If you run a business that’s all about the bottom line, and have a flat roof, you want it to be white. But here’s the real stunner: these white or “cool” roofs could even be the best choice for the planet, beating out celebrated green roofs on climate-change benefits.
That’s the conclusion of government researchers, who compared three major roof types – standard black; white; and green, aka living roofs.
The researchers did 50-year life-cycle cost analyses on nearly two dozen commercial flat roof projects. The green roofs got the advantage of an assumed 40-year service life, compared to 20 years for white and black roofs. Even so, green roofs were the most expensive. From the Berkeley Lab release:
The 50-year life-cycle cost analysis found that even the most inexpensive kind of green roof (with no public access and consisting of only sedum, or prairie grass) costs $7 per square foot more than black roofs over 50 years, while white roofs save $2 per square foot compared to black roofs. In other words, white roofs cost $9 per square foot less than green roofs over 50 years, or $0.30 per square foot each year.
But what about climate change? White roofs win there, too.
[U]nlike white roofs, green roofs do not offset climate change. White roofs are more reflective than green roofs, reflecting roughly three times more sunlight back into the atmosphere and therefore absorbing less sunlight at earth’s surface. By absorbing less sunlight than either green or black roofs, white roofs offset a portion of the warming effect from greenhouse gas emissions.
The researchers do acknowledge that green roofs can have benefits that weren’t measured in their study, said to be the first to compare the economic costs and energy savings benefits of these three major roof types.
For example rooftop gardens provide stormwater management, an appreciable benefit in cities with sewage overflow issues, while helping to cool the roof’s surface as well as the air. Green roofs may also give building occupants the opportunity to enjoy green space where they live or work.
The Berkeley Lab study appears in the March 2014 issue of the journal Energy and Buildings. The abstract is available free online here.