they can do that, they are achieving more than the top climate modellers who have not been successful at that task.
So, it’s a reasonable conclusion that the paper takes some short cuts and so it might not be correct.
I’ve an open mind. Convince me I’m wrong & I’ll write a corrected post, with unreserved apology.
I wonder about the snow removal process on a green roof like the Chicago City Hall’s. Where would one dump the excess snow?
Edinburgh Scotland. The slate tiles heat up in the sun so they fitted extraction and a pipe to blow the warm air down amongst the congregation. Saving 40 percent of the cathedrals heating bill.
Horses for courses eh?
As goes this paper……. wtf???
Ooops, sorry, Edinburgh Cathedral
Scientists work out why all the buildings in Greece are white. They are so clever are scientists. They must be gods they know so much. /sarc
Actually kenw, you were correct in the first instance. A properly constructed simple peaked roof over and attic area will have a opening in one or both gable ends and a continuous ridge vent, no matter the prevailing climate. The objective is to maintain the roof temperature as close as possible to the outside air temperature. In winter, if the underside of the roof is significantly warmer than the outside, any snow cover will melt from underneath during the day, and refreeze at night as temperatures drop. This leads to the formation of ice dams and can cause a tremendous amount of damage as water backs up below the shingles and penetrates the felt layer, causing damage to drywall ceilings and walls alike.
Now I know why my younger brother kept pigeons. Way ahead of his time, that lad.
Jimbo says: January 31, 2014 at 12:50 pm
… London is littered with dark building absorbing nice heat for a reason. Maybe I’m wrong here and stand to be corrected.
The reason London is dark is because it hasn’t gotten around to scrubbing off all that coal soot the way Paris has.
I find it humorous that the title for this blog post is: ‘Study: Green Roofs Suck at Solving Global Warming.’ And then immediately below that heading is a picture of City Hall in Chicago where, of course, they suck at solving anything.
Geeser, in cold climate (i.e. high latitude) or during winter, the sun rays are low and thus would only marginally warm a black roof vs a white roof.
In cold climates roofs (on houses) are peaked. To keep the snow from accumulating.