Among the latest in green technology demonstration projects, we checked in on the NAHB Research Center's test home in suburban Baltimore. Unlike the BASF demo house in Paterson, N.J., which I wrote about in the July 1 issue (see “Seal the Deal,” page 99), this house uses more standard, stick-built construction techniques.
Located in Bel Air, Md., and dubbed the “PowerHouse,” this house is a 2,500-square-foot, two-story colonial with a 1,100-square-foot unfinished basement. It has four bedrooms, two-and-a-half baths, and central air conditioning. The builder, Bob Ward Cos. of Edgewood, Md., wanted to apply green technology to standard building methods. Therefore, the house is not a 100 percent net-zero energy structure, but it should run on roughly half the energy of a conventional house.
According to Joe Wiehagen, senior research engineer for the NAHB Research Center, the house was framed using optimum value engineering (OVE), consisting of 16-inch-on-center framing using primarily 2x4s. Features include three-stud corners, ladder nailers where interior partitions intersect exterior walls, non-load-bearing headers on non-load-bearing walls, and insulated headers on load-bearing walls. Other features include wrapping the framing in Covalence Coated Product's Thermoply, covering it with OSB, and then applying 1 inch of high-density foam. All joints, windows, joists, and electrical and plumbing channels are sealed, and the walls and attic are insulated using blown-in fiberglass. The result is a rating R-15 for 2x4 walls, R-23 for 2x6 walls, and R-49 in the attic. All windows are Energy-Star rated.
The basement's prefabricated foundation system, Superior Wall's Xi system, consists of 2 inches of high-density concrete, 2 inches of foam insulation, and concrete studs wrapped in foam spaced 2 feet on center. The panels are poured with rebar in the factory and are bolted together, caulked, and mastic sealed on site. They go together easily and quickly, Wiehagen says, and they sit on crushed gravel as opposed to footings. (Evaluation reports are available on this system from the International Code Committee.) The result is a basement with a rating of R-12.5.
The HVAC system is based on a heat pump coupled with a multispeed blower and two-speed compressor. For heat, it is rated at a 9.3 HSPF (heating seasonal performance factor), which compares with a federal minimum standard of 7 HSPF. For air conditioning, it carries a 19 SEER (seasonal energy-efficiency ratio) rating. The duct system uses a return air system for each floor, coupled with crossover vents from room to room, and allows air to move evenly in the rooms when the doors are closed. The house also features a whole-house ventilation system with a common exhaust for the bathrooms and kitchen and an outside air intake.
The hot water system routes the hot water as close as possible to where it is used through PEX tubing routed to a central manifold. It is an electric on-demand system augmented by a 60-square-foot thermal collection array installed on the roof. The thermal side of the system should supply all needed hot water in the summer and raise the temperature of cold water by 30 degrees to 40 degrees in the winter.
Also on the roof is a 3,850-watt photovoltaic array that links to a pair of inverters in the basement and a reverse electric meter. Excluding the electricity provided by the PV system, the house should use 43 percent less energy than a traditionally built home, including 50 percent less for heating, 60 percent less for cooling, and 70 percent less for hot water, according to Wiehagen. The electric bill, he estimates, should be less than $150 per month. Wiehagen also estimates that savings on energy should cover the additional cost of all the systems when weighed against cost savings achieved through the OVE system over the life of a mortgage.
SOURCE: THE NAHB RESEARCH CENTER