From the outside, one home in Durham, N.C., looks like many others in the area— well-constructed, inviting and surrounded by trees. On the inside, however, the house has a different story to tell. Passive systems warm the interior and keep it comfortable for its occupants in a very natural, energy-efficient manner. Designed and manufactured by Youngsville, N.C.-based Enertia Building Systems, the solid-timber home stores solar energy in an interconnected sheath of space between the interior and exterior walls. Air that has been warmed through passive solar gain is retained in this outer envelope and radiates into internal walls. As warmer air on south-facing elevations rises and is pulled to the cooler side of the house, a natural convection process distributes warmth throughout the home. Even in winter, the convection process takes advantage of the mild 55 F (13 C) temperature below-grade at basement level. During the summer, hot air escapes through attic vents and sunlight is deflected by the insulated roof.
The secret, according to Enertia Building Systems’ founder Michael Sykes, is the wood. In these houses, 5 3/4-inch- (146-mm-) thick southern yellow pine timbers are milled as siding on the exterior and as paneling on the interior, eliminating the need for traditional siding, studs, insulation and wallboard. Most significantly, the southern yellow pine contains a high percentage of natural resin, which stores energy and maintains a fairly constant temperature. The company is investigating the possibility of injecting even more natural resin—a waste product of the paper industry—into the wood. “The resin is a material that goes from a liquid to a solid and back to a liquid, which stores more energy,” Sykes explains. “If you put a brick into the oven, it stores energy but just gets hotter and hotter. When you deal with something that changes phases, it stores and releases energy at a constant temperature, which is about 60 to 70 F [16 to 21 C].”
The owners of the 1,536-square-foot (143-m2), two-bedroom, two bathroom home with attached garage already have seen bottom-line savings. Their electric bill is only $35 per month with an average total energy expenditure of less than $600 a year. This amounts to about $1,000 per year less than comparable houses in the region, according to the Washington, D.C.-based U.S. Environmental Protection Agency. Solar hot-water panels fuel the water system, but the house does have a back-up natural-gas heating system for use during cloudy periods or particularly cold weather. The Durham home features a south-facing window wall, or “sunspace,” that captures most of the passive solar gain upon which the house relies. Iron grilles in the sunspace floor allow air to complete the loop as warmer air travels from the south side of the house up into the roof cavity, then through the north wall, down to the basement and, finally, back up to the sunspace. “The house is nestled back in the trees, but it gets enough sun to work,” Sykes says. “It’s a very energy-efficient house. The owners don’t like air conditioning so they use ceiling fans, and the outer shell shades the inner shell.”
Although Sykes admits a home like this costs 10 to 15 percent more than a typical stick-frame house, he says that when one factors in other life-cycle considerations, such as the cost of repairing and replacing mechanical systems, the Durham home and others like it are cost effective. The basic kit for the same model as the Durham house is sold for just more than $109,000. This includes the exterior wood, interior walls, posts, door frames, girders, beams and rafters. The owner or builder supplies the foundation, main floor, partition framing, windows, doors, stairs, flooring, ceiling, roof, plumbing, wiring and cabinets. In part because of its near-city location, the Durham house, in total, cost about $350,000.
The Durham house used solid southern yellow pine timbers, which were treated with borax mineral crystals that offer more efficient storage and release of passive solar energy. Enertia Building Systems now has switched to glulams, which are glue-laminated wood blocks that can be built out of scrap lumber and resist warping and cracking, making them more durable than logs. The wood for the glulams is sustainably harvested, and the glulam manufacturer asserts that two or more trees are planted for every one cut down. “Changing to glulams has made the kit more costly, but the builders have reported to us that the time to assemble the house is cut in half because glulams are straighter and of more perfect quality,” Sykes says. “Since every foot is usable, we cut our scrap from 30 percent down to less than 1 percent.”
Although his distinctive houses do not readily fit into the Washington-based U.S. Green Building Council’s LEED certification or most other greenrating systems, Sykes asserts that the homes are truly sustainable by their inherent design. “Once you choose to make your walls out of wood, you eliminate about five major green-building decisions, such as choosing siding, framing, insulation, inside finish and paint,” Sykes says. “All are replaced by one renewable building block. Future builders will see these blocks and make another house out of them. We’re pretty confident that these blocks will always be more valuable as structural components than as landfill fodder. This wood actually is a cradle-to-cradle component.”
Energy Costs: In 2006, the 1,526 square foot (143 m2) Durham house had a total energy expenditure of $572. Electric: $35 per month ($420 annually). Gas $80 in January and $72 in February ($152 annually).
Kim A. O’Connell writes about architecture and sustainability from Arlington, Va.
DURHAM HOUSE / DURHAM, N.C.
DESIGNER AND BUILDER / Enertia Building Systems Inc.,
Youngsville, N.C., www.enertia.com
GENERAL CONTRACTOR / South Grown Building & Timber
Frame Co., Hillsborough, N.C., www.sgtimberframe.com
SOLAR-PANEL INSTALLATION / Solar Consultants, Carrboro,
GLULAMS / Anthony Forest Products, El Dorado, Ark.,
WINDOWS / HURD Window Co., Medford, Wis., www.hurd.com
BACK-UP WOODSTOVE / Fireplace Editions, Chapel Hill, N.C.,