Planning and Building a Greenhouse
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.
Location
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.
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 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.
Structural Materials
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.
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 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.
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-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.
Air Circulation
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
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.
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 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.
Watering Systems
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.
Adapted from Fact Sheet 645 - University of Maryland Cooperative Extension
Service, David S. Ross, Extension Agricultural Engineer, Department
of Agricultural Engineering
Information provided by West Virginia Agriculture Extension http://www.wvu.edu
