The Hsu House is a small 2200 sq.ft. house designed by EPIPHYTE Lab using passive solar principles to create an inexpensive, energy efficient dwelling for a young family. Rising out of a hillside south of Ithaca in upstate New York, the bifurcating monopitch roof covers a house organized around a three story main living space, which functions as both spatial nexus and a trombe wall assisted ventilation stack. The space is bifurcated by an interior south facing cast-in-place concrete heat sink mass wall, which stores sensible heat and animates light, creating an all season solarium that, in the summer, opens and unfolds into the natural landscape. The exterior cement-board siding surface of the house was dynamically modeled to produce a gradient pattern responding to cardinal solar orientation and to animate facade color to dissolve the mass of the house in surrounding landscape. The concrete mass wall was considered necessary to damp diurnal energy fluxes and store absorbed solar radiation. It was designed with three primary criteria in mind. First, wall surface area was maximized to increase the rate of thermal transfer between the sun, the wall, and the interior environment. Second, the shape was parameterized to allow light from the solarium to pass through the structure, allowing illumination of the north side of the first floor of the house. Third, the mass wall was designed as the sculptural focus of the house, defining the entryway, the kitchen, the solarium, and the edge of the triple height main living space. The large number of parametric iterations was tested also in relation to the fabrication process. The primary concern was maintaining a level of porosity that would create a sense of lightness, allowing for views through the wall and to counteract the tendency of the concrete to read as a massive and unwelcome subdividing element in what is, after all, a fairly compact and spatially diverse living space. The Hsu House holds Silver LEED certification.