ICFs and advanced framing techniques used throughout

This energy-efficient house in San Jose, California, not only produces enough solar electricity to meet its annual energy needs, it also complies with the stringent Passivhaus standard.

The design-build team of Allen Gilliland and Bronwyn Barry of One Sky Homes were behind the spec home and succeeded in meeting the Passivhaus standard and the performance requirements of the U.S. Department of Energy’s Zero Energy Ready Home program.

To meet the DOE Zero Energy Ready Home criteria, the home had to meet the requirements of the Energy Star Version 3.0 program and comply with the minimum insulation requirements in the 2012 International Energy Conservation Code. The DOE program also requires that homes be certified to the air quality requirements of the U.S. Environmental Protection Agency’s Indoor airPLUS program. In addition, the builder chose to pursue LEED for Homes and achieved platinum certification on the home.

Home took four years to design and build

The $1.1 million house could be thought of as a building science test project. The 3,198-square-foot, two-story spec home took four years to design and build, more than twice the builder’s typical construction cycle for a custom home, because Gilliland wanted to test several new construction techniques.

“It was our intention to take a real building science approach. We wanted to measure this building in every way possible, to learn from it and share what we learned. To everyone’s credit, we were very successful. The data verified that we were ‘plus site energy.’ (The home’s 6.4-kW photovoltaic system produces enough electricity to power both the home for a year and an electric car for thousands of miles of charge.) And we’re providing the homeowner with a vastly better living experience,” said Gilliland.

One Sky Homes collaborated with Davis Energy Group, a research partner in DOE’s Building America program. Davis Energy Group helped the builder meet the DOE Zero Energy Ready Home criteria while testing several advanced building technologies, including a night ventilation cooling system that cut cooling costs by 98%.

High efficiency starts at the ground level where Gilliland used R-22 insulated concrete form (ICF) blocks to create a 3-foot-high foundation wall that formed the insulated sides of the unvented crawl space. Gilliland covered the crawl space floor with 4 inches of ¾-inch aggregate, a heavy-gauge polyethylene vapor barrier, 3-inch-thick 2-pound rigid expanded polystyrene foam (EPS) foam (R-14), then a 3-inch concrete crawl space slab. Above the crawl space are 9.5-inch-deep engineered I-joists.

Advanced framing methods saved on materials

The above-grade walls consisted of 2×6 studs spaced 24 inches on-center. Advanced framing techniques were used: for example, two-stud rather than three-stud corners, open headers over doors and windows, windows sized to the stud bay openings, and ladder blocking at interior-exterior wall intersections to reduce lumber usage and provide more room in the walls for the R-23 of dense-packed cellulose.

“Everything is sized on a 24-inch grid, which makes for faster assembly with less materials waste,” said Gilliland. Over the exterior plywood sheathing, a 1-inch layer of EPS foam board was installed. This was covered with housewrap. Then the stucco installers attached a ¼ inch plastic mesh rain screen that served as a drainage plane. Over this they installed wire lathe that was then covered with a traditional three-coat stucco cladding.

Raised-heel trusses provided plenty of room in the attic for the R-51 worth of blown-in cellulose insulation installed above the ceiling. The OSB roof sheathing has a foil radiant barrier coating on the underside facing the open attic space. Like most ventilated attics, this attic has continuous eave and ridge vents. The attic temperatures are relatively low; Davis Energy Group monitoring confirmed that interior attic temperatures never climbed more than 20 degrees above exterior ambient temperatures.

Triple-pane glazing was a requirement

All of the home’s windows and patio doors have triple glazing with low-emissivity coatings and argon gas fill, with the exception of one double-pane window installed for testing purposes. All of the windows are wood-framed. Solar heat gain coefficients (SHGC) varied depending on the orientation of the window. Higher SHGC (0.49) windows were installed on the south side to allow beneficial passive heat gain in the winter. Lower SHGC (0.29) windows were directed toward the west to reduce unwanted late afternoon heat gain. The calculated average “installed” R-value for all of the windows was R-5 (equivalent to U-0.20).

Thanks to rigorous air sealing measures, the home was able to meet the very low air leakage target required to meet the Passivhaus standard (0.60 air changes per hour at 50 Pascals pressure difference). The blower-door test showed that the home has an air leakage rate of 0.57 ach50.

Energy usage met the Passivhaus standard

The average U.S. home uses 26,252 kWh of energy (electricity, natural gas, or other fuels) annually. A typical 3,000-square-foot home might use as much as 40,000 kWh annually, with half of that consumed by heating and cooling. The One Sky zero energy home uses only 10,000 kWh of energy a year and only 2,000 kWh of that is used for heating and cooling. A split-system central air source heat pump with a cooling efficiency of 21 SEER and a heating efficiency of 10 HSPF is located in an upstairs closet. The system’s ducts are located in conditioned space, in the open-web trusses between the first and second floors.

To keep the air healthy in the nearly airtight home, a heat-recovery ventilator (HRV) was installed to draw in fresh air and exhaust stale air. The air passes through a MERV 13 filter then is supplied to the bedrooms and common areas. Exhaust air is pulled from the bathrooms and laundry. The HRV uses 3-inch ducts that are completely separate from the heat pump’s 8-inch ducts. “With the heat pump, we’re moving 100 cfm of air per register. That is an order of magnitude of difference from the 10 cfm of air moving through each HRV duct. It’s hard to get 10 cfm of air to move where you want it to in an 8-inch duct,” said Gilliland.

The home uses Night Ventilation Cooling, a ventilation system tied to the central air handler that uses temperature sensors, dampers, and electronic controls to draw cool nighttime air from outside through a duct to the return side of the air handler fan when temperature sensors indicate that the outside air is cooler than the inside air and the system is calling for cooling. The technology was initially developed in the late 1990s, in part by Davis Energy Group, and is similar in principal to the economizers used on commercial cooling systems. The system is ideal for climates with dry summers and large day-night summer temperature differences. The Night Ventilation Cooling system’s performance was extensively measured and showed dramatic results. It completely eliminated overheating and met 98% of the home’s total cooling demand, with a measured coefficient of performance (COP) of 14.

Solar thermal collectors heat water

The home has three roof-mounted solar thermal collectors that send heated water to a storage tank, which the auxiliary gas-fired 96% efficient storage water heater draws from to supply domestic hot water. To reduce water use, plumbing fixtures are water-conserving and hot water fixtures are equipped with a recirculation pump that is button- or motion sensor-activated to speed hot water to the fixture.

Another unique feature of the home is the graywater recycling system. During the irrigation season, graywater from the home’s sinks and showers goes to an underground 50-gallon tank, from which it is pumped to several irrigation zones around the yard. During the rainy season, the graywater goes directly to the sewer. The graywater system meets 80% of the home’s landscape irrigation needs; the water is distributed via a below-grade drip irrigation system.

The 6.4-kW PV system (28 panels, each rated at 230 watts) produced 11,000 kWh in 2013, meeting 113% of domestic demand; the extra power was used for charging the homeowners’ electric car. One electric car charging station is installed in the garage and wiring is in place to install a second one. The PV system is grid-tied, but could be upgraded with an inverter that will support independent operation in case of grid failure. Without the PV system, the energy efficiency upgrades would save a homeowner a calculated $1,095 compared to a home built to the 2009 IECC. With the PV system, homeowners are expected to save about $2,900 a year compared to owners of a code-built home.

All of the home’s appliances, including the clothes washer, dishwasher, and refrigerator, are Energy Star rated. The home’s lighting includes 40% CFL, 40% LED, and 20% halogen kitchen task and accent lights. Vacancy sensor controls were installed in all rooms. Closet lights, heat lamps, and other utility lighting were installed with count-down timer controls. All exterior lighting is Energy Star rated and is controlled by timers with photocell shut-offs.