Davis, California is hot--with a summer design temperature of 103 degrees F. So, researchers at Pacific Gas & Electric Co. made a bold move when they proposed to build an experimental house without an air conditioner. The project was called ACT Squared.

"The goal was to apply an integrated package of the latest technologies to a house design to see how far we could go with energy efficiency," says Lance Elberling, research associate at PG&E. "The result was a building shell so effective at resisting summer heat gain that no air conditioning equipment was required."

Designers at Davis Energy Group were hired to help. They started with an energy package that complied with California's Title 24 energy code. Then, they evaluated 86 energy efficiency measures. Twenty-seven of these were selected for the project. The first demonstration house was built in Davis in 1993. Another house was completed in Rockland, California early this year.

Perimeter Changes

Several design changes involved the house perimeter. The shape of the building was made more compact. Researchers estimated that energy use shrank 0.31 percent for every foot of perimeter reduction.


The final ACT Squared design offers 1656 sq. ft. in an efficient compact plan.

Seventy-three percent of the window area ended up on the south wall to improve wintertime solar heat gain. In summer, the roof overhang shades these windows, so moving windows to the south also reduced cooling needs. West facing windows were reduced to only 4-1/2 sq. ft. Overall, window area was reduced to only 11 percent of floor area.

The final plan was a fairly simple rectangular shape with 178 linear feet of exterior wall. The longer axis extended from east to west, creating a large south-facing wall for windows. These changes to the perimeter and glazing reduced energy use by 23 percent.

Cooling Options

The original design assumed a typical forced air space heating and cooling with a gas furnace and electric air conditioner. Designers evaluated three alternatives:

All three options reduced construction costs and energy use compared to the original design.

The no-cooling package was found to be the most cost-effective for the Davis House. One key feature of the local climate made this approach possible: air temperature drops substantially at night. This allows heat to be flushed out with cool night air. Eliminating cooling equipment reduced construction costs by $1500. Energy savings were estimated to be 3,200 kWh of electricity and 370 therms of natural gas. That was 62 percent less than the original design.

"This package contained several energy saving measures that were considered too costly in earlier evaluations," says Elberling. "But overall, the package turned out to make economic sense, because the entire labor and materials cost for cooling was eliminated."

The other measures include: multiple ceiling fans, double-drywall in the ceiling, tile floor, whole-house attic fan, insulated doors and low-e, gas-filled windows. These features were added to the high levels of wall (R-26) and ceiling insulation (R-38) that had already proven cost-effective.

Evaporative Cooling

"The Rockland site doesn't cool off at night," says Elberling, "so night ventilation alone wasn't acceptable." The design solution was a unique heating/cooling system.

Heating energy is provided by a water heater that circulates water through a fan coil attached to a duct system.

Cooling is provided by an evaporative cooler. The system operates primarily at night when the evaporative cooler runs at greatest efficiency and electric rates are lower. The cool water circulates through 1000 ft. of 2-in. tubing located below the concrete slab floor. Cooling the 500 gallons of water in the tubing and the slab's thermal mass at night helps keep the house cool the following day. Temperature controls prevent the slab from dipping below 60 degrees F, which reduces the chance that condensation will form. If daytime cooling becomes necessary, water from the tubing circulates through the fan coil. This system alone can maintain the house temperature below 78 degrees F. The evaporative cooler's fan also provides night ventilation, when appropriate.

Savings at the Rockland site were projected to be 5,200 kWh of electricity and 330 therms of natural gas.

Wall Innovations

Both houses used an innovative wall system built from Oriented Strand Wood. Walls were filled with rigid foam insulation boards for an overall insulating value of R-25.6.

Marketability

At the Davis site, the final plan wasn't the most energy efficient package developed by the designers. Another plan reduced wall perimeter even further and offered four percent greater energy savings. However, that design presented problems with its roofline and room proportions. The final plan--four bedrooms, a family room and balanced daylighting in most rooms in only 1,656 sq. ft.--offered the best balance between energy efficiency and marketability.