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Although all greenhouses are "solar" in that they use the heat of the sun to provide a growing environment for plants, the term "solar greenhouse" typically refers to a passive solar greenhouse. Passive solar greenhouses are different from traditional greenhouses in a few significant ways. A passive solar greenhouse uses only heat and light from the sun, and a traditional greenhouse uses gas, propane, or electric heat at night or during the winter to provide sufficient warmth to the plants. Passive solar greenhouses must be oriented toward the equator. They also use stone or large containers of water to absorb the sun’s heat by day and disperse it by night, and they use some type of insulation and double-glazed glass.
Traditional greenhouses use electrical heat lamps, floor heaters, propane heaters or other types of heating devices during the winter and, if necessary, at night to provide enough warmth to the greenhouse plants, but a true passive solar greenhouse relies upon the sun and natural components. Many solar greenhouses are constructed with the non-equator-facing wall made of masonry to absorb daylight heat and release it throughout the night; the remaining two walls typically are partially or entirely masonry, as well. Every square foot (929 square cm) of equator-facing glass requires 150 pounds (68 kg) of stone or masonry material. Placing buckets, tubs or pools of water strategically throughout the greenhouse allows the water to absorb and release the sun’s warmth in the same way. Each square foot (929 square cm) of equator-facing glass requires 4 gallons (15.1 liters) of water; containers should be dark in color and strategically placed throughout the greenhouse.
For a solar greenhouse to be effective, some kind of insulation is required in most environments. The unvented portion of the roof and all walls should be covered with insulating fabric or other material with a thermal resistance value, or R-value, of up to 19. If the passive solar greenhouse is sitting on a foundation, it, too, must be very well insulated. When the greenhouse is on a slab, the slab should be is insulated both where the greenhouse rests on it and anywhere there is a crack or opening.
A passive solar greenhouse requires double-glazing with night insulation. Single-glazed glass won’t be able to retain enough heat for the other passive elements — the containers of water and stone — to do their jobs. The only part of the glass portion of the greenhouse that shouldn’t be glazed is two-thirds of the roof, in order to permit the sun’s full effects during the winter months. Protection might be needed in the form of whitewash or fabric during summer months.
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