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Unformatted text preview: r light transmission qualities. Both can
be used on curved surfaces; the polycarbonate material
can be curved the most. As a general rule, each layer
reduces light by about 10 percent. About 80 percent of
the light filters through double-layer plastic, compared
with 90 percent for glass.
Film plastic. Film-plastic coverings are available in
several grades of quality and several different materials.
Generally, these are replaced more frequently than other
covers. Structural costs are very low because the frame
can be lighter and plastic film is inexpensive. Light
transmission of these film-plastic coverings is comparable to glass. The films are made of polyethylene (PE),
polyvinyl chloride (PVC), copolymers, and other materials. A utility grade of PE that will last about a year is
available at local hardware stores. Commercial greenhouse grade PE has ultraviolet inhibitors in it to protect
against ultraviolet rays; it lasts 12 to 18 months.
Copolymers last 2 to 3 years. New additives have
allowed the manufacture of film plastics that block and
reflect radiated heat back into the greenhouse, as does
glass, which helps reduce heating costs. PVC or vinyl
film costs two to five times as much as PE but lasts as
long as 5 years. However, it is available only in sheets 4
to 6 feet wide. It attracts dust from the air, so it must be
washed occasionally. Environmental Systems
Greenhouses provide a shelter in which a suitable
environment is maintained for plants. Solar energy
from the sun provides sunlight and some heat, but you
must provide a system to regulate the environment in
your greenhouse. This is done by using heaters, fans,
thermostats, and other equipment. Heating
The heating requirements of a greenhouse depend
on the desired temperature for the plants grown, the
location and construction of the greenhouse, and the
total outside exposed area of the structure. As much as
25 percent of the daily heat requirement may come
from the sun, but a lightly insulated greenhouse structure will need a great deal of heat on a cold winter
night. The heating system must be adequate to maintain the desired day or night temperature.
Usually the home heating system is not adequate to
heat an adjacent greenhouse. A 220-volt circuit electric
heater, however, is clean, efficient, and works well.
Small gas or oil heaters designed to be installed through
a masonry wall also work well.
Solar-heated greenhouses were popular briefly during the energy crisis, but they did not prove to be economical to use. Separate solar collection and storage
systems are large and require much space. However,
greenhouse owners can experiment with heat-collecting
methods to reduce fossil-fuel consumption. One
method is to paint containers black to attract heat, and
fill them with water to retain it. However, because the
greenhouse air temperature must be kept at plant-growing temperatures, the greenhouse itself is not a good
Heating systems can be fueled by electricity, gas, oil,
or wood. The heat can be distributed by forced hot air,
radiant heat, hot water, or steam. The choice of a heating system and fuel depends on what is locally available,
the production requirements of the plants, cost, and
individual choice. For safety purposes, and to prevent
harmful gases from contacting plants, all gas, oil, and
woodburning systems must be properly vented to the
outside. Use fresh-air vents to supply oxygen for burners for complete combustion. Safety controls, such as
safety pilots and a gas shutoff switch, should be used as
required. Portable kerosene heaters used in homes are
risky because some plants are sensitive to gases formed
when the fuel is burned.
Calculating heating system capacity. Heating systems are rated in British thermal units (Btu) per hour
(h). The Btu capacity of the heating system, Q, can be
estimated easily using three factors: Foundations and Floors
Permanent foundations should be provided for glass,
fiberglass, or the double-layer rigid-plastic sheet materials. The manufacturer should provide plans for the
foundation construction. Most home greenhouses
require a poured concrete foundation similar to those in
residential houses. Quonset greenhouses with pipe
frames and a plastic cover use posts driven into the
Permanent flooring is not recommended because it
may stay wet and slippery from soil mix media. A concrete, gravel, or stone walkway 24 to 36 inches wide can
be built for easy access to the plants. The rest of the
floor should be covered by several inches of gravel for
drainage of excess water. Water also can be sprayed on
the gravel to produce humidity in the greenhouse. 5 Rigid-frame. The rigid-frame structure has vertical
sidewalls and rafters for a clear-span construction:
There are no columns or trusses to support the roof.
Glued or nailed plywood gussets connect the sidewall
supports to the rafters to make one rigid frame. The
conventional gable roof and sidewalls allow maximum
interior space and air circulation. A good foundation is
required to support the late...
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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