Green and Cool Roofs
Reduced / Sequestered
(To Implement Solution)
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.
In modeling cool roofs and green roofs, we account for climate-appropriate regional applications of each technology. If cool roofs grow from 5 percent of its relevant roof market at an annual 8–10 percent rate (CAGR) to 2050 and green roofs similarly grow from 1 percent at 9–11 percent annually, a total of 47–75 billion square meters of efficient roofing would be in place globally. Combined, these technologies could reduce carbon dioxide emissions by 0.6–1.1 gigatons at a cost of US$624–954 billion, with lifetime operating savings on heating, cooling, and maintenance of US$330–593 billion.
Note: August 2021 corrections appear in boldface.