Air envelope keeps this solar home warm -- and cool

A home builder here sees its passive solar design as the wave of the future in energy-conserving houses. "What it amounts to is sophisticated insulation -- a Thermos-bottle effect," says Brent Millhollen of Sun Energy Builders, a five-year-old building firm located in Carmichael, a Sacramento suburb.

"What we have done is improve a double-shell house which has been around for a while."

The real key to the system is that it does not call for exotic modifications and allows the unit to resemble a conventionally designed house.Most people do not want to buy a house that is daringly unconventional in design, he adds.

Sun Energy used the sunloop design for a house it built last summer and plans to feature it in a subdivision still on the drawing boards.

The house has been performing "just as the computer said it would perform," reports Mr. Millhollen. "The people living in the house say they are happy with it. Their energy use was very little last winter and the back-up heater seldom had to come on."

Temperatures in the area vary from 100-plus F. in the summer to winter lows in the 30s.

A passive solar system, in contrast to an active system, contains no moving parts. Instead, it depends on the positioning of the building, the materials, and landscaping to take full advantage of the sun.

An active system employs a mechanical apparatus, such as collector panels, to collect, transfer, and store solar energy for both heating and cooling a structure.

As the cost of energy continues to rise, home buyers are asking for energy-efficient houses, reminds Mr. Millhollen.

Why is he so optimistic about the sunloop design?

Because, he says, the system calls for "nothing special" in machinery and technology; therefore, it is no more costly to build than a conventional house.

Also, it looks like a conventional house.

The designers added a sunporch on the south side of the structure to capture the sun's rays.

"We have always gone for aesthetics," asserts Mr. Millhollen. At the same time, he reports, the houses have to be affordable to people or they won't buy them, so "we also have to work for the maximum in cost efficiency."

Some passive solar systems call for extraordinary features which many people dislike. One system, for example, includes the erection of water- or rock-filled towering cylinders in front of large, south- facing windows.The cylinders absorb heat that is then released into the house throughout the day.

The sunloop system, by contrast, works by sealing the house in an envelope of air.

The sunroom collects heat during the day. As the room heats up, the warm air rises and flows through the roof trusses and across the attic.

When the air reaches the cooler north wall -- a double wall with an air space in between -- it flows down the air space toward the crawl space beneath the house. The loop is completed when the air travels under the house and back to the sunroom to begin the process all over again.

The house is warmed, not by heat radiating from the moving air, but by the air holding heat within the structure.

In the summertime the air blanket curbs the heat from penetrating the building.

The sun porch is separated from the rest of the house by sliding-glass doors.

"It is usually usable in the morning and the evening," Mr. Millhollen says. "Generally, we put the sun porch off the living room or kitchen. It can be used as a breakfast area when it is off the kitchen. If it is situated off the living room, it can add to th e living space in the evening by opening the doors."