More people than ever before are growing vegetables, and they want them year round. Greenhouse manufacturers have capitalized on the boom and increased sales all over the country. Economies of scale and the growing use of plastics, fiberglass and other inexpensive materials have brought greenhouse prices within the reach of millions—some small units sell for as little as $100. Even the grander, aluminum-and-glass styles cost only around $1,000. Furthermore, a marriage of experience and modern technology has enabled an increasing number of companies to offer greenhouse kits that can be built and installed in a single weekend. For the adventurous and handy consumer, these offer an opportunity to save a tidy sum in construction costs.
The multiplicity of choice available to the home gardener nowadays is mind-boggling. Structural alternatives include inflatable, lean-to, free-standing, quonset-hut, gothic, dome-shaped—-and a hundred variants in between. Structural materials range from steel to aluminum to wood; coverings from glass to soft plastic to rigid plastic and other futuristic materials. Accessories include heaters, humidifiers, water systems, time controls, vents and many more.
The good news is that greenhouses are a practical proposition just about everywhere in the United States today (though, with recent and continuing increases in the price of energy, special attention needs to be given to keeping heat loss to a minimum).
Temperature is a key factor in the operation of a greenhouse. As a general rule, for each square foot of surface of a greenhouse that faces the outside, you need 1.4 BTUs (British Thermal Units) per hour for every degree Fahrenheit of difference between the outdoor temperature and the level of heat you wish to maintain within. For example, if a greenhouse has 80 square feet of surface area and you want to keep the inside temperature at 50° F, you will have to produce over 3,360 BTUs per hour if the outside temperature is 20° F. Double glazing and other forms of insulation will reduce this figure.
In general, then, the greater the surface area of your greenhouse, the greater will be the heat loss. (Other factors like materials and insulation will be discussed later in the chapter.) A quonset-like structure uses less than the regular rectangular greenhouse of the same length because it is curved and therefore has less surface area. By the same token, a round or dome-shaped greenhouse also has less surface area than a square structure whose sides are equal to the diameter of the round greenhouse.
Another factor affecting heat loss is the greenhouse's placement on your property. You can minimize heat loss by placing the greenhouse in a protected area—the downwind side of a row of trees, hedges, a fence, or any other barrier. If you're on more or less flat land and the wind is an important weather factor, it might be worthwhile to construct or plant some kind of windbreak. Take care, though, not to restrict the greenhouse's access to light. Some people add a piece of clear plastic to the north side of their greenhouse, thus reducing heat loss without cutting down significantly on the light.
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