The University of Georgia
Cooperative Extension Service
College of Agricultural and Environmental Sciences
Transplant Growing Facilities
Paul E. Sumner
Extension Engineer
General
Greenhouses and plant growing facilities include three main categories of components:
1. Structures
2. Coverings
3. Environment
Certain data and information concerning these three categories are summarized. Several
Extension publications and blueprints are available giving more comprehensive information on
these topics.
STRUCTURES
Location and Orientation
Proper greenhouse site selection, location, and orientation are very important for profitable
commercial production or satisfactory hobby or part-time operations. The following factors should
be considered:
- Orient and locate the house for maximum sunlight. In southern latitudes, the ridge should run
north-south and northern latitudes east-west.
- Avoid placing the house near objects east, west or south which will shade the house. Keep away
a distance equal to at least three times the height of the object.
- Place in an area sheltered from northerly and north-westerly high winds if possible.
- Locate on a soil which is well drained and where surface water does not run into the house. Use
drain tile if necessary.
- Avoid sloping beds or floors in the greenhouse.
- Locate the greenhouse near adequate and reliable sources of utilities -- electricity, water and gas.
- Provide good access roads, parking, and turn-around area.
- Position headhouses or supporting facilities on the north side.
- Arrange initial construction so that the range can be expanded.
Uses and Types
The basic purpose of a greenhouse structure is to provide a reliable enclosure within which an
environment favorable to plant growth and production can be created. This section summarizes
some of the main features and data on Greenhouse Structures.
There are several types of greenhouse structures and methods of construction with special
characteristics that are widely adaptable or sometimes suited only for certain uses. Most all houses
should have the following basic features.
BASIC FEATURES DESIRED
Strength
- To be structurally adequate and resist local wind and snow loads.
- Usually recommend design for 70 mph winds and 12 p.s.f. snow load.
- Adequate foundation to support the structure.
Covering
- Easy covering attachment and proper support, sometimes with double-layer covering for heating
economy.
Light
- Minimum overhead structural members, or size of members to minimize shading.
Openness
- Clear-span design desirable for easy working and easy equipment or operating maneuverability.
- Wide doors and height as necessary for equipment and crop operations.
Environment
- Capable of utilizing efficient and economical heating and ventilation systems.
Cost/Life
- A cost/life ratio that is most suitable for your needs. Maintenance should be considered.
Insurance and Taxes
- These values vary depending on temporary or permanent structures and flammable or nonflammable materials.
Wood Types, Life, and Preservative Treatment
Wood is commonly used in construction of benches, flats, doors and structural frames for
greenhouses. Since its life and durability are important for long-term usage, proper species and/or
treatment can be most valuable in initial construction. The following information should be
considered.
TABLE 1. TYPICAL LIFE OF UNTREATED HEARTWOOD IN GROUND CONTACT
Type____________________________________________________________________
Black Locust
Osage-Orange
RY RESISTANT (Over 15 Years Average Life)
Red Cedar
Redwood
Cypress
White Oak
RESISTANT (7 - 15 Years Average Life)
Ash
Beech
Birch
Cottonwood
Douglas Fir
Hemlock
SLIGHTLY RESISTANT (2 - 7 Years Average Life)
Hickory
Larch
Maple
Red Oak
Pine
Spruce
Yellow Poplar
Proper preservative treated wood has a much longer life than most untreated species and does not
interfere with normal construction, painting, or management procedures. The oil borne
preservatives such as creosote and penta are not recommended due to toxicity to plants and
covering materials. Instead, the water-borne salt-type preservatives, which are just as effective,
readily available, and suitable for greenhouses should be used.
Brush-on treatment is rather ineffective for long-term protection and not recommended. Proper
"pressure" treatment by a commercial company is recommended. Expected life of pressure treated
wood is 20 to 40 years. Southern Yellow Pine and Douglas Fir are most generally treated but other
softwood species and some hardwood can be equally treated.
GREENHOUSE COVERINGS
One of the most important parts of a greenhouse facility is the covering. Since sunlight is generally
the limiting factor in wintertime greenhouse production, a covering that transmits maximum sunlight
in the plant growth spectrum is essential. Physical durability and optical stability are other critical
factors.
Several types of covering materials are presently available. Which one is best, or most economical
over the long term, is not easy to state. Glass has been the long-time standard and is still the most
stable but other film and rigid plastic materials are offering lower cost coverings but with varying
levels of dependability and life.
TABLE 4. TYPES OF COVERING MATERIALS
Types
Films
Rigid Plastics
Name
Polyethylene, UV
Acrylic/Polyester
Polyester
"Fiberglass"
Acrylic
Polycarbonate
Glass
Description
4 and 6 mil
"Flexigard"
"Mylar" or equal Weatherable
Fiber reinforced Polyester
"Plexiglass" or equal
"Lexan" or equal
Regular
Double Strength
Low Iron
General Characteristics and Performance of Coverings for Greenhouse Use
Polyethylene (Regular and U.V.)
1. Lowest cost covering
2. Widely available, some manufacturers report recently they have stopped production of this
product for
greenhouse use. (Thus, be cautious of buying any product of unknown quality
for greenhouse use.)
3. Relatively short life in sun: 9 to 11 months for regular, one to three years for U.V.
4. Splits more easily at the folds. Should use unfolded or lay-flat rolls for maximum life.
5. Transmits approximately 85 to 88 percent for solar energy available at the earth's surface.
6. Transmits all wavelengths of action spectra required for plant growth.
7. Transmits the wavelengths of thermal radiation which allows the house to cool more rapidly at
night.
8. The strength of new 4 and 6 mil film is one to two times that of 1/8 inch standard glass.
9. Permits double-layer covering which results in 35 to 40 percent reduction on heat loss,
reduced
condensation, and only 8 to 10 percent reduction in light due to the second
(if clean) layer.
10. Provides a "tighter" house with less air leakage which causes somewhat higher inside humidity
conditions.
11. Film most useful for low cost temporary or seasonal coverings.
12. Polyethylene film reinforced with synthetic fibers is also available at a cost four to five times
that of regular film but generally this material is not used for greenhouses.
13. Double-layer covering on top side of structure with centrifugal fan developing pressure
between the two
layers is a way to reduce labor and installation costs. Life equal or better than
conventional installation
methods.
Acrylic/Polyester
1. Combine weatherability of acrylic with high service temperature of polyester.
2. Good transmissivity.
3. Nonreversible, acrylic must be installed to the outside.
4. Susceptible to wind flapping.
5. Estimated life 10 years plus.
Polyester
1. Excellent transmissivity.
2. High service temperature.
3. Low impact resistance.
4. U.V. degradable unless treated.
5. Estimated life 7 to 10 years.
Polyvinylflouride
1. The Tedlar® film has proven to have excellent weatherability but is too costly to compete with
existing films as a covering. It is now being used as a surface coating which is molecularly bonded
to fiberglass panels to improve their weatherability.
Plexiglass®
1. An acrylic plastic which has been available for many years but has not been widely used as a
greenhouse covering due to high cost except for special Climatic or Conservatory type facilities.
2. It is more resistant to impact than glass.
3. Transmits approximately 90 to 92 percent of available sunlight and is available in UV
transmitting and absorbing types.
4. Has long life and weathering resistance comparable to glass.
5. Softer than glass, it is easily scratched and is sensitive to some solvents.
6. Cost appreciably more than glass and other possible covering materials.
7. Flexible enough to be used as curved panels in glasshouses.
8. Strong enough to resist snow and ice loads near gutters of connected houses.
9. Expands and contracts greatly with temperature changes and should not be directly nailed or
screwed down, but held under a cover strip with soft mastic sealer to allow movement.
Fiberglass Reinforced Rigid Plastics (FRP)
1. Many brands of the basic polyester resin reinforced with fiber glass are available in flat and
corrugated forms. Corrugated form adds strength.
2. Made in "weights" from 4 to 8 ounces per square foot, widths up to 51 1/2 inches (48 inch
coverage ) and lengths pre-cut up to 30 or more feet (special order). Use minimum number of
joints and laps to reduce chances of dust and dirt accumulation between panels and also air/water
leakage. Use proper clear sealer on laps for tightness.
3. Two to four times more resistant to impact and lateral loading than glass. Crazing (not
shattering) usually results from impact, but this crazing has no harmful effect unless the panel
surface is cracked or broken.
4. The polyester of the panels burns freely and rapidly; entire houses have burned in
approximately 10 minutes. Flame retardants and good weatherability have not been successfully
used together. Insurance on fiberglass is not easily obtainable.
5. Clear or "frosted" panels of greenhouse quality material transmits approximately 78 to 90
percent of available light when new. Non-greenhouse formulations, especially colored panels,
should be avoided.
6. Panels with 15 percent acrylic additive have proven more durable than straight polyester
formulations.
7. Acrylic modified polyester panels need cleaning at least annually, and generally re-surfacing
with an acrylic liquid sealer every 4 to 5 years to restore weathered surfaces to near-new
transmission and surface condition (except Tedlar Coated). The durability of the sealer coat is
questionable and undergoing more study at present.
8. Some manufacturer's guarantees are rather nebulous. Until accurate evaluation procedures and
quality standards are established, judge a product more on its performance and company
reputation rather than the "guarantee".
9. Proper attachment to the structure and sealing/fastening of lapped joints is essential for
resistance to wind forces. (Use fasteners every 8 to 12 inches on ends and sides, or per
manufacturers specifications.)
Regular Glass
1. Single strength and small panes not used much on newer designs and constructions.
Replacement of panes in existing houses could be double-strength for more resistance to
breakage.
Tempered Glass
1. Two or three times stronger than regular glass.
2. Frosted or "hammered" types available for better light diffusion, reduced shadow, and non-seethrough properties.
3. Larger pane sizes for reduced structural members, hence less shadows.
4. Requires special structural members and glazing methods to give water and air-tight
construction.
ENVIRONMENT
The successful operation of any greenhouse requires the maintenance of an inside temperature
near the optimum level for plant growth. The exact inside temperature to be maintained will depend
on the crop being grown. Generally, designs are for 65oF inside capability with thermostatic
adjustment for exact conditions per horticultural recommendations. Other climatic factors include
relative humidity, air movement, and carbon dioxide. Temperature and relative humidity are
normally controlled by the heating and ventilation equipment. Continuous air circulation, especially
in the wintertime, is important for distribution of heat and uniformity of inside conditions by
preventing air stagnation and stratification. Carbon dioxide enrichment of the air for greater plant
growth and production is sometimes profitable.
Heating
Heat Requirements and Fuel Costs
Factors primarily affecting the heat requirements of a greenhouse are:
1. The external environmental condition
2. The size of the greenhouse.
3. The structural nature of the greenhouse.
4. The number of layers of glazing material used to cover the house.
The following permit determination of heating requirements and annual fuel costs for a particular
greenhouse type, size, and single or double-layer covering.
Heat Requirement Calculations for Greenhouses
It is useful to know some heat loss calculation procedures to predict heating loads and identify
areas of the greenhouse with the most heat loss. Heat loss by conduction may be calculated with
the following equation.
Q = U A (Ti - To)
where
Q = heat transfer rate, BTU/hr.
U = heat transfer coefficient, BTU/hr.-ft.2 oF
A = surface area, ft.2
Ti-To = air temperature difference between inside and outside, oF.
Sometimes "R" values (the resistance to heat flow) are listed instead of "U" values. The relation
between "U" and "R" is:
U=1
R
The conduction heat transfer equation using "R" can then be written as:
Q = A(Ti-To)
R
Frequently, it is more convenient to work with "R" values when dealing with insulation, as the
added effect of insulation can be determined quickly by simply summing the "R" values of materials
in the heat flow path. For example, from Table 5., the "R" value for a single layer of glass is
equivalent to 0.88 and for 1-inch-thick styrofoam, 4.00. Adding styrofoam to a single-layer glass
surface will give the wall an insulation value of R = 0.88 + 4.00 = 4.88 (or a "U" value of 1/4.88 =
0.204). Note that high "R" values and low "U" values indicate less heat flow.
TABLE 5. HEAT TRANSFER COEFFICIENTS FOR CONSTRUCTION MATERIALS
R Value**
Materials (hr.oF sq.ft./BTU)
Glass, single layer
0.88*
Glass, double layer, 1/4 in. space
1.54*
Glass, triple layer, 1/4 in. space
2.13*
Clear polyethylene film, single layer (2, 4, or 6 mil)
0.87*
Clear polyethylene film, double layer, separated(2, 4, or 6 mil)
1.43*
Polyethylene film, double layer, separated over glass
2.00*
Fiberglass
1.00*
Double acrylic (Acrylite SDPTM)
1.78*
Double polycarbonate (Tuffak-TwinwallTM)
1.61*
Face Brick, 4 in. thick
0.44
Concrete block, 8 inch
1.96
Concrete block, 8 inch plus l inch foamed urethane
7.69
Concrete block, 8 inch plus 1 inch foamed polystyrene
5.55
Concrete, poured, 6 inch
1.33
Cement asbestos board, 1/4 inch
0.91
Cement asbestos board, 1/4 inch plus l inch foamed urethane
7.14
Cement asbestos board, 1/4 inch plus l inch foamed polystyrene
4.76
TM
Microfoam 1/4 in. thick
1.08
Polystyrene (beadboard or loose fill), 1/2 in. thick
2.10
Polystyrene (beadboard or loose fill), 3/4 in. thick
3.05
Polystyrene (beadboard or loose fill), l inch thick
4.00
Extruded polystyrene (Styrofoam) 1 inch thick
5.40
Polyurethane foam (applied at site), 1 inch thick
7.30
Plywood 1/2 inch
0.62
Plywood 1 inch
1.25
1 inch nominal softwood
1.79
Expanded vermiculite (6-6 lb./cu.ft., 1 inch thick)
2.20
Curtain Materials
Al/TempTM, aluminum down
aluminum up
Al/BlacTM
Duracote #2425 (FoylonTM)
Black Sateen
Black poly, 6 mil
1.43
1.18
1.37
2.63
1.54
1.05
ReemayTM, spunbound polyester, 2016
0.83
Vinyl (aluminized polyester laminated vinyl) 4.5 mil
2.15
** The R value represents the resistance to heat flow at the thickness listed. The higher the R
value the better the insulating property.
* Includes effects of surface coefficients.
Acrylite S.D.P., TM CY/RO Industries Microfoam, TM DuPont
Al/Blac, TM Simtrac, Inc. Reemay, TM DuPont
Al/Temp, TM Simtrac, Inc. Styrofoam, TM Dow Chemical
Foylon, TM Duracote Corp Tuffak-Twinwall, TM Rohm and Haas Co.
Infiltration heat loss can be significant and should be calculated and added to conduction heat
losses. The equation for infiltration heat transfer is:
Q = 0.02 x Vol x NC x (Ti-To)
where: Q = heat transfer rate, Btu/hour
Vol = greenhouse volume, ft3.
NC = number of air exchanges per hour (Table 6).
Ti-To = air temperature difference between inside and outside, oF.
TABLE 6. NATURAL AIR EXCHANGES FOR GREENHOUSES
Construction System
Air Exchanges per Hour*
New Construction, glass or fiberglass
0.75 to 1.5
New Construction, double layer plastic film
0.5 to 1.0
Old Construction, glass, good maintenance
1 to 2
Old Construction, glass, poor condition
2 to 4
* Low wind or protection from wind reduces the air exchange rate
Once dimensions are known and listed, the area and volumes can be calculated with the following
equations for the appropriate greenhouse
style:
Single gable greenhouse (Figure 1.)
Wall area = 2 (F x C)
End area = (2xFxB) + (GxB)
Roof area = 2 (DxC)
Foundation Area = 2 (ExC) + 2(ExB)
Volume = AxBxC + 1/2 (BxG)xC
Gutter-connected gable greenhouses (N = number of greenhouses) (Figure 2)
Wall area = 2 (FxC)
End area = [2 (FxB) + 2(GxB)] x N
2
Roof area = [2 (DxC)] x N
Foundation area = 2 (ExC) + [2 (ExB) x N]
Volume = [AxBxC + 1/2
(BxG) x C] x N
Gutter-connected, curvedroof greenhouses (N = number of greenhouses) (Figure 3)
Wall area = 2(AxC)
End area = [1.3(HxB) + AxB] x N
Foundation area = 2 (ExC) + [2 (ExB) x N]
Roof area = (DxC) x N
Volume = [4/3 (HxBxC) + (AxBxC)] x N
Quonset-style greenhouse (Figure 4)
End area = 4/3 (HxB)
Roof area = DxC
Volume = 4/3 (HxBxC)
Estimating Annual Heat Cost for Greenhouses
It will be useful for the grower to estimate annual heating cost in a typical heating season. Fuel
usage can be determined by the following equation.
Fuel Usage = 100 x A x HD x T
R x HC x EFF
where:
Fuel Usage = Units, gallons, cubic feet
A = Surface area, ft2
HD = Heating degree days per year
T = Number of hours heating per day, hrs.
R = Resistance to heat flow, hr x ft2 x oF/Btu
HC = Heat capacity of fuel used, Btu/units, gallons, cubic feet
EFF = Burning Efficiency, %
Table 7 presents heating degree days for various locations in Georgia at various base
temperatures. Also, Table 8 gives heat content and burning efficiencies for typical heating fuels.
Knowing fuel usage, the cost of heating a greenhouse can now be calculated.
Cost = Fuel Usage x UC
Where UC = fuel unit costs, $/unit
TABLE 7. ANNUAL HEATING DEGREE DAYS AT VARIOUS BASE TEMPERATURES
County
70oF
65oF
60oF
55oF
Bacon
Bibb
Charlton
Chatham
Clay
Fulton
Muscogee
Stephens
Thomas
Tift
Walker
Wilkes
2705
3233
2204
2806
2810
4115
3311
4149
2499
2891
4787
4226
1835
2279
1417
1921
1922
3021
2356
3018
1672
1993
3617
3104
1168
1512
861
1242
1227
2128
1575
2104
1059
1299
2636
2188
693
948
491
753
733
1417
997
1370
625
796
1830
1447
TABLE 8. HEAT CAPACITIES OF VARIOUS FUEL AND BURNING EFFICIENCIES
Fuel
Coal
Natural Gas
LP Gas
Fuel Oil
Electricity
Heat Content
13,000 BTU/LB.
1,000 BTU/Cu.Ft.
92,000 BTU/Gallon
38,000 BTU/Gallon
3,413 BTU/KWH
Burning Efficiency
60%
80%
80%
70%
100%
Heating Equipment and Cooling Systems
The most popular heating equipment now used in many greenhouses are gas-fired unit-heaters or
comparable furnaces. Oil-fired units are sometimes used if fuel prices and availability are more
favorable but maintenance of this equipment is greater than gas. For large greenhouses or
greenhouse ranges of 1/2 acre or more, centrally located hot water or steam systems with gas, oil
or stoker coal fired burners can be feasible.
Air Circulation
Continuous wintertime air circulation is recommended in greenhouses to prevent air stratification
and thereby provide more uniform temperature and humidity control. Definite patterns of air
movement should be provided. Air velocities of 30 to 60 feet per minute within the crop zone are
adequate. An air circulation rate of 30 to 40 percent of the total house volume (cfm) is
recommended. The use of smoke or dust producing devices may be used to determine if
circulation and air movement are occurring throughout a house.
Ventilation
Warm season and summertime ventilation of greenhouses is essential for temperature control.
Modern, thermostatically controlled fans are the heart of present day ventilation systems. Poorly
planned systems or inferior equipment often result in serious crop damage or failures by
inadequate air exchange and high temperatures.
Figure 5 shows the effect of air exchange rate on control of temperature rise within the greenhouse
on sunny days. An air change rate of 3/4 to 1 or 1 l/4 volume per minute is recommended
depending on the type of covering and management used. From the heat requirement section the
volume of the greenhouse can be calculated. Total air flow rate of house fans can be determined.
Figure 5. The influence of air exchange rate on temperature rise in greenhouses.
Evaporative Cooling
Evaporative cooling pads are quite effective and useful in lower humidity climates but their use and
value in Georgia depend on the crop and other factors.
Recommended Greenhouse Heating-Ventilation Equipment Specifications
Ventilation Fans
A.M.C.A. (Air Moving Conditioning Association) rated to provide required CFM at 1/10 to 1/8 inch
static pressure. Totally enclosed motors, with ball bearings, thermally protected, direct or belt-drive,
one or two-speed as required.
NOTE: Non-A.M.C.A. rated fans are not recommended but if used should be over-sized 35 to 50
percent to compensate for reduced air flow under static pressure operating conditions.
Shutters
Aluminum or enamel-painted steel frame, reinforced aluminum vanes tie rod connected, bronze or
nylon pivot bearings, gravity or motorized wall type as required.
Unit Heater
A.G.A. approved vented type with aluminized or stainless steel heat exchanger. Propeller type fan
with ball-bearing or permanently-lubricated continuous-duty sleeve bearing motor, thermal or
impedance protected. Pilot with 100 percent safety shut-off controls, 115/24 V control transformer.
LP or natural gas as required for fuel available. Heater to have Btu/hour output as required.
Heat Thermostat
24 V SPST heat-anticipator or line voltage type, temperature range approximately 50 to 80oF.
Suitable for greenhouse dirt and humidity conditions. Mercury-bulb type to be firmly mounted to
prevent movement and erratic heater operation.
Ventilation Thermostat
Heavy duty, snap action, line voltage type, amperage of Hp rating larger than fan motor, SPST or
two-stage (2 SPDT switches) as required, suitable for greenhouse dirt and humidity conditions.
Evaporative Cooling Pads
Some evaporative cooling systems can cool air to 85 percent of the difference between its original
temperature prior to cooling and the coolest temperature which could be achieved if the air were
cooled to the dew point or to the point of 100 percent relative humidity.
Evaporative coolers are more effective when the humidity is low. Fortunately, relative humidities
are usually low during warm periods. Solar heat entering the house offsets some of the cooling
effect. A well designed ventilation system providing one air exchange per minute is essential for
good evaporative cooling system.
Many pad materials have been successfully used for evaporative cooling. Table 9 gives
recommended air flow through various pad type materials.
TABLE 9. RECOMMENDED AIRFLOW RATE THROUGH VARIOUS PAD MATERIALS
Pad Type
(CFM/ft2)
Airflow Rate Through Pad
Aspen fiber mounted vertically (2-4 in.) thick
Aspen fiber mounted horizontally (2-4 in.) thick
Corrugated cellulose (4 inches) thick
Corrugated cellulose (6 inches) Thick
150
200
250
350
Pads are usually confined in a welded wire mesh. Water running through a pipe with closely
spaced holes allows water to run down a sheet metal spreader onto the pads. The flowrate of the
water supplying header pipe is listed in Table 11. Water that does not evaporate in the air stream
is caught in the gutter and returned to a reservoir for recycling. The reservoir should have the
capacity to hold the water returning from the pad when the system is turned off. Table 10 presents
recommended reservoir capacity for different types pads.
TABLE 10. RECOMMENDED WATER FLOWRATE AND RESERVOIR CAPACITY FOR
VERTICALLY MOUNTED COOLING PAD MATERIALS.
Minimum Flowrate per Length
Pad Type
of Pad (gpm/ft)
Aspen fiber (2-4 inches)
Corrugated Cellulose (4 inches)
Corrugated Cellulose (6 inches )
0.3
0.5
0.8
Minimum Reservoir Capacity
Per Unit Pad Area (Gal/ft2)
0.5
0.8
1.0
Cooperative Extension Service, The University of Georgia College of Agriculture offers educational programs, assistance and materials to all people without
regard to race, color, national origin, age or handicap status.
AN EQUAL OPPORTUNITY EMPLOYER
ENGINEERING
Mis.Pub. No. ENG00-003
Jun, 2000
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agriculture and Environmental
Sciences and the U. S. Department of Agriculture cooperating.
Gale A. Buchanan, Dean and Director