Take what is essentially a house-within-a-house and give it a large expanse of south-facing windows and you have a new form of solar architecture that is somewhat mysterious and, for the moment, quite controversial.
The desing, called the double shell or solar envelope, was pioneered by the San Francisco archtectural firm, Ekose'a. At least 100 houses of this sort now are scattered around the country, and as many as 1,000 more are under construction.
The doube-shell design employes "passive" solar heat. That is, it utilizes sunlight shining through windows and natural convection, as opposed to "active" systems which use box-like solar collectors with electric pumps or fans to distribute the heat.
Passive solar techniques were discounted in favor of active systems in the early 1970s. But htey are increasingly being recognized as a cost-effective approach to energy saving in new buildings. In fact, a considerable mystique has grownup around passively heated homes. Devotees consider them more "natural ," and the most extreme even lok down their noses at active solar systems.
These solar-envelope homes are one type of design. They have an inner and an outer shell, with living space inside the inner shell. The outer shell consist of a southfacing greenhouse, a crawl space under the floor, a roof with attic space, and an outer north wall with about a 12-inch air gap. The outer spaces are interconnected.
The basic idea, as Ekose's's founder, Lee Porter Butler, originally conceived it, was that a sunny winter days hot air would rise from the greenhouse, be drawn down between the north walls into the crawl space. There the warm air would depotis its heat in the earth or gravel beneath the house.
At night the cycle reverses. As the air in the greenhouse cools off, it sinks beneath the house and is warmed by the ehat stored in the earth.
To cool the house during hot weather, many envelope homes have a duct that runs from the crawl space underground about 50 feet from the house. Attice vents are opened, letting hot air escape. Replacement air is pulled through the underground vent and so is cooled.
Advocates of the solar envelope claim a number of advantages:
* It is up to 80 percent more energy efficient than ordinary housing.
* With no furnace or blowers and a double shell to damp outside noises, the houses are extremely quiet.
If desired, the greenhouse can be filled with plants which help freshen the air.
But there are a number of experts who feel the solar envelope has been oversold.
Limited monitoring of a Rhode Island double-shell house by Ralph Jones of Brookhaven National Laboratories found no evidence of the reverse, night cycle. In this case, at least, an appreciable amount of heat was not stored in the earth beneath the house, calling into question Ekose'a's claims that these houses do not need a backup heating system.
Well-known solar expert William A. Shurcliff has compared the solar envelopw with the "superinsulatred home." This later design is simply an ordinary house so tightly insulated that it can be heated by body heat and waste heat from lights and appliances. Dr Shurcliff comes out favoring superinsulation.
Cost is another a major point of debate. advocates claim that solar envelopes can be built for about the same price as ordinary structures. But critics claim they can cost as much as $10,000 more.
"The real question is whether the solar envelope is the most cost-effective way to achieve this magnitude of energy savings," says bruce Baccei of the Solar Energy Research Institute. "It is my feeling that there are less expensive ways ," he adds.