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A south-facing heat sink mass wall stores heat and diffuses light, creating an all-seasons solarium for an energy efficient home in Upstate New York.
Dana Cupkova and Kevin Pratt, founders of Ithaca-based design and research practice Epiphyte Lab, met the client for which they built Hsu House in one of Cupkova’s classes at Cornell where she teaches design and fabrication strategies for ecologically adaptive construction systems. The client, a medical doctor, was in the class with the goal of designing his own energy efficient home in Danby, New York, but after an initial consultation, asked Cupkova and Pratt to design it instead.
The 2,200-square-foot house is designed using passive solar principles. Interiors are organized around a three-story living space, which also functions as a ventilation stack. At its center, a south-facing, cast-in-place concrete mass wall creates a heat sink for the house, absorbing heat in warm weather and releasing it when the temperature drops. “It tempers the internal environment of the house,” said Cupkova. “It’s a giant radiator and cooling device. The house has no AC.”
Designing the wall posed several challenges. The team had to create a shape with a large surface area to maximize thermal transfer from the sun to the wall to the home’s interior. But the wall also had to let daylight into the space and, as the sculptural focus of the house, have an interesting shape and texture.
The team began by testing hundreds of parametric iterations of the wall using Grasshopper software, optimizing the design for four-foot-wide concrete formwork components to keep costs down. When Cornell’s CNC mill became unavailable, they turned to Syracuse fabrication company SCCM. Because CNC-milling the high-density polystyrene concrete form liners resulted in a bumpy texture, the team had to rethink its fabrication strategy.
“Andy McDonald at SCCM was seminal in helping us figure out the project,” said Cupkova. Frank Parish, Cornell’s digital fabrication shop technician, helped oversee the work. Instead of working with the concrete form liners directly, the shop CNC milled more than 100 guides for the forms, which were then laminated and hand-cut using a hot wire-cutting knife.
Once the forms were in place at the house, the entire structure was reinforced with rebar, allowing it to support part of the floor above. The wall is 23 feet long and 14 feet high with widths varying from 5 1/2 to 16 inches. As Its conical shapes created funnels in the formwork, the concrete mixture had to be adjusted to be more fluid. According to Cupkova, the job is an atypical one for the Ithaca area’s concrete trades, and she credits general contractor Paul Hansen Construction with making the project run smoothly (the fact that she was eight months pregnant also helped her gain some sympathy). As concrete was poured the wall was vibrated, allowing the material to fill the small spaces in the forms. The entire pour was completed in three hours.
Now, the house has weathered two seasons without much use of heating or even electric lights thanks to the mass wall. “In the summer, after it was finished, the temperatures were in the 90s and it was 70 indoors,” said Cupkova. “It works.”