A Berkeley Lab report comparing the economics of white, green and black roofs calls white roofs the winner and one of the authors says black roofs should be phased out.
In the 50-year life-cycle cost analysis, white roofs cost nine dollars per square foot less than green roofs, or 30 cents per square foot yearly, and black roofs cost seven dollars per square foot less than green roofs over the 50 years.
The report titled "Economic Comparison of White, Green, and Black Flat Roofs in the United States" by Julian Sproul, Benjamin Mandel, and Arthur Rosenfeld of Berkeley Lab, and Man Pun Wan of Nanyang Technological University in Singapore has been published online and will be in the March 2014 volume of Energy and Buildings.
Previous findings at Lawrence Berkeley National Laboratory Heat Island Group in Berkeley, California:
- white roofs reflect heat and sunlight very well, better than green or black roofs
- both white and green roofs cool their buildings on sunny days
- green roofs are best at cooling buildings in summer due to plant evaporation
- green roofs are best insulation in winter for trapping heat
- green roofs do not offset climate change like white roofs because white ones reflect roughly three times more sunlight back into the atmosphere, absorbing less sunlight at the earth's surface, offsetting part of the greenhouse gas emissions warming effect
- white and light-colored roofs can reduce energy use and lower utility cost
- dark roofs cause higher air conditioning cost and increased carbon dioxide (CO2) emissions.
Note that the study is limited to economic analysis. Other factors need to be considered:
- a white roof reflects more heat while a green one absorbs it. Some of the reflected heat goes to neighboring buildings which increases their air conditioning use
- the living roof's CO2 eating plants absorb an unmeasured amount of sunlight, convert the CO2 in photosynthesis and absorb sunlight in their moisture transpiration.
- a living roof's extra insulation and thermal mass save an unmeasured amount of energy
- vegetation on green roofs aids storm water management
- studies have shown that being near greenery increases human happiness
- black roofs pose a major negative health risk in cities with high summer temperatures as evidenced by the Chicago mortality rate on the top floor of a black roofed building in the 1995 heat wave
- more health and environment factors.
The bottom line is a roof that saves more on air conditioning energy than it loses in heat energy reduces power plant emissions indirectly lessening negative climate change effects. Request a copy of an article from Ben Mandel by emailing BHMandel@LBL.gov.
The Heat Island Group’s lead scientist and a professor of Building, Civil and Environmental Engineering at Montreal's Concordia University, Hashem Akbari, says the equivalent of emissions from the world's 600 million cars over 18 years would be offset by switching to reflective roofing and pavements in global urban areas. Urban white roofs would also lower smog levels and local temperatures made higher by increased paved surfaces.
A roof should be high in both of the properties by which a cool roof is measured, its reflectance or ability to reflect energy off the roof, and its thermal emittance, ability to radiate absorbed heat. The average total square footage of a United States home is over 2,000 and a 1,000-square-foot cool roof saves air conditioning use that would emit about half a ton of carbon dioxide yearly.
The best time for installation of a cool roof is either when major roof maintenance or a totally new roof is needed. But for other times, there are white roof coatings that substantially improve the reflecting ability of an existing roof while sealing and protecting it for longer life. Akbari says the coating cost, at 50 cents to one dollar per square foot, will be paid back by lower energy bills and extended roof life.
The argument has been made that white roofs are not so great in colder climates but research shows that because winter days are shorter, skies cloudier, the sun's angle lower, and roofs sometimes snow-covered, a dark roof's heating benefits are negligible. Akbari says "The amount of heat savings you may lose in the winter would be, at the maximum, 30 percent of the summertime savings. If you need cooling in the summer and heating in the winter, no matter where you are, a white roof will most likely save you money."
Check out cool roof products in the Lawrence Berkeley National Laboratory's Cool Roofing Materials Database where the Heat Island Group has a Solar Reflectance Index (SRI) for rating cool roofs. The Heat Island Group says to reduce utility bills with an SRI rating of 30 percent or higher for sloped roofs and 70 percent or higher for flat roofs. There is also a Rated Products Directory on the Cool Roof Rating Council site.
For environmentally friendly roofs in the Greenville, South Carolina area contact Dillard-Jones Renovations, Greer, 864.527.0463; Living Roofs, Inc., Asheville, 828.252.4449; Kleiworks, Asheville; Metacrylics, 888.285.9842; Greenville Foam Roofs, 864.991.3183.