Developers of rental properties have been notably more environmentally conscious than condo builders, in large part because it benefits their bottom line to save on electricity, fuel and water. For instance, the Durst Organization, which has been incorporating green measures across its many properties and projects in the city, installed a blackwater treatment system at the Helena and Via, two adjacent residential buildings it owns on the Far West Side. The system collects wastewater, cleanses it in a 60,000-gallon vat with special microbes and recycles it to the buildings' toilets and gardens.

"The buildings are about 47% more efficient than buildings of this size in terms of their water consumption," said Sydney Mainster, Durst's director of sustainability. "It saves potable water and saves the sewer system from having to take on our wastewater, which protects the waterways."

At John F. Kennedy International Airport, MCR Development is installing natural gas–driven reciprocating engines to fully power the new TWA Hotel, which will be completely off the power grid. Waste heat from the generators will be harnessed to provide hot water, a technique that is gaining currency. Extra energy will be stored in a truck-size battery for use during peak periods.

Other builders have recently pursued even more cutting-edge standards. Justin Palmer, founder and CEO of Synapse Development Group, adopted innovative techniques to build Perch Harlem, a 34-unit, 7-story apartment building at 542 W. 153rd St. The $24 million rental project achieved Passive House certification, one of the most stringent energy-consumption standards, which limits BTUs per square foot to 38,100, less than a third of the average.

"This building uses 80% to 90% less energy than a typical multifamily building," Palmer said. "And it didn't cost more to design or implement. It's a false perception that you have to make sacrifices to be energy-efficient."

Palmer gave the building an ultra-efficient foam facade and insulated every attachment between it and the building's structure. It became, in effect, a thermos, locking in heat and cold, regardless of the temperature outside. Heating and cooling accounts for roughly half of a residential building's energy consumption, the Urban Green Council said, making climate control crucial for efficiency.

"No one was building to Passive House standards in the city, but then again, no one had really built an electric car before Elon Musk, and now he's crushing it," Palmer said. "My view was, Let's take a radically different approach and build a community around environmental responsibility."

Palmer aims to achieve Passive House standards at a 14-story rental project Synapse is planning for Williamsburg.

On Roosevelt Island, The Related Cos. is building a 26-story student and faculty housing project to Passive House standards as part of the new Cornell Tech campus. In recent years the mega-developer has sought LEED certification for all its residential projects—rental and condo—in the city.

Some other condo builders appear to be taking steps toward efficiency. Hines is equipping the 1,050-foot, pyramid-shaped condo tower it is raising next to the Museum of Modern Art with 5,747 highly insulating triple-pane windows. The German-made 12-foot-by-7-foot windows cost millions more than conventional double-pane units, said David Penick, a vice president at Hines. It may be the largest installation of ultra-efficient glass in the city.

"It makes a window behave almost like a solid wall in terms of its thermal performance," Penick said. "But it definitely costs a premium."

The firm, which has developed several other green-conscious projects in the city, including the LEED Gold–certified office building at 7 Bryant Park, has its own efficiency mandates. But it did not use energy savings in its pitch to potential buyers at 53 W. 53rd St., illustrating the lack of confidence even environmentally oriented builders have that upscale buyers will care. Instead, the windows are being touted for being draft-free and buffering outside noise.

"There's improved acoustic performance, and buyers will notice that it makes the perimeter of an apartment very comfortable," Penick said.

At 40 Mercer St., a condo it recently built in SoHo, Hines considered using geothermal energy, a technique where piping, bored hundreds of feet belowground, uses the ambient temperature of the earth to provide heating in the winter and cooling in the summer. But the property lacked the necessary basement space.

The city's building code now requires building owners to consult with energy experts on ways to cut electricity and fuel consumption. In 2020 landlords will be required to publicly post energy grades, akin to restaurants' cleanliness ratings.

A gulf, however, remains between just meeting the code and pushing the boundaries of energy efficiency. Some condo developers appear focused on simply doing what is required. Extell is building the condo spire Central Park Tower; at 1,550 feet, it will be the tallest building by roof height in the city. A spokeswoman could not identify any systems or materials that will allow it to perform better than Extell's energy dud at 157 W. 57th St.

A condo tower being raised by Zeckendorf Development at 520 Park Ave., east of Central Park, appears to be following the same development model as a ritzy project the family-owned firm raised more than a decade ago, 15 Central Park West. That property consumed 253,700 BTUs per square foot in 2016, almost double the median energy use, and had an Energy Star score of 1, the lowest possible mark on the 100-point scale.

Vornado Realty Trust, meanwhile, is finishing 220 Central Park South, an 83-unit ultra-luxury building near Columbus Circle. That building too does not appear to incorporate any efficiency measures into its design.

"Just because these buildings are newly built doesn't mean they can't become energy-efficient [in the future]," said Robbins of the Natural Resources Defense Council. "But it's going to cost extra money to upgrade them. In some senses, given the wealthy buyers in these properties, they're probably in a financial position to do everything they can to be more efficient."