| Adapted from Fact Sheet 645 - University of Maryland
Cooperative Extension Service, David S. Ross, Extension Agricultural Engineer, Department
of Agricultural Engineering Careful planning is important before a home greenhouse project is
started. Building a greenhouse does not need to be expensive or time-consuming. The final
choice of the type of greenhouse will depend on the growing space desired, home
architecture, available sites, and costs. The greenhouse must, however, provide the proper
environment for growing plants.
The greenhouse should be located where it gets maximum sunlight. The first choice of
location is the south or southeast side of a building or shade trees. Sunlight all day is
best, but morning sunlight on the east side is sufficient for plants. Morning sunlight is
most desirable because it allows the plant's food production process to begin early; thus
growth is maximized. An east side location captures the most November to February
sunlight. The next best sites are southwest and west of major structures, where plants
receive sunlight later in the day. North of major structures is the least desirable
location and is good only for plants that require little light.
Deciduous trees, such as maple and oak, can effectively shade the
greenhouse from the intense late afternoon summer sun; however, they should not shade the
greenhouse in the morning. Deciduous trees also allow maximum exposure to the winter sun
because they shed their leaves in the fall. Evergreen trees that have foliage year round
should not be located where they will shade the greenhouse because they will block the
less intense winter sun. You should aim to maximize winter sun exposure, particularly if
the greenhouse is used all year. Remember that the sun is lower in the southern sky in
winter causing long shadows to be cast by buildings and evergreen trees (Figure 1).
Good drainage is another requirement for the site. When necessary, build the greenhouse
above the surrounding 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.
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 2A), Lean-tos are useful where space is limited to
a width of approximately seven to twelve 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-tos 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 rapidly. The
lean-to should face the best direction for adequate sun exposure. Finally, consider the
location of windows and doors on the supporting structure and remember that snow, ice, or
heavy rain might slide off the roof or the house onto the structure.
Even-span. An even-span is a full-size structure that has one gable
end attached to another building (Figure 2B). It is usually the largest and most costly
option, but it provides more usable space and can be lengthened. The even-span has a
better 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 2D). The special window extends outward from the
house a foot or so and can contain two or three shelves.
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 2C). 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 manage
because temperatures in small greenhouses fluctuate more rapidly. Small greenhouses have a
large exposed area through which heat is lost or gained, and the air volume inside is
relatively small; therefore, the air temperature changes quickly in a small greenhouse.
Suggested minimum sizes are 6 feet wide by 12 feet long for an even-span or freestanding
greenhouse.
A good selection of commercial greenhouse frames and framing materials is available.
The frames are made of wood, galvanized steel, or aluminum. Build-it-yourself greenhouse
plans are usually for structures with wood or metal pipe frames. Plastic pipe materials
generally are inadequate to meet snow and wind load requirements. Frames can be covered
with glass, rigid fiberglass, rigid double-wall plastics, or plastic film. All have
advantages and disadvantages. Each of these materials should be considered--it pays to
shop around for ideas.
Frames
Greenhouse frames range from simple to complex, depending on the imagination of the
designer and engineering requirements. The following are several common frames (Figure 3).
Quonset. The Quonset is a simple and efficient construction with an
electrical conduit or galvanized steel pipe frame. The frame is circular and usually
covered with plastic sheeting. Quonset sidewall height is low, which restricts storage
space and headroom.
Gothic. The gothic frame construction is similar to that of the
Quonset but it has a gothic shape (Figure 3). Wooden arches may be used and joined at the
ridge. The gothic shape allows more headroom at the sidewall than does the Quonset.
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 lateral load on the
sidewalls.
Post and rafter and A-frame. The post and rafter is a simple
construction of an embedded post and rafters, but it requires more wood or metal than some
other designs. Strong sidewall posts and deep post embedment are required to withstand
outward rafter forces and wind pressures. Like the rigid frame, the post and rafter design
allows more space along the sidewalls and efficient air circulation. The A-frame is
similar to the post and rafter construction except that a collar beam ties the upper parts
of the rafters together.
Coverings
Greenhouse coverings include long-life glass, fiberglass, rigid double-wall plastics,
and film plastics with 1- to 3-year lifespans. The type of frame and cover must be matched
correctly.
Glass. Glass is the traditional covering. It has a pleasing
appearance, is inexpensive to maintain, and has a high degree of permanency. An aluminum
frame with a glass covering provides a maintenance-free, weather-tight structure that
minimizes heat costs and retains humidity. Glass is available in many forms that would be
suitable with almost any style or architecture. Tempered glass is frequently used because
it is two or three times stronger than regular glass. Small prefabricated glass
greenhouses are available for do-it-yourself installation, but most should be built by the
manufacturer because they can be difficult to construct.
The disadvantages of glass are that it is easily broken, is initially expensive to
build, and requires must better frame construction than fiberglass or plastic. A good
foundation is required, and the frames must be strong and must fit well together to
support heavy, rigid glass.
Fiberglass. Fiberglass is lightweight, strong, and practically
hailproof. A good grade of fiberglass should be used because poor grades discolor and
reduce light penetration. Use only clear, transparent, or translucent grades for
greenhouse construction. Tedlar-coated fiberglass lasts 15 to 20 years. The resin covering
the glass fibers will eventually wear off, allowing dirt to be retained by exposed fibers.
A new coat of resin is needed after 10 to 15 years. Light penetration is initially as good
as glass but can drop off considerably over time with poor grades of fiberglass.
Double-wall plastic. Rigid double-layer plastic sheets of acrylic or
polycarbonate are available to give long-life, heat-saving covers. These covers have two
layers of rigid plastic separated by webs. The double-layer material retains more heat, so
energy savings of 30 percent are common. The acrylic is a long-life, nonyellowing
material; the polycarbonate normally yellows faster, but usually is protected by a
UV-inhibitor coating on the exposed surface. Both materials carry warranties for 10 years
on their 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 five years. However, it is available only in sheets four to six feet wide. It attracts
dust from the air, so it must be washed occasionally.
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 ground.
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.
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.
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-heater 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 solar-heat collector.
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:
- A is the total exposed (outside) area of the greenhouse sides, ends, and roof
in square feet (ft2). On a Quonset, the sides and roof are one unit; measure
the length of the curved rafter (ground to ground) and multiply by the length of the
house. The curves end area is 2 (ends) X 2/3 X height
X width. Add the sum of the first calculation with that of the second.
- u is the heat loss factor that quantifies the rate at which heat
energy flows out of the greenhouse. For example, a single cover of plastic or glass has a
value of 1.2 Btu/h x ft2 x oF (heat loss in Btu's her
hour per each square foot of area per degree in Fahrenheit); a double-layer cover has a
value of 0.8 Btu/h x ft2 x oF. The values
allow for some air infiltration but are based on the assumption that the greenhouse is
fairly airtight.
- (Ti-To) is the maximum temperature difference between the lowest
outside temperature (To) in your region and the temperature to be maintained in
the greenhouse (Ti). For example, the maximum difference will usually occur in
the early morning with the occurrence of a 0oF to -5oF
outside temperature while a 60oF inside temperature is maintained.
Plan for a temperature differential of 60 to 65oF. The following
equation summarizes this description: Q = A x u x (Ti-To).
Example. If a rigid-frame or post and rafter
freestanding greenhouse 16 feet wide by 24 feet long, 12 feet high at the ridge, with 6
feet sidewalls, is covered with single-layer glass from the ground to the ridge, what size
gas heater would be needed to maintain 60oF on the coldest winter
night (0oF)? Calculate the total outside area (Figure 4):
| two long sides |
2 x 6 ft x 24 ft = 288 ft2 |
| two ends |
2 x 6ft x 16 ft = 192 ft2 |
| roof |
2 x 10 ft x 24ft = 480 ft2 |
| gable ends |
2 x 6 ft x 8 ft = 96 ft2 |
|
A = 1,056 ft2 |
Select the proper heat loss factor, u = 1.2
Btu/h x ft2 x oF. The temperature differential
is 60oF - 0oF = 60 oF.
Q = 1,056 x 1.2 x 60 = 76,032 Btu/h (furnace output).
Although this is a relatively small greenhouse, the furnace output is
equivalent to that in a small residence such as a townhouse. The actual furnace rated
capacity takes into account the efficiency of the furnace and is called the furnace input
fuel rating.
This discussion is a bit technical, but these factors must be considered
when choosing a greenhouse. Note the effect of each value on the outcome. When different
materials are used in the construction of the walls or roof, heat loss must be calculated
for each. For electrical heating, covert Btu/h to kilowatts by dividing Btu/h by 3,413. If
a wood, gas, or oil burner is located in the greenhouse, a fresh-air inlet is recommended
to maintain an oxygen supply to the burner. Place a piece of plastic pipe through the
outside cover to ensure that oxygen gets to the burner combustion air intake. The inlet
pipe should be the diameter of the flue pipe. This ensures adequate air for combustion in
an airtight greenhouse. Unvented heaters (no chimney) using propane gas or kerosene are
not recommended.
Installing circulating fans in your greenhouse is a good investment.
During the winter when the greenhouse is heated, you need to maintain air circulation so
that temperatures remain uniform throughout the greenhouse. Without air-mixing fans, the
warm air rises to the top and cool air settles around the plants on the floor.
Small fans with a cubic-foot-per-minute (ft3/min)
air-moving capacity equal to one quarter of the air volume of the greenhouse are
sufficient. For small greenhouses (less than 60 feet long), place the fans in diagonally
opposite corners but out from the ends and sides. The goal is to develop a circular (oval)
pattern of air movement. Operate the fans continuously during the winter. Turn these fans
off during the summer when the greenhouse will need to be ventilated.
The fan in a forced-air heating system can sometimes be used to provide
continuous air circulation. The fan must be wired to an on/off switch so it can run
continuously, separate from the thermostatically controlled burner.
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 1½ 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 single-speed fan and a two-speed fan allows three
ventilation rates that best satisfy year round needs. A single-stage and a two-stage
thermostat are needed to control the operation.
A two-speed motor on low speed delivers about 70 percent of its full
capacity. If the two fans have the same capacity rating, then the low-speed fan supplies
about 35 percent of the combined total. This rate of ventilation is reasonable for the
winter. In spring, the fan operates on high speed. In summer, both fans operate on high
speed.
Refer to the earlier example of a small greenhouse. A 16-foot wide by
24-foot long house would need an estimated ft3 per minute (cubic
feet per minute; CFM) total capacity; that is, 16x24x12 ft3 per
minute. For use all year, select two fans to deliver 2,300 ft3 per
minute each, one fan to have two speeds so that the high speed is 2,300 ft3
per minute. Adding the second fan, the third ventilation rate is the sum of both fans on
high speed, or 4,600 ft3 per minute.
Some glass greenhouses are sold with a manual ridge vent, even when a
mechanical system is specified. The manual system can be a backup system, but it does not
take the place of a motorized louver. Do not take shortcuts in developing an automatic
control system.
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 roll-up 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
air to change water from liquid to a vapor. Moist, cooler air enters the greenhouse while
heated air passes out through roof vents or exhaust louvers. The evaporative cooler works
best when the humidity of the outside air is low. The system can be used without water
evaporation to provide the ventilation of the greenhouse. Size the evaporative cooler
capacity at 1.0 to 1.5 times the volume of the greenhouse. An alternative system, used in
commercial greenhouses, places the pads on the air inlets at one end of the greenhouse and
uses the exhaust fans at the other end of the greenhouse to pull the air through the
house.
Controllers/Automation
Automatic control is essential to maintain a reasonable environment in
the greenhouse. On a winter day with varying amounts of sunlight and clouds, the
temperature can fluctuate greatly; close supervision would be required if a manual
ventilation system were in use. Therefore, unless close monitoring is possible, both
hobbyists and commercial operators should have automated systems with thermostats or other
sensors.
Thermostats can be used to control individual units, or a central
controller with one temperature sensor can be used. In either case, the sensor or sensors
should be shaded from the sun, located about plant height away from the sidewalls, and
have constant airflow over them. An aspirated box is suggested; the box houses each sensor
and has a small fan that moves greenhouse air through the box and over the sensor (Figure
5). The box should be painted white so it will reflect solar heat and allow accurate
readings of the air temperature.
A water supply is essential. Hand watering is acceptable for most
greenhouse crops if someone is available when the task needs to be done; however, many
hobbyists work away from home during the day. A variety of automatic watering systems is
available to help to do the task over short periods of time. Bear in mind, the small
greenhouse is likely to have a variety of plant materials, containers, and soil mixes that
need different amounts of water.
Time clocks or mechanical evaporation sensors can be used to control
automatic watering systems. Mist sprays can be used to create humidity or to moisten
seedlings. Watering kits can be obtained to water plants in flats, benches, or pots.
CO2 and Light
Carbon dioxide (CO2) and light are essential for plant
growth. As the sun rises in the morning to provide light, the plants begin to produce food
energy (photosynthesis). The level of CO2 drops in the greenhouse as it is used
by the plants. Ventilation replenishes the CO2 in the greenhouse. Because CO2
and light complement each other, electric lighting combined with CO2 injection
are used to increase yields of vegetable and flowering crops. Bottled CO2, dry
ice, and combustion of sulfur-free fuels can be used as CO2 sources. Commercial
greenhouses use such methods.
Alternative Growing Structures
A greenhouse is not always needed for growing plants. Plants can be
germinated in one's home in a warm place under fluorescent lamps. The lamps must be close
together and not far above the plants.
A cold frame or hotbed can be used outdoors to continue the growth of
young seedlings until the weather allows planting in a garden. A hotbed is similar to the
cold frame, but it has a source of heat to maintain proper temperatures. |
