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
“chris y says:
January 31, 2014 at 2:32 pm
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.
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and all while saving money.
Radiators work through both convection and radiation. Surface area is the main factor, and the radiator is vertical because it takes up less floor space. A horizontal radiator with the same surface area would work just as well in heating the room. In either case, convection is enhanced when a fan circulates the air.
White roofs work well in the tropics to keep the house cooler, but, as others have pointed out, are worthless if they can’t be kept clean and white. Green roofs are even harder to maintain, and require a much sturdier construction beneath the soil, both in weight-bearing and waterproofing.
Well-placed shade trees are a cheaper and easier to maintain solution in areas of sufficient rainfall.
We are living under a pale green corrugated roof atm. The outside temp. is in the high 30’s. Thank God for the A/C! First thing today the temp. was below 20C and we felt cold…there is a lack of insulation in the house. When will the average Australian house catch up with the vagaries of the climate here?! ( We are in Victoria).
I miss our cosy little Yorkshire cottage; we could keep warm in winter and cool in summer without much trouble.
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.
WHICH Global Warming? Never happened in recent years. Measuring reflexion is one thing, but no computer program on Earth can measure 1 meter above ground and at the same time 1 meter under ground/water surface…..
Btw. it’s close to 43 years since I had my Exam as a Systemprogrammer. Studied many other subjects over the years as well among them Mathematic, Mathematic Statistic and History plus some courses in Geology…
Which is better, white roofs or roofs covered with solar cells? (rhetorical question)
This paper looked at the economics of some industrial roofs and it makes sense that a white roof would be better than a green roof economically. But how did the study determine that white roofs are three times more effective than green roofs “at cooling the globe”? Did they measure anything to get that conclusion? Should we extrapolate that and cut down forests and dig up lawns? How would a green grass roof and an green astroturf roof compare?
This is from one of the links Mosher recommended:
http://www.coned.com/newsroom/pdf/Columbia%20study%20on%20Con%20Edisons%20roofs.pdf
The earth’s surface, the part which is not water or desert or ice, is mostly densely covered by green solar collectors (plants), little engines which influence the climate in ways not appreciated.
Now I think it is time to consider mirrors on the roofs! But I suppose some environmentalists will soon find out that it is harmful for birds. If some ducks mistake that for water, well …
“Steven Mosher says:
January 31, 2014 at 12:16 pm
The impact while small in degress C, has the benefit of saving money.
………………..
Are people making a case for conserving global energy OR local energy as within buildings?
I thought the paper’s authors were pushing the former.
It is not always that a white roof will save energy. If it results in local heating of air that has to be sucked into a nearby aircon unit’s inlet, it will cause a load. That is not a money-saving benefit. Property rights, good neighbour stuff and so on applies.
Surely it is only a real saving if the global balance is affected in an energy-saving way. Otherwise, it’s not necessarily a saving, it’s just a redistribution. That in turns leads to arguments of what is good and what is bad. Excess heat can be a discomfort for people but an aid to edible plant yield. It’s surely far too big a consideration to be analysed in terms of roof size, shape, colour and abundance.
Angels dancing on heads of pins?
I put a green roof on my house. Corrugated, galvanized steel. Light green.
Not done yet though. Still need to paint the disruptive patterns to confuse the aerial surveillance and sattelite imagery used by local government to extort higher rates from residents.
P.S. Non Nomen Insects in have been observed on larger PV installations to be confused by the surface of the panels, causing them to deposit their eggs on the shiny surface, apparently believing it to be water.
‘Green toilets’ don’t do much to prevent global-warming either. I guess it’s just as well we haven’t had much for about a decade or two.
scarletmacaw says:
January 31, 2014 at 10:18 pm
“Radiators work through both convection and radiation.”
I do rather know that you know. I will still contend that a radiator of the same area laid flat will be less efficient in transferring heat to the air than one placed vertically. The air speed on the top of the two arrangements is quite different.
A flat object produces quite gentle air flows. A vertical one quite a fast ones.
As I said above, if you wish to gain the most effective transfer of solid to air one would use the roughest surface in a ‘large’ sense as one could manufacture.
This will allow the vertical surfaces to promote air flow which will then help to cool the flat portions. Like a set of cubes, of better still rectangles, placed on end. If you arrange such a surface so that the rectangles are randomly placed on the horizontal area you will allow the maximum surface area for energy to be collected especially outside of the tropics where the input angle for most of the day will be below the critical angle where the effective area of the horizontal surface exceeds that of the vertical one over the whole year. Cos and sin might help you work out the effective as opposed to actual surface areas involved when doing your calculations.
If you achieve a surface which has larger vertical surfaces (i.e. rougher) , you will, in general, improve energy transfer.
Please note that green roofs are mostly only useful for flat roofs and mainly for buildings that are made of reinforced concrete due to the large mass that such structures entail. There have been roofs constructed out of living grass, etc. but the extra cost of the reinforced roof structure needs to be taken into account.
White roofs, as noted by others above, require continuous maintenance to remain effective.
Lastly, the total area of roofs (as opposed to other flat collecting surfaces) is so minute that if will probably not show on most standard, fixed point display calculators as factor to be added.
The only way the figure gets to be of interest is if you add in roads, pavements, parking areas, etc. to the mix.
Then it might, just might, creep into the right hand side of such a calculator as a ……..01.
The world is very big and we occupy such a small area of it. Now the Sahara Desert. If we covered that with shiny metal we could really gets some legs behind theidea. Don’t fly planes over it afterwards.
IMHO
Steve:
I know there are lots of studies. Can you please point me to where they have taken into account the above comments?
RichardLH says:
February 1, 2014 at 2:20 am
“White roofs, as noted by others above, require continuous maintenance to remain effective.”
Our church had an extensive area of plantings on the roof, and guess what? The roof membrane that kept the rainwater from invading the space below eventually leaked and a gargantuan mess ensued. The only solution was to take out the plantings and the soil below to allow access to the membrane to replace it. Very expensive. Personally, I would not consider a green roof knowing I faced that project in the future.
“Our church had an extensive area of plantings on the roof, and guess what? The roof membrane … leaked and a gargantuan mess ensued.”
I always thought a church is the right place for a mess…
I personally live under a black roof that is made of some sort of moulded foam concrete(I think) and underneath it is hot in summer and cold in winter. I suppose that a reasonable insulation helps more than anything else.
I live in the tropics….. A white roof is the only one I’d have.
Bernd Felsche says:
February 1, 2014 at 12:49 am
I put a green roof on my house. Corrugated, galvanized steel. Light green.
Not done yet though. Still need to paint the disruptive patterns to confuse the aerial surveillance and sattelite imagery used by local government to extort higher rates from residents.
P.S. Non Nomen Insects in have been observed on larger PV installations to be confused by the surface of the panels, causing them to deposit their eggs on the shiny surface, apparently believing it to be water.
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Love the camo idea:-) I have a lightgreen corugated roof while the rust on one spots a start..I LIKE your idea a LOT .
In Vic as the lady above.
was under 20 but wouldnt say that was cold this am
got to the 40+ late arvo, inside? was 35 and thats with a middlin high pitched roof and insulation..
the house is wood and the WALLS have no insulation..if/when I can rip boards off and pack batts in there I bet its heaps better.
last home old stone walls 12 ins thick winter inside got to 3c skillion roof and no insulation
with batts? it didnt drop below 10c still cold but heaps better.
Steven Mosher says: January 31, 2014 at 10:13 pm
bill_c says:
January 31, 2014 at 1:27 pm
@ur momisugly Steve Mosher – how far down wind of a city can we see UHI effects?
Depends on
1. the strength of the wind. anything above 7m/sec and you wont have any UHI
So 2.2 MPH is called wind…and that mitigates UHI, Please!
As I type I have am under a corrugated sheet roof! Honestly.
Maybe I should have been more precise in my hast and said that “people living under corrugated roofs” instead of “sheets“. The difference between the two can be plain sheets upheld by some light wooden beams and triangular roofs with a ceiling. The plain sheets are murder, the roofs with spacing allow for ventilation in the space created and thus you can’t ‘feel’ the heat being radiated down. Don’t worry I know about corrugated sheets.
Oooops! My last reply was meant for
markx says:
January 31, 2014 at 6:08 pm
Not all corrugated abodes are in same or in the same country. 😉
My corrugated home is the left plain silvery colour (aluzinc) which I think is better for reflecting much of the heat compared to coloured sheets. On the other hand some people paint their with red oxide to give it a better look, while some go for different colours on factory painted sheets. My home is painted white. I personally would avoid dark colours due to the heat absorption. I also have an air-conditioner and fans in the bedrooms and living room, windows nearly always open with thick dark curtains with a light secondary back covering to reflect the sun’s rays a little. No floorboards just cool tiles. So markx, I do know a little about keeping cool under heat.;-)
Cost is the key factor. Public health and local environment is a minor after thought. Or should be. Unless you are going to plant all your streets, sidewalks and parking lots, your roof top benefit available mainly only in a light shower anyway is pretty much entirely pointless. We’ve got a couple of green posturing roofs in the Midwest. How are those minimal benefits affected during dormancy. “Green” is an attempt to distract the mind, but it sounds like lighter color roofs makes economic sense.
Dark roofs should be phased out in warm climates for public health purposes…..
bite me…
first they came for the roofs
Coach Springer says:
““Green” is an attempt to distract the mind, but it sounds like lighter color roofs makes economic sense.”
For internal comfort, better roof insulation is likely to be cheaper and more effective. As are thicker walls and underfloor insulation.
I used to live in a Cotswold Cottage. Big thick walls. Always cool in summer and, provided you kept the heating going, always warm in winter. If you let the heating stop though, it could take weeks to get it back to ‘normal’.
Green roofs don’t work. Neither apparently do “green” roofs:
http://toronto.ctvnews.ca/engineers-work-to-secure-rooftop-solar-panel-after-partial-collapse-1.1664419
So if the climate is warm, we should have white houses and roofs. Derrr, I think the Greeks knew about this some 3,000 years ago.
Santorini village, Greece.
http://upload.wikimedia.org/wikipedia/commons/thumb/b/b1/Oia-Santorini-Greece.jpg/639px-Oia-Santorini-Greece.jpg
I suppose they will be telling us next that is it rains a lot, we should have gutters. And if it snows a lot, we should have snow guards of the roof. And if it is very cold, we should insulate.
Please forward a grant application form to this email address, because i could revolutionise town planning with all these brilliant ideas.
/sarc.
Ralph
Oh, and a piece if advice for the greenies out there. If you plan on making a lovely green roof do calculate the weight of the rain it will accumulate, otherwise your attempt to save the planet will turn into a deadly disaster.
Riga ‘green roof’ collapse.
http://www.bbc.co.uk/news/world-europe-25045879
As ever with green ideas, the fantasy sounds wonderful, while the reality is a catastrophe for everyone.
Ralph
Did the study consider the difference in greenhouse gas emissions for HVAC between the white and green roofs?
I’ve seen a few claims that you want a dark roof in winter. This will obviously depend on your locality, but in Montreal, you don’t have a choice: Mother Nature always paints your roof white in winter. There’s a couple weeks in spring and fall when you have the heat on but no snow on the roof; the rest of the time, your roof is either white by snow cover, or you hope your roof is white to keep the heat down. So, get a white roof!
(I rent, so I have a dark roof, sadly.)