The tallest and largest Passivhaus structure in the world, the 26-story House at Cornell Tech in New York City, has won certification from Germany’s Passivhaus Institut after completion earlier this year and is now home to hundreds of university students and faculty.

According to a post at 475 High Performance Building Supply, the project was finished under budget and ahead of schedule. A blower door test, completed with a single blower door fan, measured 0.13 air changes per hour at a pressure difference of 50 pascals, 475 said, making it far tighter than the Passivhaus standard requires.

The project was built by a partnership of Hudson Companies and Related Companies and designed by Handel Architects. Developers broke ground in June 2015 at a site on Roosevelt Island, which is in the East River between the New York City boroughs of Manhattan and Queens. The building contains 352 units of housing.

To reduce costs and complexity, designers used conventional materials and techniques wherever possible, a report on the project by Handel says. The building is cast-in-place concrete with a high-performance skin made from prefabricated metal panel; the R-value of the exterior skin average 19, with values ranging from R-5 to R-40. Glazing was capped at 23% of the exterior surface, and windows were installed and sealed before the panels left the shop.

Handel said the panelized system had a number of advantages, including cost effectiveness, better quality control and faster on-site assembly. But getting the system to meet performance requirements was “incredibly challenging,” according to Lois Arena, director of Passive House services at Steven Winter Associates, which helped on the project.

The 270,000-square-foot project had a budget of $115 million.

Ventilation was a major hurdle

It took months for designers to decide how to provide ventilation and heating and cooling to the building while meeting energy use limits in the Passivhaus standard, Handel’s report says. Installing an energy recovery ventilator in each apartment would have added $1.5 million to construction costs while increasing long-term maintenance so designers settled on a centralized system that needed only a small number of rooftop air filters that were easy to maintain without disturbing tenants. Two custom ERVs serve the entire building.

Heating and cooling is provided by a variable refrigerant flow (VRF) system, Handel says, with condensers for the refrigerant located on a thermally broken balcony on the side of the building. There are more than 500 evaporators to provide conditioned air to apartments. Engineers had determined none needed to have a capacity of more than a quarter-ton (3,000 Btu/hour), but nothing on the market had a capacity of less than a ton. “We hope the market will respond with smaller capacity units in the near future,” Handel said.

Other challenges included a public design review process in which the city’s Public Design Commission originally insisted on an all-glass building facade, and conflicting ventilation requirements between the Passivhaus standard, LEED, Energy Star, and city building codes.

As Handel’s report explains, builders also had to be willing to fix things on the fly.

“Going into the project, we knew that even if the design worked perfectly on paper, there were so many unknowns that could come up during construction,” it says. “While we tried to work out everything ahead of time, many of the details that were planned in preconstruction ended up being changed onsite.”

Managers with Monadnock Construction, which built the tower, went through Passive House contractor training, and the company built a 10-by-20-foot wall mockup with both a window and a door to make sure they understood the details of air-sealing. A consultant trained subcontractors how to apply sealing tapes properly. Workers on the job were told not to lean any building materials against the walls and to alert a supervisor should the air barrier be punctured.

The project is part of a $2 billion, 12-acre campus for Cornell Tech, as detailed in this report published by Curbed New York.