Greenhouses. Planning A Home Greenhouse

The even span has a better shape than a lean to for

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Unformatted text preview: shape than a lean-to for air circulation to maintain uniform temperatures during the winter heating season. An even-span can accommodate two to three benches for growing crops. Window-mounted. A window-mounted greenhouse can be attached on the south or east side of a house. This glass enclosure gives space for conveniently growing a few plants at relatively low cost (Figure 2). The special window extends outward from the house a foot or so and can contain two or three shelves. rounding ground so rainwater and irrigation water will drain away. Other site considerations include the light requirements of the plants to be grown; locations of sources of heat, water, and electricity; and shelter from winter wind. Access to the greenhouse should be convenient for both people and utilities. A workplace for potting plants and a storage area for supplies should be nearby. Types of Greenhouses A home greenhouse can be attached to a house or garage, or it can be a freestanding structure. The chosen site and personal preference can dictate the choices to be considered. An attached greenhouse can be a half greenhouse, a full-size structure, or an extended window structure. There are advantages and disadvantages to each type. Attached Greenhouses Lean-to. A lean-to greenhouse is a half greenhouse, split along the peak of the roof, or ridge line (Figure 2). Lean-to’s are useful where space is limited to a width of approximately 7 to 12 feet, and they are the least expensive structures. The ridge of the lean-to is attached to a building using one side and an existing doorway, if available. Lean-to’s are close to available electricity, water, and heat. The disadvantages include some limitations on space, sunlight, ventilation, and temperature control. The height of the supporting wall limits the potential size of the lean-to. The wider the lean-to, the higher the supporting wall must be. Temperature control is more difficult because the wall that the greenhouse is built on may collect the sun’s heat while the translucent cover of the greenhouse may lose heat Freestanding Structures Freestanding greenhouses are separate structures; they can be set apart from other buildings to get more sun and can be made as large or small as desired (Figure 2). A separate heating system is needed, and electricity and water must be installed. The lowest cost per square foot of growing space is generally available in a freestanding or even-span greenhouse that is 17 to 18 feet wide. It can house a central bench, two side benches, and two walkways. The ratio of cost to the usable growing space is good. When deciding on the type of structure, be sure to plan for adequate bench space, storage space, and room for future expansion. Large greenhouses are easier to 2 Ventilation Ventilation is the exchange of inside air for outside air to control temperature, remove moisture, or replenish carbon dioxide (CO2). Several ventilation systems can be used. Be careful when mixing parts of two systems. Natural ventilation uses roof vents on the ridge line with side inlet vents (louvers). Warm air rises on convective currents to escape through the top, drawing cool air in through the sides. Mechanical ventilation uses an exhaust fan to move air out one end of the greenhouse while outside air enters the other end through motorized inlet louvers. Exhaust fans should be sized to exchange the total volume of air in the greenhouse each minute. The total volume of air in a medium to large greenhouse can be estimated by multiplying the floor area times 8.0 (the average height of a greenhouse). A small greenhouse (less than 5,000 ft3 in air volume) should have an exhaust-fan capacity estimated by multiplying the floor area by 12. The capacity of the exhaust fan should be selected at one-eighth of an inch static water pressure.The static pressure rating accounts for air resistance through the louvers, fans, and greenhouse and is usually shown in the fan selection chart. Ventilation requirements vary with the weather and season. One must decide how much the greenhouse will be used. In summer, 1 to 1 1/2 air volume changes per minute are needed. Small greenhouses need the larger amount. In winter, 20 to 30 percent of one air volume exchange per minute is sufficient for mixing in cool air without chilling the plants. One single-speed fan cannot meet this criteria. Two single-speed fans are better. A combination of a singlespeed fan and a two-speed fan allows three ventilation rates that best satisfy year round needs. A single-stage Cooling Air movement by ventilation alone may not be adequate in the middle of the summer; the air temperature may need to be lowered with evaporative cooling. Also, the light intensity may be too great for the plants. During the summer, evaporative cooling, shade cloth, or paint may be necessary. Shade materials include rollup screens of wood or aluminum, vinyl netting, and paint. Small package evaporative coolers have a fan and evaporative pad in one box to evaporate water, which cools air and increases humidity. Heat is removed from the...
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This note was uploaded on 03/27/2013 for the course BOT 101 taught by Professor Drake,d during the Spring '08 term at University of Hawaii, Manoa.

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