Six years of planning, design, and construction led to a 2,302-square-foot trilevel home that uses a fraction of the energy that a conventionally constructed house in Colorado might use. With energy-efficiency measures and a 7 kW solar-electric system rooftop canopy, the all-electric home is on track to produce as much energy as it uses each year and receive LEED Platinum certification (see sidebar.)
Key to the home’s energy footprint is a passive solar design that minimizes active heating and cooling needs. The German Passivhaus standard served as a guiding force during the design process.
The home’s structural, thermal, and aesthetic spine is an exposed concrete mass wall, with all the floors and ceiling joists attaching to it. The wall bisects the floor plan in such a way that every room in the house, from top to bottom, benefits from the passive conditioning. The thermal mass absorbs heat during the day and releases it throughout the evening. Although the wall appears continuous, it actually addresses three different envelope conditions: interior only; interior/exterior; and exterior only. The two 8-inch-thick concrete faces are separated by 4 inches of insulation only in the interior/exterior portion to prevent thermal transmission. The contiguous face is also thermally broken where the roof attaches to the wall.
Wall thickness was driven by the structural requirements of the second condition (interior/exterior) more than any other consideration. Based on the glazing areas, however, it was calculated that 697 cubic feet of interior mass would be needed to adequately absorb passive solar gain. This amount is attained by the interior mass wall, and by the concrete and tile floors in the office, bedroom, and living room that receive direct solar gain.
The 9-inch-thick wood-framed walls and 12-inch roof cavities, as well as the space beneath the on-grade floor slab, contain polyisocyanurate spray-foam insulation—achieving an average wall R-value of 40 and an average roof R-value of 60. Fiberglass-framed, dual-paned Serious Materials 925 windows were selected to help maximize passive solar gain on the south side, with solar heat gain coefficients ranging from 0.35 to 0.45. The south-facing master bedroom window and southwest-facing windows and doors on the first and second floors admit solar gain, which is absorbed by the floor slab and radiated when the house’s air temperature is lower than the slab’s temperature. Though also partially shaded by an architectural overhang, the southwest windows require sun-blocking shades to minimize solar gain during the summer. The first floor’s south wall is slightly angled (at 80°), creating a sculptural angle and a tapering overhang that provides more shade as the sun gets higher in the sky.
As project engineer, Corbin developed multiple energy models and analyses to determine the home’s passive solar design and optimal mechanical makeup, taking into account the local climate and the area’s abundance of sun (66% of all daylight hours are sunny and clear). An initial analysis determined that the home, as originally designed, would require supplemental cooling 11% of the year and supplemental heating 77% of the year. This ratio was a factor in several design decisions, including the glazing ratio.
As a result of the energy models, the glazing area was increased to approximately 28% of the floor area, mainly on the south side of the home—a decision that reduced the building’s heating load at the expense of increased summer cooling demand.
“There’s always a trade-off when you design a home for passive heating and cooling,” Corbin says. “The large glazing area contributes to heating the building, but during the summer, it works against you. The key is finding the right balance.”