Building Energy Efficiency Into a House Brings Together High-Tech, Low-Tech Options

An array of photovoltaic solar-cell roof panels for a 2,400-square-foot house costs about $25,000. (Photos By Susan Biddle -- The Washington Post)

In conversations about home building these days, I often hear the terms "sustainable approach" and "sustainability." What exactly do they mean?

When I asked a number of architects, builders, engineers, environmentalists and ecologists, I got some interesting answers.

A sustainable approach is another name for "green building," they said. The aesthetics are as varied as those for conventional construction, but the environmental impact of a house in both the near- and far-term is equally important. This has produced material-selection criteria that are much more extensive. Energy efficiency is also a central concern.

A material can be selected for many reasons besides price and durability. For example, a locally produced material requires less energy to get to your building site than one manufactured across the country. A natural material, such as quarried stone, requires less energy to process than fired brick.

Carpets made from plastic milk jugs and soda bottles keep these out of a landfill. Beautiful floors made with reclaimed logs that fell to the bottom of a river en route to a sawmill are not affecting old growth forests. Nor are reused doors that were plucked from a disassembled building. Materials that are chemically stable are better for healthy indoor air than those made with substances that off-gas volatile organic compounds (VOCs). The most objectionable VOC is formaldehyde, which is classified as a human carcinogen by the World Health Organization.

In many cases, material choices are not clear-cut. For example, bamboo has become a popular flooring material, in part because it seems to meet the sustainability criteria. Because it is a grass and not a tree, bamboo regenerates quickly. It grows with amazingly speed, maturing in three to five years. But, on the negative side, bamboo flooring is produced in Southeast Asia. It must be shipped across the Pacific, and then across the country to get to your house. In addition, each floorboard is made up of smaller pieces that are laminated together with glues that can contain formaldehyde.

When it comes to energy use, a sustainable approach maximizes efficiency with a mix of high-tech, low-tech and conventional heating equipment. From the beginning of the project, energy use is an important consideration. When a builder and architect walk the site with the owner to find the best views, they are also checking the feasibility of the least-costly energy option -- the free warmth of the sun for winter heating. When the living spaces can be oriented toward the south, the sun will warm the rooms in winter. An overhang will shade these same spaces in summer, keeping the sun out.

As the design of the house begins to take shape, the energy focus shifts to the building envelope and the ways to minimize heat gain -- the summer sun coming in -- and heat loss -- warm air going out in winter.

At the same time, another low-tech energy option can also be explored -- adding "thermal mass" to the south-facing rooms to "store" the free solar heat. A thermal mass absorbs the sun's heat and stores it for many hours before it starts to radiate it back into the room. This means that after the sun has gone down, you're still getting its heat.

Although thermal mass may sound like a piece of awkward equipment you will have to somehow camouflage, it's simply a finish material that can absorb a sufficient quantity of heat. The more common ones include adobe brick, a favorite in the Southwest; exposed stone; exposed concrete; or an additional half-inch layer of gypsum drywall to make a finish wall an inch thick.

A well-designed, passive solar-thermal mass combination will not, in most cases, entirely replace a furnace. But in North America, it will, on average, provide about 20 percent of your home heating needs, said Canadian energy expert William H. Kemp. A furnace would also be needed to provide backup on cloudy days, he added.

Thermal mass provides benefits in the summer, too. With properly sized overhangs, the south-facing rooms will not get any direct sun. Solar heat will still pass thorough the walls, but the thermal mass will absorb it, keeping your rooms cooler. At night when the mass releases its stored heat, you can open the windows to catch the evening breezes.