The developer of the world’s tallest building seeking certification under the international Passive House program expects energy savings of 70% to 90% compared to a conventional building. Thanks to the 270-ft-tall building’s energy-efficient systems, designers of the 270-ft-tall Cornell Tech Residential—a 26-story dormitory for Cornell Tech’s Roosevelt Island campus in New York City—project savings of 882 tons of carbon dioxide per year over the standard building, which is the equivalent of planting 5,300 new trees.
The building is constructed to “incredibly rigorous” air tightness standards never before executed on a building this tall, says Luke Falk, assistant vice president of sustainability for the Related Cos., which is developing the 57%-complete dorm with Cornell Tech and Hudson Cos. “The new approach to ensuring air tightness, including a vapor retarder and tape products not used in New York City very often,” creates some construction challenges, adds Falk.
The Passive House Standard is a voluntary international building standard developed by the Passive House Institute (PHI), in Darmstadt, Germany. Passive Houses minimize heating and cooling loads through passive measures, including orientation, massing, insulation, heat recovery, passive use of solar energy, solar shading and elimination of thermal bridges. In addition to Passive House certification, the building team is seeking LEED Platinum certification.
Though buildings must be designed to certain energy budgets, constructed to certain criteria and commissioned, Passive House certification is not dependent on actual building energy performance.
The Cornell Tech building, which contains 352 units with 536 beds for graduate students and faculty, has an annual heating budget of 4.75 kBtu per year; a cooling budget of 5.3 kBtu per year and an energy-use budget of 38.1 kBtu per year.
Regarding the air tightness, under the standard, the building has an allowable limit of 0.6 air changes per hour at 50 Pascals of pressure, which will be verified onsite at the end of the project with a blower door test.
During construction, to ensure proper air tightness of the façade prior to closing the exterior wall with interior drywall, general contractor Monadnock Construction Inc. conducts guarded blower-door smoke tests. The procedure involves isolating a section of the floor, and pressurizing and depressurizing the section using fans. The Cornell Tech building performed a successful guarded blower door test on the fifth floor on Saturday, June 18, says Aleksandr Yelizarov, Monadnock Construction's project executive.
To help it achieve Passive House standards, Cornell Tech Residential, on schedule to open in August 2017, has a façade that acts as a thermally insulated blanket. It is composed of a prefabricated metal panel system, with 12 ft tall x 36 ft wide units, containing triple-glazed windows and up to 11 in. of insulation.
The panels are fully taped and sealed, from the inside. A vapor retarder forms a continuous jacket, also inside, that prevents air leakage and helps with façade durability, says Deborah Moelis, project manager for Handel Architects, which designed the green-building systems with sustainability consultant Steven Winter Associates Inc.
To facilitate panel taping at column locations, perimeter columns are “pulled back” 8 in. from the slab edge and 10 in. in from the façade. That created an access space between the column face and the panel’s interior face. The dimension was determined by how much space a worker needed to reach in and seal the panel, says Moelis.
Because the building is intended to be airtight, low-volume filtered fresh air will be continuously supplied to every room and stale air exhausted from service spaces. This results in balanced and controlled ventilation with high-efficiency heat exchange, according to the building team.
In another departure from convention, each of the 352 units has its own interior ceiling-level fresh-air vent, independent of the room’s heating and cooling unit along the facade.
The vents are connected to a louver system on the southwest façade that extends the height of the building. The reveal is designed to be the “gills” of the building, providing an enclosed exterior space for heating and cooling equipment and allowing the building system to breathe.
For the individual heating and cooling units, the dorm has a variable refrigerant flow system, first developed in Japan in the early 1980s. VRF systems are enhanced versions of ductless systems. They permit more indoor units to be connected to each outdoor unit and provide additional features, such as simultaneous heating and cooling, and heat recovery.
Using VRF technology, the heating and cooling system “pushes refrigerant around the building” through piping, to heat and cool each unit based on the occupant’s needs, says Falk. “Rather than on or off, the system varies the flow of refrigerant for maximum thermal comfort,” he adds.
The unconventional mechanical design has interesting challenges, as well, says Falk. But, thanks to the tight envelope and the system design, the mechanical equipment is “very small” compared to a typical building, he adds.
With a high-rise Passive House, “there is definitely a learning curve,” says Arianna Rosenberg, senior project manager for Hudson. The project has a cost premium, but Rosenberg would say only that the project is meeting its budget.
Hudson is looking at developing other Passive Houses, which typically recoup some of the extra upfront costs with low operating costs. “We expect the prices will come down,” as contractors become more familiar with the systems, says Rosenberg.
That's likely to happen. Lois B. Arena, senior mechanical engineer for Steven Winter, says her firm has 18 Passive House projects underway.
“We are plowing the way for everyone to come behind us,” adds Handel's Moelis.