Large windows placed strategically, and clerestory windows above the dining area in the great room and at the peak of the roof above the main entry allow ample natural light into interior spaces, eliminating the need for artificial lighting during the day. When open during the warmer months, the clerestory windows also provide passive ventilation, offering an escape route for warm, rising air.
With afternoon temperatures known to reach 110°F in July and August, special attention was paid to insulating the structure to avoid using lots of energy for cooling. Insulated concrete forms (ICFs) used for the exterior walls were cost-effective and achieved good efficiency. The home’s Durisol wall-forms feature interlocking blocks made from cement-bonded wood shavings and chips—a fire-resistant, vapor-permeable material.
For additional insulation, an insert of mineral wool fiber was added to each block, for a whole-wall R-value of about R-20. Once in place, the cavity of each wall was filled with poured concrete for a strong wall that functions as thermal mass and insulation. The result of the combination is a more continuous R-value throughout the walls, minimal thermal coupling between the inside and outside, and minimal air infiltration.
Structural insulated panels (SIPs)—panels of polystyrene foam sandwiched between two layers of oriented strand board—were used to construct an R-38 roof. The galvanized steel roofing panels are made from 30% recycled steel and painted with a performance finish, which helps mitigate heat by reflecting up to 70% of the sun’s light. Gaps between the roof and Durisol wall forms were sealed with urethane spray foam or plugged with recycled denim fiber filler.
For thermal envelope performance, high-efficiency double-pane casement windows complete the seal. A heat recovery ventilation system in the small attic space helps keep fresh air circulating in the house and controls humidity.
The Carstens’ all-electric home relies on several systems to maximize efficiency: a grid-tied solar-electric system, sized to generate up to 4,164 KWH of electricity annually; a solar hot water system that provides 65% of their household hot water needs; and a high-efficiency geothermal, closed-loop heat pump.
The couple received three rebates through the Oregon Department of Energy’s residential energy tax credit program and the home earned nearly $12,000 for its RE technologies from Portland-based Energy Trust of Oregon, a nonprofit organization that promotes energy efficiency and renewable resources for customers of Pacific Power, Portland General Electric, and NW Natural.
Lower utility bills will help Tom and Kathy recoup the remaining up-front costs for the RE systems. In one year, from November 2005 to October 2006, the household’s electricity bills averaged about $25 per month, with a high of $83.04 in February and a surplus of $15.81 in July. For a 2,544-square-foot, all-electric home in southern Oregon, where cloudy winters mean low solar thermal and solar-electric output, the systems have performed well.
The payback doesn’t end there, though. The Carstens received $3,650 in federal tax credits for 2006, as well as a total of $3,600 in state tax credits for 2005 and 2006. Tom couldn’t be happier with his investment. “I’m living on a military pension, and I have few monthly expenses for this home. My bills are so low that I barely notice when energy rates go up,” says Tom. “The cost of energy is only going to get more and more expensive, but with this home, the rate increases are easier to absorb.”
“The [home] design and RE systems work together to keep the interior temperature comfortable all year,” Tom says. “We’re proud to tell people that we only turned on the air conditioning three times last summer. We find that, even when the temperatures are in the 80s, we can leave our windows open all day, and the inside of the house stays cool and comfortable.”