As growing place, living space, or auxiliary heater, greenhouses cultivate a loyal following

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They used to be called glass houses, or greenhouses, and generally housed only plants. When the sun shone, they overheated; when it didn't, they consumed vast amounts of energy, trying to keep out the frost. Traditionally, not many private homes boasted one, and those that did tended to be in more affluent areas.

Nowadays, these structures are more likely to be called ''sun spaces,'' and they generally do much more than house plants. In addition, they are often used for relaxing, reading, and sometimes even dining.

While some of these sun spaces may consume a little energy when the sun isn't shining, it is never very much. Often they contribute a lot of heat to a house. As a result, growing numbers of people now own a sun space, and they're not necessarily affluent or even close to it.

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What makes the modern greenhouse, or sun space, so much more energy-efficient results from modest design changes, adequate insulation, and the ability to store heat. But, before you go ahead and build your own greenhouse, or have one built, you must decide what exactly you want it for.

Is it to be a growing place, a living space, or an auxiliary heater for the home? It cannot be expected to give 100 percent in all three directions at once.

Most homeowners would like the greenhouse to perform at least two functions. Just remember, though, that some compromises will have to be made.

The greenhouse that is constantly providing heat to the rest of the house during the day, for example, won't be able to store enough heat to keep a plant community thriving through the night. As a result, you would not include much heat-storage capacity in such a sun-room.

In contrast, the ''production greenhouse'' stores the excess energy that pours in during the day for its own use at night. It has little excess heat to give to a house during the height of a northern winter. Planting beds, which are required by a production greenhouse, also prevent its being used as effective living space, although some people have been able to carve out a little breakfast nook in what is otherwise a production greenhouse.

The greenhouse as living space is something of a compromise as it is. It provides some heat to the rest of the house during the day, but stores enough heat to maintain a moderate temperature at night. It may, on occasions, draw on the home's heating system at night, but it will never take back anywhere near the amount of heat it gave to the home during the day.

As a bright, sunny area it would also be an ideal place for houseplants, for starting seedlings indoors, and for wintering over a favorite plant or two.

In the past, the old all-glass greenhouses would overheat in the sun and quickly freeze after dark without artificial heat. In contrast, temperatures seldom rise above 80 degrees F. in the modern passive-solar greenhouse during the day and rarely fall below comfortable, or at least plant-tolerable, temperatures at night. Insulation and heat-storage capacity make the difference.

The north-facing wall (facing south in the Southern Hemisphere) and any north-sloping roof of the solar greenhouse are not made of glass, because the sun doesn't shine from that direction. The rationale is that if the glass will serve no useful purpose, why use it. In addition, these nonglass walls are insulated to prevent rapid loss of heat, while all-glass surfaces are double thickness (with a thin dead-air space in between).

Heat-storage materials (thermal mass is another term) enhance the performance of these greenhouses by absorbing all the excess heat that would overheat the place during the day. Then at night, with no heat coming from the sun, this stored heat is gently released into the greenhouse. In turn, the stored heat can be retained much longer if an insulated covering is pulled down over the greenhouse or tight-fitting insulated shades are drawn on the inside at night.

Thermal mass comes in the form of masonry (bricks, stone, concrete, and so forth), water-filled barrels or cans, or phase-change salts. These hydrated salts are by far the most effective, but they also are the most expensive. Hydrated salts are used wherever space is at a premium, because the salts are roughly 14 times as effective as an equal volume of water. In turn, water is roughly three times as effective as masonry at storing heat.

Any form of container filled with water and painted black will absorb heat. Metal makes the best container for this purpose and is the reason why so many greenhouse owners stand their growing beds on 55-gallon drums filled with water.

Long-running trials by the Rodale organization have shown that greenhouse plants thrive in some pretty chilly air temperatures as long as their roots are kept warm - another argument for standing the growing bed on top of a thermal mass.

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