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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
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
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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.
- Spring '08