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Buildings and Cities

Green Roofs

Designed by Dr. Stephan Brenneisen, the green roof of the Cantonal Hospital in Basel, Switzerland, overlooks the town and Rhine River. Constructed in 1937, the building welcomed its first green roof in 1990, which mimics the riverbank of the Rhine in design. The vegetated roof features two gravel areas to attract birds, as well as areas of sedum, herbs, moss, and large grass meadows. It is interspersed with big branches and stones to provide cover, and is monitored for birds, spiders, beetles, ladybugs, bumblebees, and more.

The average rooftop is brutal terrain, taking a beating from sun, wind, rain, and snow, and enduring temperatures up to 90 degrees higher than the surrounding air on a hot day.

Green roofs, in contrast, are veritable ecosystems in the sky. They may support a simple carpet of hearty, self-sufficient groundcover such as sedum; or they may sustain full-fledged gardens, parks, or farms. The soil and vegetation function as living insulation, moderating building temperatures year-round—cooler in summer, warmer in winter. Because the energy required for heating and air-conditioning is curbed, greenhouse gas emissions are lower, as are costs.

Cool roofs achieve similar impacts but with different methods. When solar energy hits a conventional dark roof on a 99-degree day, just 5 percent of it is reflected back into space. The rest remains, heating the building and surrounding air. A cool roof, on the other hand, reflects up to 80 percent of that solar energy back into space. Cool roofs reduce the heat taken on by buildings and the overall urban heat island effect in cities.

Construction incentives for green and cool roofs and building policy that encourages or mandates their use are the key drivers of proliferation.

References

temperatures…ninety degrees higher: Garrison, N., C. Horowitz, and C.A. Lunghino. Looking Up: How Green Roofs and Cool Roofs Can Reduce Energy Use, Address Climate Change, and Protect Water Resources in Southern California. Natural Resources Defense Council, 2012.

truck plant in Dearborn, Michigan: Klinkenborg, Verlyn. “Up on the Roof.” National Geographic. May 2009.

Brooklyn…urban agriculture: Miller, Mark J. “A Farm Grows in Brooklyn—on the Roof.” National Geographic. April 29, 2014.

energy use for cooling: Garrison et al, Looking Up.

life span [vs.] conventional [roofs]: GSA. The Benefits and Challenges of Green Roofs on Public and Commercial Buildings. Washington, D.C.: General Services Administration, 2011.

beauty and…well-being; biophilia: Grinde, B., and G.G. Patil. “Biophilia: Does Visual Contact with Nature Impact on Health and Well-Being?” International Journal of Environmental Research and Public Health 6 (2009): 2332–2343.

increased property appeal and value: Chiang, K., and A. Tan. Vertical Greenery for the Tropics. Singapore: Centre for Urban Greenery and Ecology, 2009; GSA, Green Roofs.

Singapore…green roof installation: Singapore National Parks. “Skyrise Greenery Incentive Scheme 2.0.” https://www.nparks.gov.sg/skyrisegreenery/incentive-scheme.

Chicago fast-tracks permits: Taylor, D.A. “Growing Green Roofs, City by City.” Environmental Health Perspectives, 115, no. 6 (2007): A306–A311.

San Francisco…green roof mandate: Snow, Jackie. “Green Roofs Take Root Around the World.” National Geographic. October 27, 2016.

conventional dark roof [vs.] cool roof: Hesson, Ted. “Cool Roofs.” The Atlantic. December 4, 2015.

relieve the urban heat island effect: Meichun Cao, Pablo Rosado, Zhaohui Lin, Ronnen Levinson, and Dev Millstein. “Cool Roofs in Guangzhou, China: Outdoor Air Temperature Reductions during Heat Waves and Typical Summer Conditions.” Environmental Science & Technology, 49, no. 24 (2015).

California…building efficiency standards: Akbari, Hashem, and Ronnen Levinson. “Evolution of Cool-Roof Standards in the US.” Advances in Building Energy Research 2, no. 1 (2008).

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