Del Fierro is a home designed from the ground up to be an upscale, energy-independent, passive solar residence. The intention was to create a structure capable of lasting centuries and one in which interior spaces can be periodically updated as technology, lifestyles, and trends change. The theory is that this would result in lower lifetime costs with less overall environmental impact than a similarly sized conventional home. Much of the project was experimental with an eye on future scaled-down versions for affordable housing projects.
I tackled my first energy-efficiency project on my own 1980s-era home in 2000 by weatherizing; upgrading to more efficient appliances and a high-efficiency HVAC system; using CFL lighting; and applying energy consciousness. Although I reduced energy consumption by about 40%, I concluded that there’s a limit to making an older home highly efficient. To get to the next level, I needed to start from scratch. This is particularly true in the postwar desert Southwest, where the housing techniques— 2-by-4 stick-frame construction, with minimal or no wall insulation—reflected a time when energy was relatively cheap.
After studying passive solar design, construction techniques, and materials, I was frustrated to find that little was written about the topic specific to desert climates. Most passive solar technology focuses on heating and retaining heat. While the physics of heat conduction and convection apply similarly to cooling and heating, the effects of the sun’s radiation play a significantly more important role in cooling design. In the summer in the desert Southwest, exposed surfaces can frequently reach temperatures in excess of 150°F, so keeping out the heat is more important than in other climes.
For keeping a home cool, it is critical to reduce its exposure to the sun, keeping the surface temperature of the shell as low as possible. This reduces thermal conduction through the envelope and minimizes charging of thermal mass. This can be accomplished by shading with overhangs, vegetation, the use of reflective coatings, light colors, and radiant barriers. Compromises for cooling over heating are made, since saving energy on cooling was the deciding objective. Tucson has almost twice as many cooling degree days (the number of days where mechanical cooling is needed) as heating degree days (2,954 vs. 1,678). Compare this to Ann Arbor, Michigan, which has 7,864 heating degree days and 289 cooling degree days, and you’ll see the drastic differences in passive design requirements.
Readily available earthen-based building materials, such as adobe and rammed earth, have been used in the Southwest throughout history. Modern performance-driven designs fully benefit from their inherently high thermal mass, fire resistance, and durability. I originally intended to use rammed earth, which involves compressing a mixture of earth, chalk, lime, and gravel (and sometimes a cement stabilizer). But at 3,450 square feet, costs for rammed earth were prohibitive due to the volume and complexity of the house, with its high ceilings, parapets, and intricate architecture.
A member of my design team introduced me to lava concrete (LC), a lightweight cast-in-place concrete made from volcanic cinder sand, very abundant in Arizona and other parts of the world. The construction is similar to casting rammed earth, but much less labor-intensive. Once cured, the steel-reinforced structure is strong and considerably more durable than rammed earth or other earthen building systems. With an R-value of 3.27 per inch, our 18-inch-thick walls would be nearly R-59. The LC also provides thermal mass and soundproofing. The inside was finished with a natural clay plaster from American Clay, which also contributes to the home’s thermal mass.
A cooling tower provides evaporative cooling and much-needed humidity in the dry months of March through June. A mister injects moisture at the top of the tower, wherein the cooler, dense air falls by gravity, displacing warmer household air out through open windows at each end of the house, and a small electric fan augments the airflow. The cooling tower also works at night when the outside ambient air temperature is cooler than the displaced inside ambient room air.
Another passive cooling technique—deep (8- to 12-foot) overhangs—help shade south- and west-facing walls and windows in the warmer months. Helping overall building efficiency, the roof includes a 2-inch, R-14 sprayed-on layer of urethane foam over a double layer of OSB over I-joists and trusses filled with loose-fill fiberglass insulation, for a value of R-45 to R-60. A white polymer reflective paint was sprayed over the urethane. Pella InsulShield triple-pane, argon-filled, low-e windows and doors retard heat transfer.
All appliances (range, oven, microwave, dishwasher, refrigerators, washer, and dryer) are Energy Star-rated, and CFL and LED lighting is used throughout.
Two 40-gallon batch solar water heaters are backed up with two Bosch 2400E LP on-demand propane-fired units. The SHW system can either feed the backup unit or send the solar-heated water directly to the house. We have it route through the backup units, which are temperature-modulated.
When needed, mechanical cooling and heating is provided by three high-efficiency (16 SEER-rated) fresh or recirculated air-source heat pumps, 2-, 4- and 5-ton units, respectively.
A grid-tied 5.44 kW, dual-axis, tracked PV array provides most of the home’s electricity. (The array size was based on the most PV modules I could get on a pair of Wattsun AZ-225 trackers.) PVWatts calculates that our system should generate 13,253 kWh annually. Since PV module prices have dropped, I plan on adding a ground-mounted PV system to cover the property’s entire electricity requirements, including the guest house and shop.
For the first year of data gathered, ending in July 2010, our usage exceeded our original energy budget. During that period, thermostats were set for 80°F in summer and 68°F in winter. The HVAC system measured 2,382 kWh over what was anticipated.
To evaluate the building’s thermal performance, thermocouples were embedded in eight strategic locations one inch from the inner and outer surfaces of the exterior walls. Data was recorded on a computer every 15 minutes over a period of a year. This was helpful in understanding where heat transfer was significant so adjustments could be made. One zone with south and west exposure consumed power disproportionately—29% of the HVAC power used for 11% of the building floor space. Planting shade trees on those sides of the house largely corrected this problem. It was also found that the default settings on the heat pumps caused the variable-speed fan motors to operate all the time, which was unnecessary for comfort. Filters were changed more frequently and a fan was added to the cooling tower, and used in the spring and early summer months, providing needed humidity. Implementing these strategies reduced the zone to 19%.
About a cord of wood was burned in the fireplace during the coldest months, reducing the nighttime heating load. These changes had no effect on lifestyle and helped keep us under the winter HVAC budget by about 4%.
PV production was improved by fixing a tracker problem and replacing several defective PV modules. Three module failures affected three strings of modules. With two identical systems, it’s easy to spot a problem. Trees were removed that had grown and shaded one of the arrays during the late- afternoon hours. We also cleaned the surface of the modules more frequently, resulting in improving PV production from 12,317 kWh annually to 13,307 kWh.
The SHW systems consist of two 40-gallon ICS batch units at each end of the house, backed up by Bosch propane on-demand units. The systems’ performance can be measured by the backup fuel that hasn’t been used. The 300-gallon underground propane tank was filled to 80% of capacity at move-in. It has never been refilled, and today it’s just under 60% full. That means about 60 gallons of LPG was used in 32 months (or 1.9 gallons per month). This consumption also includes a frequently used gas grill, which I suspect accounts for most of the gas usage.
This put us within 2% of our objective of meeting 90% of our energy needs without any compromise in lifestyle. We could squeeze out the last bit by managing computer systems, entertainment systems, and the phantom loads.
A UCLA physics graduate, Edward A. Marue designs off-grid power systems for remote communications and border security sites. He is also a principal in Solar Lava Development Company, specializing in advanced green design and construction.