Some people challenge themselves by competing in marathons or triathlons; for others, it's hot-dog eating contests or chili cook-offs. After completing LEED and GreenPoint Rated checklists for 20 buildings in the last 10 years, San Francisco architect David Baker knew what mountain he wanted to summit next: building a structure entirely self-sustaining in its energy use. He planned a freestanding cottage behind his home in the Mission District, confident he would be able to secure GreenPoint Rated and LEED for Homes Platinum certification. The real challenge would be in meeting the additional demands of Passive House and Net Zero Energy (certified by the International Living Future Institute). "It was a learning experience," says Baker. "I wanted to see if it was possible to do net zero in an urban setting instead of on acres of open land."
Courtesy Matthew Millman
For Baker, who has promoted urban living through numerous award-winning infill projects, the cottage is a prototype for living well within a small footprint. It is part of a tiny mixed-use development he has created on Shotwell Street, which contains three dwellings in a space the size of a standard San Francisco residential lot. The main house, which he remodeled previously, features living quarters above a rental studio and a commercial space for public exhibitions at the ground level. The new cottage, where the architect and his partner have taken up primary occupancy, is separated from the other structure by a common courtyard. The loft apartment, a modest 712 square feet over a ground-floor 430-square-foot office/workshop, combines both high-tech building science and creative architectural details. Constructed for about $600 a square foot, the design was not about return on investment but about "doing everything to the extreme," as Baker puts it, to reduce the building's environmental impact.
Of the four building programs, the most challenging for Baker was Passive House, a balancing act that requires an extremely tight, super-insulated structure to minimize supplemental heating and cooling (which typically accounts for more than half a home's energy use). An air/vapor barrier, critical for the house's airtight seal, was installed on the walls, roof, and floor. Then the house was essentially encased in a giant Styrofoam cooler, with a continuous layer of extruded polystyrene over the walls and roof and a layer of expanded polystyrene beneath the concrete floor slab. To address the moisture issues that arise with supertight structures, Baker was careful to put the vapor barrier underneath a sufficient layer of insulation so that warm interior air would never get cold enough to condense within the wall cavity. Triple-pane windows help with the inevitable heat loss that occurs through glazing. "It's crazy-tight. If this house were underwater, it would float—it's basically a boat," says Baker.
The proof is in the blower-door test, which determines airtightness: the Zero Cottage has a 0.43 ACH50, surpassing the Passive House bar of 0.6 ACH50 with ease. For comparison, typical new residential construction has a 7 ACH50 (meaning all the air in the house would be replaced seven times within an hour).
With this impermeable structure in place, the other major component of Passive House comes into play. A heat-recovery ventilator (HRV) allows the house to recapture heat that is typically exhausted and wasted—during hot showers, cooking, even the operation of the refrigerator's cooling coil—and deliver fresh, filtered air. Even in temperate San Francisco, Baker is now a complete convert to HRVs. "I've gone from wanting to have open windows all the time to enjoying fresh filtered air within this quiet sanctuary," he says. "With the windows closed, you can't tell that you are in the middle of a city." He sees HRVs as a potential solution to urban noise and pollution. "People are attracted to the suburbs because of the supposed quiet, but with an HRV you could build near a freeway. I would never build a house anywhere without one," he adds. All appliances are electric, including radiant-heating panels (a supplement to the HRV), a combo washer-dryer unit, and an induction stove. Hot water is supplied through a combination solar hot-water system and electric tankless water heater.
After collecting a year's worth of data, Baker was able to confirm he easily outstripped his baseline goal of net-zero energy. Over the course of the year, the house sent 2,636kWh back to the grid out of the roughly 5,500kWh it produced with a 3kW photovoltaic system. Cantilevered off the roof, the PV array captures more energy than traditional flat-mounted versions—an example of design restrictions becoming a benefit. Because the small roof had to be clear for fire department access, Baker came up with a structure that features double-sided solar collectors that shade the stairs leading to the front door. San Francisco's mild temperatures are a clear advantage in meeting certification, but the Zero Cottage shows the potential in moderate climates to go well beyond net-zero.
While the dwelling is what Le Corbusier called "a machine for living," it also exudes the warmth that comes with using reclaimed materials. In this house, Baker took the opportunity to debut a rainscreen system of his own design. "You want to go with the tried-and-true for clients, but innovation in construction requires a prototyping attitude," he says. The disco-fabulous rainscreen is a mosaic of mirrored, matte, and rusty salvaged metal shingles held in place by clips and caulk tape. The wall by the front door is covered with yet a different cladding, this one made of maple flooring salvaged from one of Baker's multifamily housing projects and then charred, a Japanese technique for finishing wood called shou sugi ban. The blackened slats were then attached to cedar battens for what Baker describes as "a low-tech rainscreen."
The interiors manifest a similar spirit of creative reuse. Life in the Zero Cottage is compact, with a combined kitchen and living area in the main space and a sleeping loft above. The loft has a handsome guardrail and built-in storage made from the framing of the shed that previously occupied the site. More salvaged maple was used for the floors and minimally finished with VOC-free flaxseed oil, while the gypsum-board walls are finished in VOC-free white plaster. The loft is reached via a stair with alternating treads, created for tight industrial spaces. Another staircase provides access to the roof, where a garden of succulents flourishes in planters made of old motorcycle tires. However, to climb through the roof monitor, you have to be rather small, a fact Baker discovered when a guest couldn't fit through. "Next time I'll create better roof access," he says. "There's always something, right?"