France, long the world's arbiter of haute couture, is taking aesthetics to a loftier level: the rooftop.

As it spruces up its green portfolio ahead of global climate talks in December, France approved a law last week that requires the roofs of new commercial buildings be covered—at least in part—by either solar panels or plants.

Green roofs have gained popularity in recent years as more cities worldwide promote their use as a way to save energy. Some, including Canada's Toronto or Switzerland's Basel, even mandate rooftop vegetation in building bylaws.

Advocates say these roofs—whether bedecked in sedums, vegetable plants, or wildflowers—help insulate buildings and thereby reduce the need for both heating and air conditioning.

The impact can be substantial. A study this week by Spanish researchers found that dense foliage can reduce the heat entering a building through the roof by 60 percent and act as a passive cooling system.

Green roofs help reduce runoff by retaining rainwater and improve air quality by absorbing pollutants. By taking in more heat during the day than they can release overnight, the plant-covered surfaces can also lower the "heat island" effect in urban areas that are warmed by asphalt roads and tar roofs. (Green walls offer similar benefits.)

In densely-developed cities, they also offer birds a place to nest and people a place to grow food. (In Brooklyn, rooftop garden grows.)

Green roofs cost more to install and maintain, and their price and complexity deter many homeowners and developers. Yet a 2008 University of Michigan study found that their benefits, including a longer-than-average lifespan, more than offset the extra up-front investment.