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Three houses used in the study were all built from the same stock plans and looked the same from the front (top). One house--dubbed the "builder house"--was built according to typical practices for the locale: slab-on-grade, solid-lumber framing 16 inches O.C., R-13 walls and R-30 roof. A second house--named the "retrofit house"--was given a modest package of energy upgrades, including more energy-efficient windows and other idtems that could be added easily without opening walls or excavating. The "energy-efficient" house (above) was fitted triple-glazed windows and 80 additional square feet of glazing on its south (back) side.  Walls were inslated with both spray and rigid foam. A flat-plate solar collector was installed on the roof to heat domestic hot water along with a 2.5 Kw photovoltaic array.

Recently released information from an ongoing study by researchers at the Tennessee Valley Authority (TVA) and Oak Ridge National Laboratory (ORNL) should be of interest to cost- and energy-conscious builders in warm, humid climates. The study found that a relatively modest upgrade in insulation and HVAC equipment can dramatically reduce a house’s annual energy use.

Three houses. The TVA-ORNL project is based on three houses built in 2008 in the same suburban Knoxville, Tenn., development. The all-electric homes share the same orientation and are built using identical stock plans for a conventional three-bedroom, 2,400-square-foot home. The first, termed the builder house, is typical for the area: It’s built on a slab, framed with solid lumber spaced 16 inches on-center, and insulated with R-13 fiberglass batts in the walls and R-30 loose-fill fiberglass in the attic. 

The second house, known as the retrofit house, incorporates a modest package of energy upgrades, all of which could be easily retrofitted into an existing home without opening walls or adding insulation below-grade. These include upgraded windows (sash with more efficient glazing were added to the vinyl windows found in the builder house) and an unvented attic sealed with a layer of spray foam at the roof deck. Those changes made it possible to downsize the hvac system from the two SEER 13 heat pumps used in the builder house (with a combined capacity of 4 1„2 tons) to a single 3-ton SEER 16 unit.

The high-efficiency house, as the third variant is called, was built from the ground up with a wider — and more costly — slate of improvements: a better-insulated foundation, triple-glazed windows, exterior foam sheathing and spray foam in the wall cavities for improved air sealing, a solar- heated domestic hot-water system, and a grid-connected 2.5 kw photovoltaic array.

Automated occupants. To accurately compare the differences in energy use among the three houses, the researchers took great pains to steer clear of “the occupant problem.” Normal person-to-person (or family-to-family) variations in use of water, electronics, lighting, and heating and cooling equipment can cause energy use in a given home to vary by a factor of two or more, depending on who lives there.

For this test, energy use is controlled not by actual people but by three identical, electronically simulated “occupants.” Computer-controlled switches turn lights and electronics on and off according to a programmed schedule, while hydraulic actuators open and close refrigerator doors, flush toilets, and operate washers and dryers. “The neighbors in the development call them “the robo-houses,” says ORNL researcher Jeff Christian, one of the project’s designers.

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Refrigerator and freezer doors in all three houses are periodically opened and closed with hydraulic actuators operated by timers, simulating day-to-day use without the messy unpredictability of human occupants. Similar controls flush toilets, run showers, and tumble-dry a load of towels — after misting them with a measured amount of water.

Positive cash flow. According to Christian, 12 months of accumulated data, from July 2009 to July 2010, reveals that the base-case builder house had an average electric bill of $6.46 per day over the course of the year. The retrofit house used $3.76 per day, while the more technologically advanced high-efficiency version clocked in at about a dollar a day.

Though it clearly had the lowest electricity bill, the high-efficiency house was also the most expensive, costing about $30,000 more to build than the builder house, or about three times the cost premium of the retrofit house. For many builders, replicating the high-efficiency house would involve clawing up the steep path of a new learning curve, dealing with new categories of subs, and possibly leaving technologically conservative buyers behind.

In fact, an analysis of the cost and energy-use data from the retrofit house, performed by Christian and co-author Tyler Blazer and recently published by the American Council for an Energy Efficient Economy, suggests that it’s the middle-of-the-pack retrofit house that should actually be capturing the attention of mainstream builders.

Christian and Blazer found that balancing the $10,000 price tag of the retrofit upgrades against the energy savings yielded a modest positive cash flow over the life of a 10-year loan at 6 percent interest. “Deep-energy retrofits can be very economical,” Christian says. “We came up with a positive cash flow in an area where electric rates are a couple of cents below the national average. If electric costs were higher, it would look even better.”