Greenhouse designs vary widely with the type of greenhouse being the principal decision facing growers. Two principal types of greenhouse coverings are the air-inflated, double-layered polyethylene, quonset-styled greenhouse (polyhouse) and the traditional glass greenhouse (glasshouse). There are advantages and disadvantages to both types. A key concern for all greenhouse operators is correct engineering for the location and climate.
Polyethylene. The majority of greenhouses are polyhouses. The principal advantage of the polyhouse is that it is significantly less expensive to install than a glasshouse. For this reason, it is often the choice of those entering the greenhouse industry. Another advantage of the polyhouse is that the two layers of polyethylene make it less expensive to heat than a glasshouse. The disadvantages of a polyhouse are reduced light conditions in winter and increased humidity compared to a glasshouse.
Glass. The principal advantages of the glasshouse are the high level of light transmitted through the glass and its long life. The permanency or rigidity of a glasshouse provides excellent crop protection in adverse weather. For instance, extremely wet snow may cause a polyhouse to collapse. Under similar conditions, a properly designed glasshouse may lose some panes, but would not collapse.
Costs of Greenhouse Construction
Table I shows the estimated cost of constructing a 28-foot by 95-foot, double-layered polyhouse (2,660 square feet). The design provides 1,700 square feet of bench space, utilizing 64 percent of the covered area. By producing crops on the ground, growers can increase the space utilization to 80 percent or more. Completely installed in 1990, the cost of this greenhouse with benches would have been $20,120 or $7.56 per square foot of total ground space. Although many growers start with one greenhouse, for the business to be economically viable a grower generally needs more than 10,000 square feet of greenhouse space.
Greenhouse construction can cost less than $20,120. Growers can decrease the costs of constructing the polyhouse by up to 57 percent by erecting the greenhouse themselves, substituting polyethylene for clear plastic in the end gables, connecting to an existing water supply, and eliminating benches, cooling pads and power vents for summer production. The prices in this example are estimates.
Growers who decide to cut expenses with used materials or to build a greenhouse themselves should be careful to weigh structural soundness along with cost. The following section details some of the characteristics that growers should look for in greenhouse components.
Frame. The principal consideration in selecting the frame is its load-bearing requirement. Each manufacturer will specify the appropriate bow spacing depending on the strength of the tubing. The closer the bow spacing of a given stub strength, the greater the strength of the structure, and the greater the expense.
Glazing. The particular polyethylene described in this example has a 3-year service life. To provide maximum energy savings, the two layers must be separated with an air blower. Polyethylene covering materials are rated with a 1-, 2-, or 3-year service life. Special additives can be incorporated into the plastic to reduce heat loss. Growers normally use a combination of 4- and 6-mil plastic to cover the roof.
End walls. The greenhouse in this example utilizes polycarbonate structural sheets and aluminum doors for the end gables. These materials provide more protection against wind and air infiltration and provide better insulation of the house than glass. However, single polyethylene can be substituted for the polycarbonate, and plywood doors can be constructed for approximately $250. This design change could reduce the total cost per square foot of the house by $0.67.
Cooling and ventilation systems. Greenhouses need exhaust fans to exchange inside and outside air and to equalize temperatures within the house. If the house is not used for summer production, the vent and cooling pad can be eliminated, and a motorized inlet vent installed, at an approximate cost of $195. This design change would reduce the total cost per square foot of the house by $0.77.
Floor. In the greenhouse in this example, heavy plastic is spread on the floor to provide a weed barrier. Gravel is laid on the plastic in the aisles. Growers, however, may choose from a range of floor types--from bare ground to concrete--depending on the intensity of use and availability of capital.
Installation of power and utility sources. Well-drilling costs depend on the flow rate desired and the depth of the well. Local well drillers have experience on expected flow rates and well depths in the area. The estimated cost of a pressure tank and pressure switch for irrigation is $200 (Table I). Many gas companies provide the above ground liquid propane storage tank at no charge; however, the customer must pay about $200 for the hookup.
Benches. Although plants can be gown on the ground, accurate temperature control is difficult to achieve. Benches, therefore, are recommended. There are a variety of bench styles available. Those in this example have a wooden frame and a wire mesh base. They are a relatively inexpensive style of bench. In this example, four rows of benches run the length of the house and stop four feet short of the end gable. Two 4-foot wide benches are placed next to the side of the greenhouse and the middle two are 6-foot wide benches. The middle aisle is four feet wide and the two side aisles are two feet wide. The benches sit on two 16-inch cinder blocks and a 16-inch length of a 2-inch by 4-inch board. The bottom cinder block lies horizontally on the floor. The second block sits vertically on the bottom block, and the wood lies flat across the top block upon which the bench rests. These legs are placed 4 feet on center along each edge of a bench.