My understanding (perhaps unfounded) was that solar panels actually had closer to a 25 year life-span, and contain mercury making their disposal problematic at end-of-life. I wonder if this was accounted for in the study - any thoughts?
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.
Are Cool Roofs Green? The Answer’s Not Black and White
Published April 28, 2015 Permalink Citation
Pearson, C. (2015, April 28). White Roofs in Cold Climates a Mistake, Says LCA Study. Retrieved from https://www.buildinggreen.com/newsbrief/white-roofs-cold-climates-mistake-says-lca-study
End of life for solar panels?
Granted I didn't read the full study, a couple of these conclusions seem very short sided and narrow minded in the larger green building picture.
- White roofs doing more harm than good? Well, only if you have designed a poorly insulated thermal envelope and are counting on heat from your roof to warm your interior. But this is extremely inefficient. If a properly insulated envelope and efficient HVAC system is designed, the white roof will reduce contributions to the urban heat island effect which is proven to have negative environmental consequences, therefore the white roof is having a positive impact.
- Vegetated roof impact on stormwater and air quality is "essentially negligible?" I can only assume they were studying a 2-3" sedum roof in which case, sure, I'll grant them that. However I am working on many projects with intensive green roofs that significantly reduce stormwater runoff.
Did the study take into account the pitch of the roof? A flat or low-pitched roof in a northern climate gets plenty of sun in the summer, but not in the winter. In that case, a light-colored roof is more beneficial, because it reflects the hot sun in the summer. In winter, a low-pitched dark roof will not absorb a significant amount of heat because of the angle at which the sun hits it.
Temp tarp roofs
was just wondering though some roofs might be expansive, wouldn't it be cost effective to roll out a white tarp in summer, and pull it back for black in winter?
tarp on roof
I love it—a retractable roof, for the heat rejection benefits. The legacy of Montreal's Olympic Stadium and others smiles upon this idea.
But look to that example for some caution. Taking a roof on and off is bound to run into practical problems, work, time on ladder, wear and tear... and what about roof penetrations, complicated geometry, etc.?
I think you've got to think bigger on this. Have you looked at planetary engineering to stave off global warming?
Seriously, I like the idea, but I don't think it's practical, and would have questionable return on effort.
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