PNC is banking on downtown Pittsburgh with a new building it’s billing as the “world’s greenest” tower. The financial services giant already has three skyscrapers in its hometown. When it opened its fourth on October 1, the Steel City’s newest building, dubbed The Tower at PNC Plaza, became the tallest built downtown since 1988.

And, according to its builders, it is also the city’s most efficient. The 33-story tower features a double-skin glass facade and a solar chimney that whisks fresh air through the building without a heavy-duty air handling system. Gary Saulson, PNC’s director of corporate real estate, tasked architects and engineers at Gensler, BuroHappold, and Paladino with bringing sustainable design techniques popular in Europe and East Asia back to the U.S.

Early in the project’s design, Hao Ko, a principal at Gensler, joined Saulson for a tour of ultra-efficient high-rises across Europe. They hit seven cities in seven days, Ko said. The double-skin facade that emerged in the final design is a rarity in the States, but is more prevalent overseas. “We had a game of counting how many buildings just in Germany had double skins,” said Ko, who traded blows with Saulson each time they spotted one, adapting the backseat VW Beetle-hunting game “punch buggy.”

The resulting design leaves the small space between the building’s two facades open and inhabitable, in case employees want to take a phone call in what Saulson calls “no-man’s land.” Should they want a more expansive breather, however, the building’s 28th floor offers a terrace and “indoor park” with greenery and a radiant-heated floor. At those heights the facade switches to a six-story cable wall system to cut down on visual clutter and create a more immediate connection to the outdoors, said Gensler principal Ben Tranel, technical director on the project.

“We realized we created something that’s a unique experience,” Tranel said: “looking out through a really transparent facade and feeling very connected to nature.”

Inside, outside air is encouraged to flow between open windows and the building’s central cavity and custom furniture from Haworth leaves the office floor open. Heat accumulates on the building’s roof, which is painted black, pulling air through a so-called solar chimney.

To fine-tune that concept, the designers built an inhabitable mock-up of one of the tower’s floors in a parking lot in suburban Green Tree, Pennsylvania. That model led to changes in the coloration of the blinds and other details, assuring Ko and other designers that their building would have a light airiness instead of a stack of whirlwinds.

“We wanted people to feel it, and get that benefit, that sensation of cool air coming across your face and your body,” said Ko, adding, “That’s actually harder to do in a controlled way.” The stack effect, which describes how air rushes out from high-rise buildings, worked against the idea of employee comfort.

And according to the client, comfort—not efficiency—was the primary goal. The Tower at PNC Plaza uses about half as much energy as a traditional office building and less than a quarter as much water, according to PNC, but Gary Saulson said the project’s ultimate success would hinge on whether employees want to work there.

“You need to put a stake in the ground and try to exceed what anyone has done and what you’ve done before,” said Saulson. “And we’re hoping someone else exceeds what we’ve done.”