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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.

Technical summaries for each solution will be available May 1, 2017.

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