One of the best tools for combating climate change is right over our heads.
Many cities have begun phasing out dark roofs in favor of white and green vegetated roofs, to mitigate the adverse effects of dark impervious surfaces. Dark roofs absorb heat and contribute to the urban heat island effect, while white roofs reflect the sunlight back into the atmosphere and help cool the structures they cover. Green roofs contribute a cooling effect, too, but they’re much more expensive to install, maintain and replace.
These are some of the conclusions drawn by researchers at the Lawrence Berkeley National Laboratory. Julian Sproul, Man-Pun Wan, Benjamin H. Mandel and Arthur H. Rosenfeld conducted an economic analysis of the costs and benefits of white, black and green roofs and found that white roofs are superior at cooling, at a much lower cost.
The study used a 50-year life-cycle cost analysis (LCCA), assuming a 20-year service life for white and black roofs and a 40-year service life for green roofs. Relative to black roofs, white roofs provide a 50-year net savings $2.40/ft2, and green roofs have a negative NS of $6.60/ft2.
The study found that the long lives of green roofs can’t compensate for their installation cost. However, it also pointed out that, while the 50-year NS of white roofs compared to green roofs is $8.90/ft2, the annualized cost premium is just $0.30/ft2-year. Because this annual difference is so small, the study recommended that personal preference should drive the choice between a white and green roof.
In summary, the researchers concluded: “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.”
In a related development, DOE has funded research for how color can be added to roofs without compromising their reflective quality. Read more here.
Note: Some industry experts have criticized this study for not taking into account the economic benefits of dark roofs in cold climates, among other things. Read their analysis here.