Study: Green roofs suck at solving global warming

White roofs three times as effective as green roofs

Green roof of City Hall in Chicago, Illinois.

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

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Dodgy Geezer

No shit, Sherlock?
… Dark roofs should be phased out in warm climates for public health purposes…
However, they should be extended in cold climates. It is also a good idea to open windows if you have a lot of people in a room and the weather outside is clement. If you are in a high building make sure that no one falls out.
During a gas attack it is wise to seal all channels to the outside air, and wait for the wind to disperse the agent.
Can I have a grant for writing this kind of stuff?

kenw

“Green roofs” pose a problem in areas that have deemed that the rainwater that falls on your own property to be public property.
http://healthfreedoms.org/2011/12/27/collecting-rainwater-now-illegal-in-many-states-as-big-government-claims-ownership-over-our-water/

David, UK

From a purely local point of view it makes sense. Buildings would be more comfortable in summer with less dependency on air con. Globally though? Meh. What is three times F-all?

You’d think a place as quasi-tropical as Florida would have figured out long ago that white
shingles are better than black, which you see everywhere. Another ( bigger) waste of energy is
the building code in Florida that allows ductwork to be entirely contained within the “attic.:”
That attic hits 145 degrees about 8 months of the year. And Florida’s Govs in the past have
trumpeted “energy saving” as a goal. Stupid jerks.

Alan Robertson

Your tax dollars at work, but that’s Ok, it’s all your fault, remember?

Betapug

Believe there is a free trial distribution of white roofs underway in much of the US at the moment. Not sure if this is government funded or not.

Gail Combs

Great
I need to replace my roof can I get Obama to pay for it… out of his own pocket?

Jimbo

They got funding for this?

RichardLH

“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.”
OK. Gets out calculator. Works out total area of ‘roofs’. Works out total area of Land Surface (we’ll just ignore the Water area for now – we only need a rough approximation to start with and anyway that wet stuff is too difficult to ‘roof’). Divides one by the other and, because the calculator I’m using is fixed point display, gets = 0.
Conclusion even IF global warming is a major concern, whilst there may be impacts on the individual structures that have benefits, the contribution to the total figure will be = ~0 (or very, very close to it).
Thanks for playing.

kenw

Col: If your attic is hitting 145 you have other issues. A properly ventilated attic should never be more than a few degrees warmer than the outside air.

rgbatduke

From a purely local point of view it makes sense. Buildings would be more comfortable in summer with less dependency on air con. Globally though? Meh. What is three times F-all?
Amusingly so very true. And yet, if one considers the surface area of tarmac roadways, it is actually nontrivial. Well, ok, it is still pretty trivial compared to the total area of just the land surface, but it’s a BIG trivial number, and there is little doubt that it and black roofs contribute mightily to LOCAL warming in cities, a.k.a. the UHI effect.
UHI corrupts current estimates of global warming by boosting poorly sited urban thermometers in ways that the primary temperature products do not seem to accurately correct for. So if by promoting white roofs and whitewashed roadways and developing white grass for lawns and maybe even just covering everything with white paint or aluminum foil in cities, we can lower urban temperatures, it will have an entirely disproportionate effect on global temperature estimates with their large occult UHI component. Basically, we need to paint the ground and everything on it white everywhere within half a kilometer of an official weather station. That would help a lot.
Next: How we can completely cancel global warming by whitewashing the surface of the planet to emulate glaciers (at a cost of only ten quadrillion or so dollars)…

Early man had “green roofs”. They got rid of them because they were tired of bugs falling on their heads.

Jimbo

We strongly recommend building code policies that phase out dark-colored roofs in warm climates to protect against their adverse public health externalities.

I’m in a hot and sunny clime with a red roof. Should I visit the doctor? What an utter waste of money.
There are many people living under corrugated sheets who feel the force of heat every day. What they need are fans and ACs not this tripe.

MJ

It is amazing to me how much money is spent by “science” for things that people with common sense already know, or could care less about.

And when the cold lingers, will we all be given permission to paint our roofs black and be reimbursed for it by our government? Just asking…

TomRude

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.

RichardLH

rgbatduke says:
January 31, 2014 at 12:04 pm
“there is little doubt that … black roofs contribute mightily to LOCAL warming in cities, a.k.a. the UHI effect.”
Sorry for the edit but roads are a different question.
Hmm. I’ll lay you odds that the vertical surfaces contribute more to the total UHI than the roofs do.
Especially if you take into account all the other flat bits, (excluding roads, pavements if you wish), parks, etc. at ground level.
That’s why heat sinks often have large vertical areas in them, much better at transferring energy to the air.

Walter Allensworth

Does the study consider the CO2 removed from the air by plants on green roofs and how that will reduce carbon forcing? I didn’t see that in the bullets anywhere.

Chris4692

Unfortunately the article is apparently free only to those who are institutionalized, so I cannot evaluate what they included, what they missed, and what their assumptions were. Green roofs are promoted around here more for their rainwater effects than energy savings so the conclusions don’t seem greatly off from what I would expect. I wonder if (or how) they included the extra structural costs of the green roof.

JJ

This annual difference is sufficiently small that the choice between a white and green roof should be based on preferences of the building owner.

Newsflash skippy: The color of the roof (white, green, black, purple, whatever) should be based on preferences of the building owner because its is his damn building.
That truth lies beside the fact that that the annual difference in “cooling the globe” between those roofs and any other kind of roof is sufficiently small that you arrogant %^&*tards need to find another pin upon which to hold your cotillions.

Dodgy Geezer

@TomRude
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…
Why, thank you, Rude Tom! That is an important insight.
Tell you what – let’s put in a proposal to visit lots of tourist spots around the world at different latitudes, and measure the effect of painting the walls black and white. We can go halves on the grant…

Apoxonbothyourhouses

This is the pseudo scientific $hit which gives academia a bad name and demonstrates monumental ongoing insensitivity as to how academics use our tax dollars. As if a few green roofs are significant compared to millions of hectares of pastureland. Having said that, it is the ivory tower person who approved the project who most needs to be dragged into the real world where poverty is all too common.

mib8

heh. I recall some hippie dippie solar energy books from the 1970s concluding that “elm leaf green” was surprisingly absorbing more energy than flat black in their experiments.
OTOH, I recall a History Channel bit (when they weren’t doing ancient aliens 24/7) about the development of the lawn: English greenswards, enclosure, sheep, importation of clover and such (for the grazing animals which weren’t doing well on the local plants), Shenandoah Valley bluegrass… that a grass lawn reduced temperatures… as compared with what alternatives I did not catch.
Then again, if I were wealthy and stuck in an over-populated, over-crowded city, I’d want my penthouse to have some live greenery. (Similarly, due to fire hazard in such over-crowded environments, I’d want the building to be mostly concrete and steel. With other measures where earth-quakes are common.)
I once lived in a house built in the 1940s. They thought a course of cinder-block was great insulation and it needed no more in the walls. I could feel the heat being sucked out of me in the winter. Similarly, many builders in Florida are just now beginning to catch on to the merits of insulation and attic ventilation.
Still, as a general rule, people should be able to build their homes pretty much as they wish and can afford without being attacked by the code nazis and permit fascists.

DesertYote

There is a reason why plants are green.

“Per unit area, white roofs cool the globe 3× more effectively than green roofs.”
An acre of white roof has about the same impact on climate as an additional acre of ice. With the deployment of additional acreage of white roofs, particularly at lower latitudes, we could see it causing a climate response much like what would be expected from increased glaciation. Seriously. If we get all “cool roof” in the US and Europe, it could potentially TRIGGER another glacial episode.

RichardLH

Steven Mosher says:
January 31, 2014 at 12:16 pm
“The impact while small in degress C, has the benefit of saving money.”
That may well be true, but I will lay you any odds that the contributions from the vertical surfaces outweighs all of that combined. They are just so, so much more efficient at transferring energy to the air. Think heat sinks and radiators. We don’t build them flat for a good reason.

Dodgy Geezer

4WIW, I suspect that the most important issue with a roof is whether it’s cheap and easy to build, whether it functions properly in protecting the house underneath it, and whether it lasts.
Those three parameters will have been optimised over hundreds of years for the different conditions and available raw materials in most human habitations – thousands of years in some cases.
I’m pretty sure that requiring a different roof coating to the one commonly used in an area would cause one (or more) of those parameters to be sub-optimal.
But hey, if there’s a grant involved…

hunter

One of the leading climatocract wrote about this years ago.
I beleive it was Sec. of Energy Chu:
“Today, U.S. Energy Secretary Steven Chu announced a series of initiatives at the Department of Energy to more broadly implement cool roof technologies on DOE facilities and buildings across the federal government. As part of the effort to make the federal government more energy efficient, Chu has directed all DOE offices to install cool roofs, whenever cost effective over the lifetime of the roof, when constructing new roofs or replacing old ones at DOE facilities. Additionally, the Secretary has also issued a letter to the heads of other federal agencies, encouraging them to take similar steps at their facilities.
“Cool roofs are one of the quickest and lowest cost ways we can reduce our global carbon emissions and begin the hard work of slowing climate change,” said Chu. “By demonstrating the benefits of cool roofs on our facilities, the federal government can lead the nation toward more sustainable building practices, while reducing the federal carbon footprint and saving money for taxpayers.”

http://newscenter.lbl.gov/news-releases/2010/07/19/cool-roofs-offset-carbon-dioxide-emissions/
In places witha lot of rain, green roofs are problemtaic due to the rain water management challenges: Extra structure for the roofs, water proofing, erosion resistance, irrigation for plants between rains, etc.

David'93

JJ says “Newsflash skippy: The color of the roof (white, green, black, purple, whatever) should be based on preferences of the building owner because its is his damn building.”
Best point of the day!

bill_c

RichardLH,
I’d take your odds. First it’s an awfully dense city that has more vertical surface than horizontal. And I think your reference to heat sinks is true only because they have large vertical surfaces to increase total surface area/volume where horizontal surface area is constrained. It’s the total surface area that matters. In that vein a city with lots of tall buildings and 100% paved/roof area probably has more UHI than the same city with short buildings, until it starts creating its own convection cells.

bill_c

hunter,
and was criticized by the rest of the climatocracy for daring.
dodgy geezer,
that doesn’t explain asphalt shingles. I think the primary reason for the shingle shape is aesthetics/ imitating slate/tile/wood shingle designs…..not because that design is the most efficient for the asphalt material (tho it’s not bad)

Jimbo

Steven Mosher says:
January 31, 2014 at 12:16 pm
The impact while small in degress C, has the benefit of saving money.
http://www.theatlanticcities.com/design/2012/04/just-fraction-more-white-roofs-could-have-huge-global-impact/1764/

Yep, only where cooling is your issue. What about Scotland, Northern England, Canada et al? Higher heating bills due to white roofs?
OK many countries in hot climes paint their walls white. London is littered with dark building absorbing nice heat for a reason. Maybe I’m wrong here and stand to be corrected.

Berényi Péter

Why, sane people on warm sites choose white since millenia and put plants into the garden where they belong. One does not need a PhD to figure that out.

RichardLH

bill_c says:
January 31, 2014 at 12:47 pm
“I’d take your odds. First it’s an awfully dense city that has more vertical surface than horizontal. And I think your reference to heat sinks is true only because they have large vertical surfaces to increase total surface area/volume where horizontal surface area is constrained”
You may well lose though. It’s all down to energy transfer to the air. Even if the area is smaller, the efficiency of transfer is so much higher when you can use convection to carry away the heat.
Flat surfaces are OK, but quite a long way down in efficiency terms. Radiators might have been a better example rather than heat sinks.
The sun is also likely to illuminate the vertical surfaces better as well. Only on one side of the building may be. But outside the tropics very important surfaces.
And as for leakage from inside the building – no contest. Walls, windows, etc. every time.

Jimbo

From Mosher’s link.

If you combined all of the world’s urban areas into one dense nugget of urbanity, it would cover about 2 million square kilometers, or 1.3 percent of the land area of the planet. About 60 percent of that – roughly the size of California and Texas combined – is made up of pavement and rooftops.
These roads and roofs can be problematic surfaces. Often dark in color, they soak up sunlight, increasing the temperature of the building and surrounding area in what’s known as the urban heat island effect. This in turn contributes to a vicious cycle: the hotter a city feels, the more we energy we tend to use to cool it down, which leads to more greenhouse gas emissions, which contribute to climate change, and so on.
http://www.theatlanticcities.com/design/2012/04/just-fraction-more-white-roofs-could-have-huge-global-impact/1764/

It depends on where your city is. Some buildings need central heating in the summer. The article assumes all cities need urgent summer cooling. LOL. Tell that to Glasgow and Edinburgh.

kadaka (KD Knoebel)

kenw said on January 31, 2014 at 12:03 pm:

Col: If your attic is hitting 145 you have other issues. A properly ventilated attic should never be more than a few degrees warmer than the outside air.

So far here in central Pennsylvania in winter, where we’ve broken decades-old records with outside temperatures of single-digit and even negative numbers (°F), your idea of “properly ventilated” is frightening.
It might work for “advanced” future construction methods, where the real house is a flat-roofed properly insulated and sealed “eco-box”, to which a traditional-looking lightweight roof structure is mounted for shedding snow and rain. If a hurricane or tornado gust rips it out of the breakaway mountings, the reinforced real house remains. Add a weathertight roof hatch to use the shell space for storage. Then your attic can be as ventilated as you want, although preferably dry. The shell space would also be good protection for your AC or heat pump central condenser unit. And perhaps a small pot farm.

Andreas

And when it rains too much followed by a freeze and snow, how ever will it withstand the extra weight of all that water?
The mall that collapsed in Riga, Latvia didn’t it have one of those green roofs and probably had too much weight in soil water??

u.k.(us)

Probably should have read to comments first, but the picture in the post comes out of my county taxes.
Compare that “green space” to the heat loss/gain of the buildings exterior.
It’s Chicago, so no surprise.

MarkW

RichardLH says:
January 31, 2014 at 12:11 pm
——
Heat sinks have large vertical surfaces because that’s the only way to increase total surface area without taking up more real estate.

Green grass, sod, rooves are traditional here, with goats to trim the grass. Unfortunately a local business man has Service Marked the use of goats so local business now have shaggy rooves.
We also have hundreds of acres of common water reed Phragmites that thatches a roof to R 40(!) with nearly unlimited lifetime with proper maintenance.

MarkW

RichardLH says:
January 31, 2014 at 12:57 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.

bill_c

RichardLH,
I 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. At least for a white vs a black roof, where there’s no evapotranspiration, we’re just talking about the % of sunlight absorbed, and of course no combination of vertical surfaces can amplify the heat gain beyond 100% of the incident solar.
Now, the building-level energy savings from installing a white roof doesn’t compete from retrofitting windows and walls for leakage in a tall building. Sure – but that’s apples and oranges here, unless I’m really not understanding the problem setup.
Most of the increased energy absorption from a dark roof is going to be radiated and convected away to the atmosphere, contributing to UHI more or less depending on meteorological conditions…

bill_c

@ Steve Mosher – how far down wind of a city can we see UHI effects?

Bob Rogers

Here in the South (i.e, southern part of USA), we have fire ants. If you put dirt on your roof, fire ants will find it, because of the lack of predators. This might not be a problem, but they will burrow down through the dirt, and through the EPDM layer, and the roof will leak. Then you have to remove the dirt to fix the layer, and start spreading ant poison.

Jimbo

I lived in London for many years and I can never remember a time when I wanted air conditioning in my apartment, never. Sure it got a little warm a few weeks in SOME summers but I didn’t even think about a fan. It just doesn’t get hot for long enough to bother with. The only places with A/Cs are stores, diners, offices etc which are often ‘sealed’ (closed windows / doors) with bright lights and lots of machinery giving off heat.
I never complained about the ‘heat’ in summer while walking in the street, never. I welcomed it sorely. 😉

hunter

bill_c,
Yes, and we skeptics pointed out that if AGW is so small as to be signficantly mitigated by white roofs, then AGW is not really a big deal. Of course we were dismissed as well.
The key ingredient to credibility in the AGW community seems to be two part:
1) The claim must come from an acceptable source, and the claim is nearly always paid for by tax payer grants or direct tax money
2) The claim must never question or minimize the underlying crisis that the AGW movement believers require.

R. de Haan

BS, I prefer black roofs. White and Green, so out of date.

Everybody knows that most roofs will be bi-racial in the future.

Newsflash skippy: The color of the roof (white, green, black, purple, whatever) should be based on preferences of the building owner because its is his damn building.

It is nice to see that there are those here who still believe in property rights. Damn nice.