July 2007

Volume 16, Number 7

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Enertia Double-Envelope Home Still Has Problems

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Under construction, this southern Vermont home designed by Enertia Building Systems uses passive solar design and thermal mass from its solid-wood walls to even out temperature swings and reduce heating and cooling costs. However, its design entails a fire risk and other compromises.

Photo: Mark Piepkorn

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.

The Enertia design’s use of solar orientation, natural ventilation, and thermal mass are timeless and unimpeachable, and with proper implementation should reduce heating and cooling costs in any house. However, Sykes claims that the double-envelope system offers advantages over other passive solar homes. “We’re picking up solar energy and getting it in touch with the maximum amount of wood,” said Sykes. The Enertia design differs from prior double-envelope designs in its use of solid wood, which adds thermal mass; with the timbers prefabricated and cut by Enertia, it offers a log-home-style installation that is fairly straightforward and requires fewer steps than stick-frame walls.

No Enertia homes have extensive performance data, making it difficult to say how well the design works. Tom Miller of 36 Degrees South tested an Enertia home in Durham, North Carolina, under Energy Star protocol, and gave it a score of 67 in the Home Energy Rating System—better than the Energy Star threshold of 85 and the standard home score of 100 but far from the theoretical net-zero-energy score of zero. Miller said that the home’s score may not fully reflect its performance: only the outer wall of the double envelope could be counted, for a relatively low insulation value of R-6, and the beneficial effects of thermal mass aren’t counted in the protocol.

According to data published on the Enertia website, the Durham home has logged a 66% energy savings over a regional benchmark. However, Miller said that when he visited the house it was 80°F (27°C) and humid inside, whereas many people would be more comfortable with the thermostat at about 73°F (23°C) and with reduced humidity.

Miller’s report echoes a pattern that EBN found with other Enertia homeowners—they were willing to tolerate less comfortable conditions than the average homeowner. That kind of habit change is great for the environment, but it makes an apples-to-apples comparison with other homes harder, since any home can use less energy if the occupants are willing to compromise their comfort. Part of Enertia’s apparent virtue may simply be the virtue of its occupants.


Enertia’s double-envelope construction uses two parallel walls of stacked glue-laminated timbers, shown in this mock-up, with an air cavity between them for circulation of air and distribution of heat.

Photo: Mark Piepkorn

Energy performance aside, the Enertia design also suffers from the same liability as the old double-envelope design. Those homes were subject to rapid flame spread throughout the house in the event of a fire inside the cavity between the two walls. Sykes claims to have this problem under control, with fire detection in the envelope cavity and with the solid wood wall providing one-hour fire protection for the living area. However, in the double-wall design, windows in the exterior wall are lined up with windows in the interior wall, to provide views and ventilation. But while those interior windows are part of what he calls a one-hour firewall, Sykes told EBN, “I wouldn’t know how they would perform” in a fire. Asked by EBN about this use of windows, Bob Howe, assistant state fire marshal for the State of Vermont, said, “You just can’t take a window from the supply yard and put it into a fire-rated wall and maintain your rating.”

The Enertia design also uses a lot of wood, which Sykes says is certified by the Sustainable Forestry Initiative. According to Sykes, the use of wood, along with the need for two windows where most homes would have one, drives up the cost of materials for the home so that the overall cost is 10%–20% greater than that of a home with a conventional stick frame.

For that cost premium and higher resource use, one would hope for a significant return in terms of reduced energy costs. Enertia’s “no fuel, no power” marketing has led to accounts that Enertia requires no heating or cooling equipment. However, of the three Enertia homeowners EBN spoke with for this article, in Maryland, New Hampshire, and Vermont, all had radiant-floor heating systems, which are needed particularly when cold, cloudy winter conditions prevail for more than a day or two and the circulation of stored solar heat comes to a standstill. Sykes also said that it was standard, especially in southern climates, for Enertia homeowners to install a small air-conditioning system.

Similar to the way in which Enertia’s design leads to reduced comfort, improving its performance, it also appears to sacrifice indoor air quality for energy performance. In the Durham home, Miller said that a blower-door test revealed a fresh air exchange rate of 0.3 air changes per hour (ACH)—just above the threshold at which Energy Star would require mechanical air exchange, he said. Gaskets between the stacked timbers help provide an airtight home, but under certain conditions, especially during cold winter periods, that airtightness may prevent adequate intake of fresh air, and there is no mechanical ventilation system to make up the difference.

Without better data on the performance of Enertia homes, it’s tough to say how good a design it is. As Joe Lstiburek, P.Eng., of Building Science Corporation told EBN, arguing that he could only judge a house design with actual energy data, “I don’t want to hear about computer simulations, computational fluid dynamics, or natural ventilation.” However, the currently available data strongly suggests that the design underperforms expectations, while presenting additional concerns, including fire safety. A design that carefully and purposefully incorporates both building science and modest energy consumption—for example, a superinsulated, airtight home with a small mechanical heating and ventilation system and a photovoltaic array—may be safer while requiring fewer functional and aesthetic compromises for homeowners looking for energy independence.

– Tristan Roberts

For more information:

Enertia Building Systems

Youngsville, North Carolina



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July 10, 2007