Casa Aquila defies wildfires with resilient systems.
The elegant single-story stucco-and-glass house atop a hill above Ramona, Calif.’s San Pasqual Valley may not look very castle-like, but the home was built with a host of features that make it a veritable fortress against wildfires, the common enemy of Southern California homeowners.
When the Witch Creek fire took their previous home in 2007, along with 1,000 other homes in the neighborhood, the owners were determined that one day they would build a home that could resist the fires, and have the energy and water self-sufficiency to hold out through the multi-day power outages that often accompany wildfire season in the Southern California hills.
“Casa Aguila” includes fire- and power outage-resistant features such as stucco and concrete walls, stone and tile ground covering, metal roofing and posts, high-impact windows, energy-efficient construction, on-site wind and solar power generation and storage, and on-site water collection and recycling systems. For energy efficiency, Alliance Green Builders certified the home to the U.S. Department of Energy’s Zero Energy Ready Home (ZERH) criteria.
This uber-efficient home also achieved a LEED platinum certification from the U.S. Green Building Council and was the first home in San Diego County to be certified by the Passive House Institute U.S. In its pursuit of water and energy self-sufficiency, the project team received the county’s first on-site wastewater treatment permit. They also designed a rainwater collection system to supply all indoor water uses. With the solar PV tracking system, solar thermal water heating, 45-foot wind turbine and battery storage, the homeowners are well on their way to achieving grid independence.
Start With the Shell
Even with large solar capacity, energy self-sufficiency would be hard to achieve without first constructing a very energy-efficient shell. For this, the builder turned to the DOE ZERH program.
A bio-based phase-changing material lines the walls and ceiling to help even out day-night temperature swings.
The builders exceeded code insulation requirements with double walls consisting of a 2-by-6 exterior wall and a 2-by-4 interior wall, set 7 inches apart to form a 16-inch cavity stuffed with R-57 of dense-packed cellulose. The inner walls and ceilings are covered with a bio-based phase-changing material that helps stabilize temperatures inside the home. The phase-change material consists of sheets of plastic containing pockets of a nontoxic gel with a defined melting point; the material cools the room as the wax melts, and warms the room as the wax re-solidifies. The sheeting is hidden behind a volatile organic compound (VOC)-absorbing drywall that lines the walls and ceilings. The outer wall is sheathed with 3/8-inch OSB that is coated with a liquid-applied waterproofing and air barrier product, which takes the place of housewrap to cover the walls and window and door openings with a seamless layer of protection. The walls are clad with traditional three-coat stucco applied over metal lath and one layer of stucco wrap. The home’s wood framing is treated with fire- and termite-resistant borate.
Working a Way Inside
The windows, which are set deep into the 16-inch-thick walls, are a key to the home’s fire defense system. It turns out that exterior wood is not the chief source of ignition for house fire damage during wildfires in Southern California. Rather, most fires start inside from embers blown through broken windows. The Santa Ana winds that cross California during wildfire season can get whipped up by the fires to such high velocities that they can pick up rocks and pelt windows with enough force to shatter glass. To protect against this threat, Alliance installed triple-glazed, tempered glass windows with an outer pane of impact-resistant laminated glass. The R-5 insulated windows also provide year-round energy efficiency with coatings to reduce heat transfer.
The roof consists of 2-by-8 rafters topped with 1/2-inch OSB coated that is coated with the liquid-applied waterproofing membrane that also covers the wall sheathing to form a continuous air and water barrier. This is topped with 6-inch-thick structural insulated panels (SIPs) that have a polyurethane core with a higher melting point than most foams. The SIPs are covered with a roofing underlayment and a standing-seam gray metal roof with a solar reflective index of 52 to help resist solar heat gain. Netting is attached to the underside of the roof rafters, and the space is filled with blown cellulose. Below that is a 2-inch gap and then 2-by-6 ceiling rafters that are covered with the phase-change material and drywall. The gap provides more space for insulation above the recessed can lights.
The home’s slab-on-grade foundation is wrapped in 1 inch of expanded polystyrene (EPS) insulating foam. Embedded in the concrete is plastic piping for the home’s radiant heating and cooling system.
Forward-Thinking Temperature Control
The home actually has several heating and cooling systems. Most of the hot water for the radiant floor heat is provided by nine 4-by-10 (360 square feet) solar thermal collectors mounted on the roof. An air-to-water heat pump provides backup water heating and water cooling. Additional water cooling, should it be needed for extremely high summer temperatures, is available via a ducted fan coil system (with cold water from the air-to-water heat pump). Per the owners’ request, a third system was installed consisting of a minimally ducted mini-split heat pump with fan coils to provide back-up heating and cooling to the bedrooms. This redundancy may seem excessive for a passive house, but according to builder Jeff Adams, “the system has been designed for a climate that could be dramatically different in 100 years or more.”
Casa Aguila performs far better than net zero, thanks to a super-efficient building shell, a wind turbine, three sun-tracking solar photovoltaic arrays and solar thermal water heating.
The solar thermal system is also the primary source for the home’s potable hot water. The air-to-water heat pump and an 80-gallon heat pump water heater provide backup domestic hot water. The hot water distribution system was designed to cut water loss to 1 cup before hot water reaches any bath or kitchen fixture.
The hilltop home is self-sufficient for water. Five 10,000-gallon tanks collect rainwater from the roof to supply 100 percent of the family’s indoor water use. Four more 10,000-gallon tanks collect stormwater from the site, and one additional 10,000 gallon tank collects graywater (from the shower, baths, sinks and washing machine). This stormwater and graywater is filtered and used for irrigation and fire suppression. A separate system collects blackwater from toilets and kitchen sinks in a septic tank; from there, it is aerated and filtered, and the clean water is pumped into another 1,000-gallon holding tank for drip irrigation.
The homeowners requested an electric production system that can allow the home to eventually go off grid. Three large dual-axis solar trackers were installed on a leveled area south of and below the home. Each tracker holds 474 square feet of photovoltaic panels, for a total capacity of 21 kW. The panels can rotate in multiple directions; sensors direct them to the brightest point in the sky, even on cloudy days. They can generate power for more than 12 hours per day and are expected to produce about 44,000 kWh per year.
In addition, a 45-foot-tall helical 3.2-kW wind turbine was installed on the hill just above the home, to take advantage of what is considered one of the best wind-generation sites in San Diego County. The wind turbine is expected to produce 5,000 kWh per year of power.
The home now operates off of 20 12-volt solar batteries, but is grid-tied to use the grid for backup power. When more battery storage is added, the home will be able to be completely energy self-sufficient.
Rated for Low EDR
Solar thermal panels provide hot water for the pool, spa, domestic hot water and radiant floor heat.
Like all DOE ZERHs, the house was evaluated by an energy rater. California uses the California Energy Design Rating (EDR) rather than the national Home Energy Rating System (HERS) score, for verification to the state’s Title 24 energy code. This home scored a very low 43 EDR without the renewable energy systems included. With the solar and wind resource included, the home achieved a remarkably low EDR of -92.
The home’s performance testing included blower door testing, which revealed an extremely tight building envelope with only 0.32 air changes per hour at 50 pascals pressure difference (ACH 50), nearly twice as tight as the 0.6 ACH 50 required by Passive House. To bring fresh air into the home without significant energy loss, Alliance installed a heat recovery ventilator (HRV). The HRV draws in fresh outside air through a filter and ducts the air to bedrooms and living spaces, while exhaust ducts pull air from the kitchen and bathrooms. The incoming and outgoing air ducts cross in a heat exchanger, which transfers heat from the warmer stream to the cooler stream, helping to warm incoming air in the winter and cool incoming air in the summer. Alliance tested a new model of HRV from Italy that also allows for some humidity control.
With the filtered ventilation system, and other features like low-VOC finishes or good moisture management, the home met the clean air requirements of the EPA’s Indoor airPLUS.
The homeowners are happy to share the lessons of their home with others. Alliance Green Builders partners Jeff Adams and Rich Williams, founders of the San Diego chapter of Passive House U.S., have used the home for numerous tours and consider sharing lessons learned the “icing on the cake” for this project. “Designing arguably the most energy- and water-efficient home in San Diego [...]is incredibly rewarding,” Adams says.
A summer kitchen on the patio helps keep heat and humidity out of the home, as do solar-reflectant metal roofing, SIP roof panels, deep overhangs and triple-pane Passive House-certified windows.
- DOE Zero Energy Ready Home Path: Performance.
- Walls: Staggered 2-by-4s, 12-inch o.c. on 2-by-6 plates, 2-stud corners, ladder blocking at wall intersections, right-sized insulated headers, window and door framing aligned with 12-inch o.c. wall studs, R-25 dense-packed cellulose, 0.5-inch coated OSB sheathing, air-sealed housewrap, fiber cement. Window and door openings get flexible flashing and beveled sills.
- Roof: Coated OSB sheathing, 13-inch raised heel trusses, 24-inch overhangs, 30-pound felt, ice-and-water shield at eaves. Energy Star light asphalt shingles.
- Attic: Vented: all top plates foamed, 16-inch R-60 blown cellulose, few penetrations.
- Foundation: Basement only 4 feet deep for daylight, insulated on exterior with 3-inch rigid XPS, 2-inch XPS beneath slab, drain mat over exterior insulation.
- Windows: Vinyl-frame triple-pane, low-E, U=0.22-0.18, SHGC=0.24.
- Air Sealing: 0.98 ACH 50.
- Ventilation: Air-cycler-controlled fresh air intake balanced to timered exhaust fans.
- HVAC: 96 percent AFUE gas furnace, 14 SEER AC. Ducts un-insulated, in conditioned space.
- Hot Water: Tankless gas EF 97.
- Lighting: 95 percent LED.
- Appliances: Energy Star dishwasher, clothes washer.
- Renewables: 2.8-kW PV.
- Water Conservation: 100 percent WaterSense fixtures.
- Energy Management System: None.
- Other: No-/low-VOC primer, paint, cabinets, flooring. Pre-wired for electric car charger.