More Than a Million Plants

Cal Academy_Building_exterior

The green roof at the California Academy of Sciences still inspires.


By Maria Shao

[dropcap]T[/dropcap]he field wears a blanket of green. Rustic grasses, flowers, ferns and other plants cover not only flat sections but also several mounds. It is an undulating two-and-a-half acre verdant terrain that seems like a landscape from another planet.

It’s actually the green roof atop the California Academy of Sciences in Golden Gate Park. This carpet of life—70 species of native Californian plants plus insects and birds—is a symbol of San Francisco and an important exhibit of the science museum.

[quote]Museum architect Renzo Piano envisioned the roof as the result of lifting up a section of Golden Gate Park and sliding a building beneath.[/quote]

“The mission of the California Academy of Sciences is to explore, explain and sustain life. Having a green roof ties in closely with our mission,” said Ari Harding, director of building systems for the San Francisco museum that includes an aquarium, rainforest and planetarium. “The green roof is one of the icons of sustainability.”

Museum architect Renzo Piano envisioned the roof as the result of lifting up a section of Golden Gate Park and sliding a building beneath. The public can view the expanse, with its seven hills blanketed by 70 native California plant species, from a rooftop terrace. The roof, installed in 2007 when the museum was built, is one of the world’s most complex green roofs and has helped the museum twice earn LEED Platinum ratings.

Initially, 1.7 million plants in over 50,000 biodegradable trays made of coconut husk fiber were installed. Nine species, including beach strawberry and self-heal, were planted; dozens more species have been planted since. Surrounding the roof’s perimeter are 60,000 photovoltaic cells that contribute 5 percent or more of the museum’s energy needs and also provide shade.

Because of the vegetation, the roof stays an average 40 degrees cooler than a standard roof, helping to keep temperatures inside exhibit halls 10 degrees cooler than with a conventional roof.  The sharp slopes of the rooftop hills—up to 60 degrees—and automated skylights strategically placed in the mounds help funnel cool outside air into the central piazza inside, naturally ventilating the surrounding exhibit spaces. The skylights also allow natural light, reducing the museum’s artificial lighting needs. Conversely, hot interior air rises and is released through the skylights. The exhibit halls do not have conventional air-conditioning, only radiant heating beneath the floors.

The living roof also keeps rainwater out of the city’s sewer system. The vegetation absorbs most precipitation, while excess is diverted to a recharge station under the building’s loading dock, then sent to an aquifer under the park to be used to irrigate the park. “One hundred percent of rainwater is either used immediately by plants or is diverted into the aquifer. We get to keep it and use it for irrigation again in a closed loop system,” Harding said.

The roof provides a vibrant habitat for dozens of insect and bird species that have been studied by scientists. But because the roof needs to stay green year round, the museum uses about 1.5 million gallons of water a year for irrigation during the dry months, an expense that Harding said is worthwhile: “It’s teaching science and botany. It’s bringing people in the door, so it’s been a good investment,” he said. “The more people see a green roof, the more they’ll be interested in installing green roofs and thinking about sustainable architecture.”

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