BioCycle December 2005, Vol. 46, No. 12, p. 46
Winning house in the 2005 Solar Decathlon uses soy-based foam insulation with a fiber panel made from recovered paper products; blocks from recycled wood chips and fly ash; plus a waterless remote composting toilet.
Jennifer Weeks

EIGHTEEN college and university teams took up the 2005 Solar Decathlon challenge in October which involved: Design an 800-square foot house that runs exclusively on solar energy, powers a suite of bio-based appliances plus a plug-in electric car, maintains comfortable interior temperatures, and does not run out of power even if it rains for days, build it on the National Mall in Washington, DC and explain it to thousands of visitors.
Sponsored by the U.S. Department of Energy, the competition tests entrants in ten categories, from overall livability to hot water production using solar energy and mileage accumulated on their electric cars (for details, see www.eere.energy.gov/solar_decathlon/). Architectural styles varied from functional rectangles to a Frank Lloyd Wright homage, but all teams used energy-efficient building materials and appliances to minimize power requirements. They also showcased such sustainable building choices as fast-growing tropical woods, like bamboo and eucalyptus, for flooring and interior trim, and lushly planted green roofs that captured rainwater and provided cooling.
COLORADO’S CHAMPIONSHIP HOUSE
The winning house was designed and built by a team from the University of Colorado, which also won the first Solar Decathlon in 2002. The UC team took as its mission: “To integrate natural materials and innovative technologies in an environmentally conscious, publicly accessible, modular solar home design.” Its product, dubbed “the Bios(h)ip,” showcases low- to nonpetroleum building components in a bright, well-equipped, and easily movable home that produces more energy than it consumes and meets all Colorado residential building codes.
The home’s nickname refers to Bio-SIPs, a twist on structural insulated panels (SIPs) developed by the team’s faculty advisor, Julee Herdt, in partnership with the U.S. Department of Agriculture’s Forest Products Laboratory in Madison, Wisconsin. SIPs, which can be used for floors, walls, and roofs, contain foam insulation sandwiched between large slices of lightweight building material such as oriented strand board. Building with SIPs costs roughly the same as traditional wood frame construction, but offers better insulation because the panels are fastened together with splines that allow very little air to penetrate. In contrast, studs and braces in conventional walls create gaps that allow heat to leak through.
CU’s Bio-SIPs contain soy-based, water-blown polyurethane foam insulation produced by Bio-Based Systems between 3/4-inch layers of Sonoboard, a lightweight honeycomb fiber panel made from recovered paper products. The panels have an insulating factor of approximately R-7 per inch, twice that of fiberglass blankets or batts or loose-fill cellulose (R-value is a measure of resistance to heat transfer, with higher numbers connoting better insulation). Full-scale versions of the panels (measuring 8 feet tall, 4 feet wide, and 7 inches thick) were strong enough to use as load-bearing walls in the solar house, where they provide insulation values of approximately R-36. The soy foam was also used in the house ceilings and as insulation beneath its steel mobile home chassis.
Herdt and the CU solar decathlon team are working with the university’s technology transfer managers to patent and commercialize Bio-SIPs. They expect to produce a new generation of the panels within about 18 months that will have different configurations than the “ice cream sandwich” design used for the Solar decathlon house.
In addition to Bio-SIPs, other natural materials were added to the durability of the house, which was designed for Colorado’s climate. The north living room wall is made of FASWALL blocks produced from treated recycled wood chips, Portland cement, and fly ash (ash deposited in chimney stacks when coal is burned). FASWALL forms provide high insulation and strong resistance against weather, fire and pests. The home’s decking and railings are Trex, a blend of recycled wood and plastic made primarily from plastic grocery bags, reclaimed pallet wrap and waste wood. Its wood components protect the plastic from sun damage, while the plastic protects the wood from moisture and insects. Trex – which comes in smooth, wood-grain, or tropical hardwood finishes – costs more than pressure-treated wood but resists weathering and pests and does not require staining or painting.
The CU house interior is light and warm, filled with natural colors that reflect more use of biobased materials. For interior windowsills, the team chose Environ Biocomposite panels made from soy and newsprint. Environ boards have a granite-like appearance and are 50 percent harder than oak, but can be cut, sanded and finished like hardwood. Applications for Environ include furniture, store fixtures, table surfaces, and cabinetry. The platform for a sleeping loft opposite the kitchen is made from WheatSheet, an industrial-grade particle board made from recycled wheat chaff and an emission-free binder. Richlite kitchen and laundry countertops are natural-fiber composites, which are made by treating pulp derived from sustainably harvested forests with phenolic resin and baking it into solid sheets. Underfoot, Forbo natural linoleum, made primarily from linseed oil, is the main flooring material throughout the house.
Herdt also designed and built “The Farmhouse,” her own 4,700 square foot house with 3 bedrooms and 3 baths in North Boulder, using largely biobased, reused, and recycled materials (http://thefarmhouse.org). According to Herdt, biobased products like those in the Farmhouse and the decathlon house are generally as expensive as conventional building products, but are superior on environmental and health counts. Because products like WheatSheet are not treated with volatile organic compounds, houses that contain them have better indoor air quality. And many biobased products add natural colors and patterns to interiors, Herdt says. “They look familiar, which makes the house more attractive without using rainforest hardwood and marble and other rare materials. They feel nice, too, and they smell good. My house smells like a cereal box inside.”
HOME ENERGY SYSTEMS
The BioS(h)ip’s energy is produced by 34 rooftop SunPower 200-watt photovoltaic (PV) panels, backed up with a bank of 40 Deka batteries designed to store enough power to operate the house for four days. Having such a large battery array was key to Colorado’s success in the Decathlon: weather during the contest week was overcast and rainy, but the CU team’s large power storage capacity combined with a highly efficient photovoltaic system gave it enough electricity to score well across the ten contest categories. Like other Decathlon entries, the CU house carried more PV cells than would normally be required to run a grid-connected solar house, particularly of this size, because teams were required to generate 100 percent of their energy requirements throughout the contest and thus needed extra capacity to be able to produce significant amounts of power at the outset (commercially-built solar homes typically carry approximately 1.5 to 4.5 kilowatts of PV cells).
A Thermomax evacuated tube solar thermal collector system on the southwest wall of the CU house heats a glycol solution to provide hot water, some of which is piped through a radiant in-floor heating system. Using radiant heating eliminated the need for ductwork and made more interior space available for living areas. The hinged roof retracts to a 16-foot height when the house is to be moved and can be raised to 18 feet for maximum interior light when the home is stationary.
Built on a steel mobile home chassis, the long, narrow form allows daylight and ventilation to reach every part of the house and makes transportation easy. Making the home mobile eased travel to Washington, DC for the decathlon – the house was moved from Boulder and back using 100 percent biodiesel – and also fits the lifestyle that the CU team envisions for its owners: people who work, travel, spend a large fraction of their time outdoors (the house has front and back decks), and may change jobs and locations several times while they live in the BioS(h)ip.
CU’s design addresses waste management as well as energy production. The house uses an Envirolet Waterless Remote composting toilet, with the waste treatment center installed directly below the unit and attached to the home’s steel chassis (in a nonmobile home, the unit installs in the basement or on the ground outside). This feature reduces the home’s water use by about 100 gallons per day compared to a conventional flush toilet. The house also comes with a worm composting bin in the kitchen and outdoor recycling bins.
WELCOME ABOARD
The Bios(h)ip currently is installed on the UC campus at Boulder, and can be toured by arrangement (visitors should e-mail Julee Herdt at Herdt@ecoisp.com). In the spring of 2006, it will be moved to a permanent site in the New Urbanist community of Prospect New Town in Longmont, Colorado. New Urbanism is a response to suburban sprawl that works to create close-knit, human-scale communities through design features such as narrow, tree-lined streets and houses that are located close together and within walking distance of stores, parks and offices. The BioS(h)ip will also be open to tours in Prospect New Town.
The decathlon house was built and furnished for $305,000, although some elements were donated. CU estimates the full retail cost at $433,000, but this includes features that were unique to the contest such as a larger-than-usual solar photovoltaic array. Herdt estimates that a version with a smaller solar energy system could be built for as little as roughly $110,000. The CU team plans to work with Genesis Homes in Colorado (a division of Champion Homebuilders Corporation), which helped the CU team build a custom steel chassis for the decathlon house, to produce future versions of the model, which has been certified by the state as a new housing type – solar mobile home. And for buyers who need something bigger, “The Farmhouse” in Boulder is currently on the market for $930,000.
Jennifer Weeks is a Massachusetts writer specializing in energy and environmental issues.