The air-envelope, or double-shell, house relies on passive solar heating and natural cooling methods to do the job. In fact, the air-envelope design combines standard energy-efficient measures, such as as attached south-facing greenhouse and thick insulation, with a new and controversial technique -- a plenum that channels temperate air between the skins of four sides of the house cube and connects directly with the earth below.
The greenhouse, a one-foot span of air over the entire roof and down the north wall, and the basement or crawl space interconnect to form the plenum.
They system, developed by Lee Porter Butler of San Francisco and Hank Huber of New Ipswich, N.H., has generated controversy since its first appearance in solar-design circles in the mid-1970s.
Mr. Huber, who led an air-envelope workshop at Heartwood Owner Builder School in Washington, Mass., last summer, acknowledges that on one knows exactly how the air envelope works.
The fact is, he contends, it doesm work.
What actually happens in the plenum to produce the tempering effect?
According to their theory, solar heat collected in the greenhouse in winter circulates over the roof, down the north wall, and into the basement. Floor vents allow the cooler basement air to be drawn into the greenhouse to replace the rising air, thus creating a complete, constantly moving flow of warm air.
The principle of convection is at work, Mr. Huber says. In other words, hot air rises and cold air falls, driving the system like a fan.
The flow of air reverses during winter nights, he says. Heat stored during the day in the basement and in the wooden and sheetrock structure of the plenum is released into the air passage as the flow is drawn toward the greenhouse, the portion of the system that now loses the most heat through the glass walls.
In summer, the system is short-circuited. Vents in the peak of the greenhouse and roof release the hot air gathered in the system. Cool air is drawn in by way of the north wall vents.
In warms climates additional cooling can be provided by an earth cooling tube -- 60 feet of pipe buried four feet underground -- which naturally cools and dehumidifies the flow as it draws fresh air from an outdoor vent through the pipe toward a basement opening.
So far very little data a verify the theory has been scientifically gathered on the 70 houses now in operation.
Data on the envelopes does attest to an indoor climate that, even on the coldest days, remains in the 60 to 70 degree F. range during the day and rops no lower than the 50s at night.
Yet the air flow that produces these temperatures is evidently more complex than "hot air rises, cold air falls."
In an attempt to untangle this complexity, energy-efficiency consultants from the Solar Energy Research Institute in Golden, Colo., are monitoring several double-shell houses in a project sponsored by the federal government.
The researchers will also look into a second controversial question about the system. Mr. Porter claims the envelopes can prove 100 percent energy self-sufficient, yet most solar houses have always required some amount of backup heat.
Homeowners aren't necessarily complaining, however.
Mr. Porter's first house, owned by Tom Smith and built at Lake Tahoe in 1978, has required the use of a wood-burning stove during the coldest winter months. Yet Mr. Smith is gleeful, nonetheless. He burned only two- thirds of a cord of wood during the entire first winter, he said, while his neighbors, who also live in modern, well-insulated homes, burned 8 to 10 cords.
Other envelope dwellers reported the use of similar amounts of fuel, or even less.
Yet if the 100 percent solar-heated house is the goal, the double shell does have room for improvement. Mr. Huber, who has lived in his own envelope for two years and who designs and builds them for New England clients, is experimenting with the addition of a small fan or two to increase air flow and thus enhance the distribution of heat.
Other designers are probing the exposed earth connection, a third area of controversy.
Proponents argue that not only can the earth store and release heat as needed , but it also contributes moisture to the system, which, added to the humidity afforded by any plants in the greenhouse, creates an ideal interior climate.
Critics, however, argue that the earth, with only a shallow surface exposed to the bottom level of the convection sandwich, is a poor storage mechanism. Builders are experimenting with crushed rock, sand, or water as alternative storage units.
Although these questions of thermodynamic functioning remain, advocates of the double-shell design -- those who have built and lived in these experimental houses -- strongly support further development and refinement of the system.
Certainly there is much to commend this new design. The houses can be built in any architectural style and generally cost no more than conventional housing, since the cost of the extra materials and labor is offset by the elimination of a large central heating system. These are quiet dwellings which provide most of their own heating and cooling requirements.
In addition, the greenhouse can provide an indoor garden -- lovely to look at or dine in through the year. The garden can also produce fresh vegetables when stores offer only tasteless tomatoes and wilted greens.
To the burgeoning number of people concerned with the conservation of natural resources and greater self-sufficiency, the airenvelope house is a welc ome new idea in the solar housing field.