News Brief

White Roofs in Cold Climates a Mistake, Says LCA Study

While roofs with solar panels are a plus, researchers suggest white roofs in Canada do more harm than good because of steep heating penalties.

April 28, 2015

This white roof might be increasing energy use in its cold Vermont climate.

Photo: Candace Pearson
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.

Read more

Are Cool Roofs Green? The Answer’s Not Black and White

Better Choices in Low-Slope Roofing

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Comments

May 13, 2015 - 10:32 am

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?

May 13, 2015 - 10:32 am

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. 

May 19, 2015 - 4:57 pm

Hi Nash,

The study was assuming insulation levels at the level required by Canadian building code. You are right that insulating beyond code would minimize the energy penalty observed in this study. And I completely agree that minimizing the urban heat island effect can be good rationale for white roofs if the location does in fact see those trends. More on that discussion here: https://www2.buildinggreen.com/article/are-cool-roofs-green-answer-s-not...

To clarify, the "essentially negligible" impacts of green roofs referred to specifically to toxicity. As the article mentions, the growing media can contain fertilizers that add nitrogen and phosphorous to stormwater. But in terms of reducing the quantity of stormwater runoff, the study did find that to be a beneficial impact.

May 13, 2015 - 10:33 am

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.

July 20, 2015 - 5:02 pm

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?

July 20, 2015 - 9:26 pm

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.