Passive Solar Design: A Cool Idea for Hot Savings

Solar panels are getting a lot of attention lately as a way to turn the energy of the sun into electricity we can use in our homes. With another solar technology, passive solar design, builders also can create homes that require significantly less energy to heat and cool.

“I like to define it as a heating system with only one moving part—the sun,” says Dan Chiras, an instructor at the University of Colorado in Boulder and director of the Evergreen Institute LLC, in Evergreen, Colo., which offers workshops focused on residential renewable energy and green building through their Center for Renewable Energy and Green Building in Gerald, Mo. “It’s a way to heat a home with minimal use of a heater or boiler and minimum outside energy.”

How Does it Work?

Successful passive solar begins with two primary building components—windows and thermal mass. By orienting a house so that the longest side faces south and filling that wall with windows, builders can provide homeowners with an ample supply of free heat from the sun. Thermal mass, in the form of concrete slab or tile floors, or substantial stucco interior walls, can help store that heat for release after sundown.

Adding to an effective plan are high levels of wall and roof insulation, along with south-facing overhangs. The insulation helps keep the heat inside—and helps keep cool air in during hot summer days—while the overhang keeps the high-angled summer sun from heating up interior spaces in warmer months.

“The overhang is the on/off switch of your passive solar design,” Chiras says. This means the overhang should be sized to allow direct sunlight to begin entering the home at the time of year when homeowners might first be tempted to turn up their thermostats, and to block direct sun when warm weather returns.

The sun is not only helping to cut energy bills; it also allows builders to specify smaller mechanical equipment, because the furnace or boiler is now a back-up resource, not the primary heat generator.

“You can now save a whole lot of money on your mechanical system, in both initial costs and over the long term,” Chiras says.

Passive solar homes are very sensitive to their setting. First, builders need to ensure the home is oriented to true south. Next, they also must match the overhang’s length to the sun’s passage in their area. In addition, ventilation must be designed to address local conditions to reduce air-conditioning needs.

Ken Haggard, a principal in the San Luis Sustainability Group, notes high desert areas might experience hot days and cool nights in the summer. Haggard’s company is a sustainable-design architecture firm based in San Luis Obispo, Calif. that has been designing passive solar homes since 1976.

Because of the desert’s weather conditions, owners can open their windows in the evening to cool their interiors and cover them in daytime to reduce heat gain. In southern climates, he adds, open windows and ceiling fans may be the key to daytime comfort.

Despite this necessary added attention to details, Haggard says passive solar homes remain an affordable alternative to today’s standard designs. The key is to design the house with the sun in mind, from the ground up, to ensure the structure stays tuned to its environment all four seasons of the year.

“The myth is that green buildings cost more, but if you start from scratch, it should cost less—that’s been our experience,” he says. “It comes down to convincing yourself that it’s important. Then it becomes easy.”