Green Roofs Reduce the Urban Heat Island Effect

A new green roof project organized by Columbia University and New York power company Con Edison adds to a growing body of research that demonstrate green roofs reduce the urban heat island effect. Using Con Edison’s training center in Long Island City, Queens, the researchers found that a layer of roof-friendly soils and plants reduce the rate of heat absorption by 84 percent in the summer, bringing down building cooling energy costs as a result.   

The urban heat island effect refers to localized urban warming caused by lots of paved, dark-surfaced rooftops, streets, and parking lots. Given much of any city is covered in these low-albedo surfaces, cities can experience temperatures significantly higher than nearby green areas. Just in New York City, say Columbia researchers, perhaps “two-thirds of New York’s localized warming over the last century” is due to surfaces like conventional black rooftops, which absorb and then re-radiate light from the sun as heat. 

Reducing the urban heat island effect isn’t just about lowering urban heat levels though, it’s crucial for reducing high inner-city asthma rates, which are caused by high-particulate hot days, and limiting deaths from heat exhaustion (see earlier post).  In parts of Europe, the dark, absorptive surfaces of urban landscapes along with increased smog have exacerbated the effects of heatwaves over the past few summers and contributed to rising death rates from heat exhaustion, mostly among the elderly or people with existing lung problems. 

According to Stuart Gaffin, a researcher at Columbia, urban roofs can reach temperatures of 175 F mid-day, even during cooler days. Gaffin told The Guardian (UK): “These [conventional roofs] are almost dangerously hot spaces. That’s a huge heat load that we can get rid of.” As has been demonstrated through a range of green roof demonstration projects, including the one on top of ASLA’s headquarters, one solution for reducing rooftop temperatures is plants. Rooftop plants regulate a roof’s temperature through evapotranspiration. Gaffin explains: “They evaporate copious amounts of water That takes a lot of energy and means it’s a great way to stay cool.” Reducing extreme shifts in temperature also means lower maintenance costs. With lower temperatures, roofs no longer expand and contract as much as they do when ranging back and forth between wildly different temperatures during each day. 

While oscilating heat temperatures can’t be used to prove climate change is occuring (it’s more about broader shifts in climate trends), micro-level meteorological stations can be added onto roofs and streets to track urban temperature change and the potential impact of green roofs on urban micro-climates, argues The Guardian. “The U.S. National Weather Service recommends placing sensors at least 100 feet from paved or concrete surfaces,” a goal that may truly be difficult in many inner-city areas.

White roofs are another solution seen as lower-cost than green roofs (see earlier post). Both have costs and benefits though. White roofs may be cheaper to install at first, but need to be periodically repainted or they get dirty and fail to reflect light well. Green roofs may be more expensive up front, but also deal with stormwater runoff, which is a growing and expensive problem in many cities (see earlier post). On the flipside, someone needs to maintain the green roof (and water it during long dry spells). The benefits of both can be maximized by combining options in one roof. 

Overall though, both significantly reduce energy usage, which is what Con Edision is interested in. In an earlier study, Lawrence Berkeley National Lab researchers found that painting roofs white, very light grey, or with an infrared reflective paint can yield a 20 percent energy savings. Green roofs have been found to yield a 15-25 percent savings in summertime energy costs and catch 40-60 percent of stormwater, reducing flow into a city’s sewers. 

Read the article, check out the study and green infrastructure resources, and see long-term performance data from ASLA headquarter’s green roof.

Also, with all the talk on creating new jobs, local green or white roof installation jobs may be a good direction. Learn more about Majora Carter’s plans for creating green community infrastructure with jobs like these.

Image credit: Con Ed  

2 thoughts on “Green Roofs Reduce the Urban Heat Island Effect

  1. WillG 11/03/2010 / 2:46 pm

    Great article! I am happy to see a broader, scientific discussion of the reduction of urban heat islands through green roofs. I am also glad you mentioned storm water runoff. In some cities, taxes are assessed on the run off, though buildings with green roofs get reduced rates.

    Another benefit of green roofs is the ability to trap and sequester fine, particulate air pollution. For high air pollution cities, like Shanghai, green roofs can prove a low-cost alternative to expensive air pollution control devices.

    Finally, I read an article recently about combing PV solar cells with green roofs ( It turns out, that the cooling provided by the green roof makes the PV cells more efficient and last longer.

  2. Milad Zabeti 02/13/2014 / 12:36 pm

    Reblogged this on DESIGNABILITY and commented:
    My very interesting future research; Mitigating Urban Heat Island via Green Roofs!

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