Enertia Double-Envelope Home Still Has Problems

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Those who remember the 1970s houses with two parallel exterior walls and an airspace between them circulating solar heat around the house may be doing double-takes in response to the Enertia house. A double-envelope house design by Enertia Building Systems has been buoyed by the 2007 Modern Marvel of the Year award from the History Channel (cosponsored by the U.S. Patent and Trademark Office) and attention from green lifestyle websites. Although the company, based in Youngsville, North Carolina, has built only about 80 homes in its 22-year history, according to Enertia inventor Michael Sykes, it is positioning itself for greater production.

That positioning starts with Enertia’s trademarked slogan, “No fuel, no power … no problem.” The basis for that slogan is Enertia’s passive solar design, coupled with passive thermal circulation. Enertia homes are oriented to the south, with numerous windows bringing solar thermal gain into a sun porch. Rather then funneling the warm air straight into a home, as do many passive solar designs, the Enertia design is supposed to circulate it upwards through a cavity between the home’s ceiling and its roof, which is insulated with structural insulated panels. That air then circulates downwards through the back, northern wall of the home through an 8” (20 cm) cavity between two parallel surfaces—an inner wall and an outer wall. The air moves through the basement and back up to the sun porch through registers.

The thermal mass of the house, whose inner and outer walls are made of 6” (15 cm) glue-laminated southern yellow pine, absorbs the thermal energy of the warm air as it moves through the double envelope, evening out daily temperature swings with its “thermal inertia.” In hot weather, the Enertia system works similarly, except that it flushes hot air through an attic window and takes in night air through a basement window; the cool thermal energy of the night air is stored in the thermal mass for the daytime. Buyers of an Enertia home get a design, a kit containing prefabricated and cut timbers with installation instructions, and installation advice as needed. Sitework, including the foundation, windows, utilities, and other components are provided by local suppliers and contractors. A typical home has 1,500 ft2 (140 m2) of living space on a main level, with a basement of the same size, which can be used as living space as long as airflow is not reduced too much.