Heading for Net-Zero
The Rose House is a study in the balance of efficiency, aesthetics, and comfort. For designers, one of the most significant aspects is that its tiny footprint yields such gracious space. Measuring a mere 800 square feet (74 square meters), the two-bedroom home seems to take advantage of every inch.
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The open kitchen, dining, and living room space is flooded with south light from generous eye-level and clerestory windows. East and west windows are limited to areas large enough to provide adequate balanced daylight. The overall effect is a testament to the notion that designing with climate can enrich spatial experience.
Why 800 square feet? The couple wanted to live in a small, efficient space, of course. Also, because they built the house as an "accessory dwelling unit" in the backyard of one of their children, the Portland zoning regulations set the upper limit at that size. Actually, since the walls are eight inches (20 centimeters) thick, the usable interior space is only 704 square feet (65 square meters).
As with any project, the first green line of defense is in reducing energy loads.
"The way to get to a zero-energy building is to start with a good envelope," says Christopher Dymond, who was the Oregon Department of Energy's senior energy analyst on the project. "Then add all the little things."
Framing the eight-inch walls with staggered studs eliminated thermal bridging except in the top and bottom plates. The added thickness, combined with blown-in cellulose insulation, resulted in a wall that performs 50 percent better than the standard R-21 wall.
The roof of the house uses structural insulated panels (SIPs), which further reduce thermal bridging, and also reduce the amount of structural lumber required. With all that, plus its high-efficiency fiberglass windows, the house requires only half the energy of a standard home of the same size built to code.
The Rose House is basically a passive-solar building with active supplements. Because the house is thermally tight, the team opted for a high-efficiency heat-recovery ventilator to supply fresh air. The unit recovers about 95 percent of the waste heat from the exiting air.
Space conditioning is provided by a complex air-to-water heat-pump system. Part of the warm water is distributed to in-floor hydronic heat in the central area of the house. Water from the heat pump is also circulated to a coil in the heat-recovery system so that incoming fresh air is warmed by both recovered heat and by the heating coil.
In an innovative move, the system uses heat from a plenum between the photovoltaic (PV) panels and the roof. Most systems exhaust this space, since PV panels operate more efficiently when they are well ventilated. So, using this heat rather than discarding it makes both the PV and the heating system work more efficiently.
The PV array supplies electricity for the house and uses "net metering." The house's electric meter logs watts both coming in and going out; the utility company buys excess power during sunny times and sells watts when needed.
With net metering, solar systems can add wattage to the grid during daytime peak periods and draw power at night when demand is low. The trick is designing a system in which demand and supply are balanced or in which solar is producing more watts than the house is using.
The PV array on the Rose House is rated at 3.3 kilowatts peak output. In actual use, it has generated a long-term average output of about 6 kilowatt-hours per day.
In what Brockman refers to as "climate-responsive elevations," the building has a carefully tuned fenestration and shading system to get sunlight when and where it's needed for heat and illumination. Of course, window placement is critical — getting enough south light, tuning east and west light for balanced daylighting, and minimizing north heat loss.
In addition, one of Eldon Haines's inventions, the Copper Cricket solar panel, provides hot water heating.
While the Rose House was designed to achieve net-zero energy consumption, it has yet to meet that benchmark on a long-term basis.
The total external energy consumption of occupancy, in actual use, shows a highly efficient dwelling, at about 18 kilowatt-hours per day, or on an area basis, 8.2 kWh/ft2/year (88 kWh/m2/year).
Offset by the PV array output of 6 kilowatt-hours per day, the actual net energy use to date has been about 12 kilowatt-hours per day, or on an area basis, 5.5 kWh/ft2/year (59 kWh/m2/year). That's very good — but it's not net-zero, yet.
Much of the overshoot in energy consumption relative to the design of the project is attributed to the house's biggest operating glitch.
The specialized air-to-water heat-pump unit hasn't operated up to its specifications, and as a result, excessive use of relatively inefficient electric resistance backup heating during cold periods drives up the overall annual energy consumption.
"Japan has an efficient heat pump that can produce 120-degree Fahrenheit (50-degree Celsius) water from 17-degree (-8 Celsius) air," Brockman says. That system uses liquid carbon dioxide as a coolant, which has the bonus of sequestering the greenhouse gas and putting it to work. But the units aren't available in the United States yet.
Another lesson is that the multi-source heat-pump system seems to be overly complicated, according to Brockman. "But the beauty is that Eldon really is a rocket scientist," remarks the architect, noting that Haines is able to tweak the system to achieve maximum efficiency.
With continuing improvements to the systems of the house, Haines hopes that the Rose House will ultimately achieve true net-zero energy performance.
Builder Dave Heslam used the Rose House project to experiment with details that reduce thermal bridging. The staggered studs actually use 15 percent more wood than typical framing, since the studs end up being 12 inches (30 centimeters) on center. When walls exceed eight feet (2.4 meters) tall, as they do in the Rose house, codes often require additional fire blocking, which adds more wood.
In subsequent projects, Heslam has used what he calls "strap wall." That approach has vertical two-by-six studs on the exterior and two-by-two horizontal nailers on the interior. The system does have tiny areas of bridging at the attachment points, but it reduces the amount of framing lumber significantly.
Built as a demonstration house for the Oregon Department of Energy, the Rose House helped define a baseline for the state's High Performance Home tax credit. According to Christopher Dymond, the combination of state standards for high efficiency and the incentives makes Oregon a leader in promoting "net-zero" buildings.
Since the house was built, six other passive-solar homes have sprung up in the area. The neighbors are now organized into the Ainsworth Collective, named after the street where they reside. The group is setting the trend for solar neighborhoods in Portland. Measured by the highly efficient Rose House — and its continuing quest for net-zero — it's a trend worth setting.
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Michael Cockram is a freelance writer and an adjunct assistant professor of architecture at the University of Oregon. More by Michael Cockram