Chicago is out ahead of New York again. First, the Windy City built the world’s first skyscraper, the Home Insurance Building, in 1885.

Then last year, Mayor Richard M. Daley began his crusade to make his city the greenest in the country by installing half a football field’s worth of flowers, grass, vines, and shrubs on the roof of City Hall. So far, the aerial prairie has saved the city $6,000 in annual heating and cooling bills, and is inspiring New York architects and environmentalists to take notice of the untapped potential of the city’s miles of rooftops.

“The benefits of this for New York City are going to be exceedingly wholesome,” says Ron Shiffman of the Pratt Institute Center for Community and Environmental Development, which along with the Environmental Business Association of New York State sponsored the first-ever international conference on rooftop development in early April. Topping Shiffman’s agenda: getting the Bloomberg administration to recognize the connection between the shortage of affordable housing and high building-operating costs. Promoting solar power projects, he argues, is critical to reducing that overhead.

A few pioneers are trying to use New York’s sunlight to their advantage. The Metropolitan Transportation Authority is installing a solar glass roof at the Stillwell Avenue subway station in Coney Island. Greenpoint Manufacturing and Design Center, home to more than 60 small manufacturing firms, plans to install solar panels across its North Brooklyn roof. And on 23rd Street, along the East River, the Community Environmental Center will cover the top of its new headquarters with plants and solar panels.

“This is probably going to cut our building-wide energy costs by 50 percent,” says David Sweeny, Greenpoint executive director.

But replicating these models on a wider scale is difficult. Sweeny plans to spend $600,000 for technology that will cover just 20 percent of his massive building’s energy needs. Without the ability to sell excess power back to the grid, which state law does not require utility companies to allow, he says “You’re never going to create a high-efficiency system.” And on top of those costs, the Department of Buildings’ permitting process is known to create long delays.

Advocates for greener projects are determined to push for administrative and legislative changes to make these projects more feasible. For now, Shiffman says their hope sits with City Council Speaker Gifford Miller, who organized a panel on conservation development last August. But getting the mayor’s ear is not out of the question: “The biggest threat to low-income housing is operating costs,” says Robert Politzer of Green Street Construction. “And when you have a building that’s leaking water and leaking energy, that puts pressure on the cash flow, which puts pressure on the landlord-tenant relationship.”