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.”
Economic comparison of white, green, and black flat roofs in the United States Julian Sproul,Man Pun Wan, Benjamin H. Mandel, Arthur H. Rosenfeld
• 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.
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