This was a question tour leader Matt Postal asked about half-way through last Saturday’s Sustainable Design in Midtown walking tour. We were standing at the S.E. corner of 42nd St. and Sixth Ave., looking at skyscrapers in three directions, but the green roof was behind us — Bryant Park. In the early 1990s, 86 miles of underground book stacks were constructed behind the New York Public Library and underneath the park which was itself being redesigned and reconstructed.

The rest of the stops on the tour were more expected. We began at The New York Times Building, which has a number of sustainable features, but didn’t try for LEED certification. (LEED is a green building certification process, which is time-consuming and can be costly.) The owners of The Times contend that they didn’t want to pay $100,000 for the honor. For other buildings, LEED status can be advantageous as proof of their commitment to sustainability. Among those is the Bank of America Tower which has received considerable attention as the first skyscraper designed to earn LEED Platinum certification. It features a gray water system, carbon dioxide detectors to determine when more ventilation is needed, concrete made with slag (a by-product of blast furnaces), and a cooling system that makes and stores ice during off-peak hours.

Our heads were filled with high-tech wonders as we arrived at our last stop, only to have a revolving door malfunction and trap a tour participant inside. (The irony did not escape us). Once the tour taker was freed, Joseph Lauro LEED AP led us through the offices of Gensler architects, which demonstrate the firm’s ability to design attractive interiors out of sustainable materials. At Gensler, clients choose the level of sustainability they want their project to achieve, but all benefit from the fact that Gensler has incorporated environmentally-sound practices in to the way it does business every day, from waste stream management to “green” wall paint.

As usual, Matt Postal raised questions more often than he offered opinions, leaving each tour taker to his or her own assessment of the state of sustainable building today — and eager to see more. Visit MAS.org/tours for more information about MAS walking tours.