White Roofs in Cold Climates a Mistake, Says LCA Study
A recent life-cycle comparison between white, vegetated, and solar roofs found white roofs have a negative impact on the environment—at least in cold Canadian climates—while solar roofs provide the greatest environmental benefit.
Taking into account impacts from manufacturing, transporting, and installing materials, as well as toxicity, the study published in The Journal of Industrial Ecology found white roofs had some smaller impacts compared to a conventional gray roof during the use phase. They contained less-toxic ingredients and fewer respiratory toxicants, and contributed less to acidification. However, the heating penalty that white roofs imposed by reflecting solar rays during the winter ultimately increased energy use in the northern locations that were studied, so the benefits didn’t come close to offsetting upstream impacts (for more detail on these issues for other climates, see Are Cool Roofs Green? The Answer’s Not Black and White).
By comparison, extensive vegetated roofs provided net benefit for most impact categories, though surprisingly, the green roofs’ influence on stormwater and air quality were “essentially negligible,” according to the study. Green roofs reduce eco-toxicity by avoiding pollutants like metals and polycyclic aromatic hydrocarbons, which can seep from shingles and tar on a gray roof. However, nitrogen and phosphorus can leach from the growing medium, especially in the first year that it is installed.
Solar roofs clad in photovoltaic panels provided the greatest net benefit over a 50-year lifespan by orders of magnitude one to three greater than either white or vegetated roofs for all impacts studied. The authors note, however, that their results are highly dependent on climate and location. For example, white roofs do save energy in warmer locations, and the insulative benefits of green roofs might be more apparent if applied to buildings that don’t already have as much insulation as Canada’s building code requires.
Published April 28, 2015