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Zero-energy building makes housing even more affordable
Imagine a struggling family whose dream of homeownership finally comes true only to learn that they cannot afford to heat and light their new home. Habitat for Humanity formed a partnership with the National Renewable Energy Laboratory and the U.S. Department of Energy’s Building America program to make sure that doesn’t happen in any of the houses the nonprofit organization builds and sells to low-income families. “The houses should be affordable to live in as well as to buy,” said Real Estate Developer Andy Blackmun of Habitat’s Metro Denver affiliate. “For that reason, all of the homes we have built since 1999 have been energy-efficient houses. What we are trying to do with NREL is bridge the gap between high-end technology and day-to-day measures that will help families save money now.” The partnership is building Habitat for Humanity’s first true net-zero energy home in Wheatridge, Colo. “The advantage of working with Habitat is that it builds so many houses. It gives us the opportunity to get new technology out of the lab and into homes,” said Ron Judkoff, director of NREL’s Center for Buildings and Thermal Systems. “Bringing energy use down makes sense with Habitat’s mission, too, since reducing electric bills will really help Habitat families,” he added. Cost, easy installation considered The house is the second the partnership has built. In 2002, Habitat Metro Denver asked the DOE to help it improve its design and construction process. The result was a highly efficient demonstration home in Westminster, Colo., that improved on standard home benchmark energy consumption by more than 40 percent. The long-term goal of the collaboration is to make zero-energy housing cost-efficient by 2020. “We look for grants and in-kind donations to cover some of the increased expense of energy-efficient building,” explained Blackmun. “When it comes to direct cost, though, we have to consider whether an incremental gain in efficiency is going to take resources away from building another house.” In addition to being cost-effective, the zero-energy Habitat house had to be volunteer-friendly to build, which ruled out certain types of technology. For example, “We looked at a recovery ventilation unit tied into the heating system, but it would have to be installed by highly trained contractors,” said Blackmun. “Then, if the ventilator went down, the house would have no heat. We don’t want to saddle homeowners with systems that require expensive upkeep.” The streamlined building process NREL researchers developed focused on improving the efficiency in four areas:
These measures bring the house’s average energy consumption down to 64 MBtu annually compared to 90 MBtu for the 2002 energy-efficient Habitat home. The standard benchmark is 160 MBtu. The grid-tied, 4-kW solar array is expected to produce enough electricity in the summer to offset energy use in the winter, earning the 2005 home its “zero-energy” status. Orientation, design, systems equal efficiency The thick walls have an R-value of 40—protection against outside noise as well as the elements—creating window reveals deep enough to hold toys, books or knickknacks. The floor plan places the living room and two bedrooms on the southern side of the house where most of the windows are. The bathroom, kitchen and utility room are on the north side. This layout allows the high-traffic rooms to take advantage of natural lighting and passive solar gain. Low-e windows were used throughout the house. Judkoff noted, “We installed different ‘flavors’ of window depending on which way the wall faced. The south-facing windows allow more sunlight in.” The roof of the house has a raised heel truss, allowing room for insulation to R-60 and improving ventilation. The floor is insulated to R-30—more effective than insulating the crawl space walls. The heavy insulation eliminates 80 to 85 percent of the heating load. The single-point, natural gas heater supplies the heat, with electric baseboard heaters installed as a backup. “The hope is that the family will never need to turn those wall units on,” said Judkoff. Because heat leaves the building so slowly, the HVAC system does not require dedicated ductwork. An energy recovery ventilator located in the utility room will circulate the heat. “When a building is this tight, air quality becomes a concern,” Judkoff explained. “This unit maintains healthy ventilation and recovers air thermal energy.” The ventilator draws fresh air from outside and exchanges heat with exhaust from the kitchen and bathroom. The outside air needs to be warmed up by only 10 percent to reach the indoor air temperature. Heating water consumes 28 percent of the average home’s energy, so the solar hot water heater donated by the Institute for Sustainable Power is key to the zero-energy home strategy. The system consists of a super-insulated, 200-gal. storage tank in the utility room, solar collector panels on the roof, a coil to exchange heat with the pressurized water in the plumbing system and a pump to circulate the water. A natural-gas flash heater brings the water up to the desired temperature on demand and acts as a backup. “We’ve had some problems with tankless heaters in the past,” Blackmun said. “If this tankless heater performs well, we might use it in other houses. Lessons for today, future Blackmun said that Habitat tries to take lessons away from each building project, but it may be six or seven years before some of the systems on the zero-energy home become standard. However, some practices can be applied much sooner. “We have always paid attention to the building’s solar exposure, but the combination of window placement, orientation, and shading really minimizes the heating and cooling needs of the building,” he said. Working with volunteers to build double-stud walls was another valuable experience. “It only adds $1,000 to $1,500 to the cost and it makes a big difference on utility bills,” Blackmun observed. The Habitat for Humanity zero-energy home was completed in September and the family has settled in for the winter. NREL will monitor the systems performance for a year. “We want to see if the house comes as close to zero-energy as predicted,” said Judkoff. “Also, whenever you put technology in a building, you want to know how it works from the occupants’ point of view.” Judkoff shares Secretary of Energy Samuel Bodman’s belief that the zero-energy home is, “a glimpse into the future of home construction in America,” as Bodman stated in a press release. “We are in business to bring down the cost of these technologies so they can go into any home, not just demonstration models,” Judkoff declared. |
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