The greenhouse covering, or glazing as it is referred to in the industry, represents the greatest decision in selecting the design for the greenhouse. There are a number of different materials to choose from, each with their own inherent advantages and disadvantages. The glazing will most drastically affect the amount and type of sunlight to reach a crop. The glazing will also determine heat loss of the structure. Of the various glazings available the most common include polyethylene films, polycarbonate structured sheets, and glass panels. It’s advisable to review all of the different choices before selecting one for a new enterprise.
Light Transmittal Properties
Greenhouse glazing protects plants from excess cold, hail, and rain. In doing so however it still has to allow sunlight to pass through for photosynthesis. Light transmittal is the amount of light that passes through the covering. A percentage of the sun ray’s will be reflected from the covering, some with be absorbed, and the remainder will be transmitted through. All coverings are compared to glass which transmits 90% of the photosynthetically active radiation (PAR; in the visible spectrum).
Polyethylene Films
According to the National Greenhouse Manufacturers Association (NGMA) poly film is still the first choice for glazing, primarily because of its low cost (1). Air-inflated double-polyethylene film greenhouses can represent up to 80% of new greenhouse construction the United States (2). Growers often start out with poly coverings and latter move up to more rigid glazings. Poly film is porous to both carbon dioxide and oxygen, thus allowing for plants to survive within a tightly sealed structure such as a greenhouse. It typically ranges in thickness from 3–8 mils for greenhouse applications (1 mil is 0.001″). Normal installation involves two layers (4–6 mil for the outer, and 2–4 mil for the inner), which provides a total thickness of 6–10 mil (3) Air is blown between the layers to inflate them to provide air for insulation. Polyethylene film is available with life expectancies of one to four years, depending whether it has been treated with UV inhibitors. With a UV stabilizer the plastic will last longer before it yellows and cracks. Poly film can also be ordered with an anti-drip coating, which will reduce condensation damage on the plants below.
In terms of heat conservation, a double layer of poly will reduce heat loss during the winter by 30–40% in comparison to glass, thus lowering fuel expenses. As for light transmittance a double poly glazing will reduce light transmittance by 15%. Inside a poly covered greenhouse the sun’s rays will be scattered and thus can be received by the plants from all directions. With this advantage of diffuse light growers can hang flowering baskets over greenhouse benches without the lower tier of plants being heavily shaded.
Polyethylene film is very light in weight. Thus the film does not require a structural support system. The poly covering simply needs to be stretched to all 4 edges of the greenhouse structure and securely attached. However due to its tight seal, growers will have to watch that relative humidity levels do not build up to the point that the plants begin to suffer from plant diseases. The ease of replacement of poly glazing is an important factor in reducing the man-power requirements for greenhouse maintenance. It is estimated that 8 workers can cover 1 acre of gutter-connected greenhouses with double film poly tubes (both inner and outer layer tied together).
Poly glazing is considered to have a moderate resistance to hail damage (4) If it does tear small holes can be filled with silicone sealants. Larger gashes will require the use of special greenhouse tape, liberally sealed with silicone. In terms of flammability, polyethylene is considered only moderately susceptible. Lower flammability rates can help reduce insurance premiums. However, due to its shorter life span the fire rating is often over-looked.
Glass
Glass glazing was commonly used up until the 1950’s when other coverings (poly, fiberglass) were developed. Originally glass panels were relatively small: 16″ by 18″. Today tempered glass for greenhouse construction is available in sizes of up to 39″ by 65″. The larger size panels are actually less fragile. If the aim is to build a long-lived structure that is visually appealing, especially to the public, glass may be the solution. In all of the choices for glazing glass still has the highest life expectancy (25+ years). As glass is inherently resistant to ultraviolet radiation, it does not degrade over time and only needs to be replaced due to loss caused by cracking and chipping damage. The support structure required for glass still surpasses that required for other glazing materials. Safety glass is available, and municipalities often have regulations for the type of glass permitted in roof in roof applications. As for flammability, obviously glass will not burn.
The percentage of direct radiation transmitted though glass is the highest of all greenhouse glazing options. Its high clarity, though, does lead to shadowing within a structure, as well as an amplified heat loss. Glass is often used in propagation houses where high levels of light are required from plant growth. In retail garden centers the heat load on shoppers beneath can become uncomfortable, thus requiring the use of shade panels on the hottest days.
In order to reduce heat loss inherent to glass during the winter months, themopane glass can be used. When the 2 panels are separated by a ¼” space of trapped air the total insulation value of the house is increased significantly. In Europe glass greenhouses are quite common in both Holland and Britain, especially where sites are windy, but snow is uncommon.
Rigid Plastic Glazings
Rigid plastic coverings include corrugated and flat fiberglass (FRP), corrugated and structured polycarbonates, and structured acrylics. Glass fiber-reinforced (FRP) panels were first introduced in 1947 for greenhouse glazing (4) Like poly film, FRP panels can scatter sunlight throughout the greenhouse. Light transmittal through FRP is considered very good (80-90% of clear glass). Most panel manufacturers warrant their product for 10 years. FRP received poor ratings during its early life as it tended to turn yellow and later darken to tan and brown. More recently a clear PVF film known as Tedlar has been developed which can preserve the light transmittance of FRP.
The biggest advantage of FRP is its impact resistance, without shattering. FRP panels can with-stand severe hail impact without breaking like glass or other rigid plastic panels. It is often used in greenhouse end walls because of its superior strength.
Structured Polycarbonates
These semi-rigid sheets are available in several standard sizes and configurations. The two primary products are acrylic (polymethyl methacrylate) and polycarbonate. Both of these come in two primary configurations: single layer and multi-layer sheets. The layer configurations refer to the number of walls separated by layers of air. Greenhouse applications generally see single layer (no air layer), double layer (two walls separated by air), or triple layer (three walls and two air separations). Architectural applications have a variety of other configurations available.
The air between the sheet layers helps reduce heat loss. However, as the number of sheet layers increases the amount of transmitted light through the panels’ decreases. The grower must decide which is more important – light transmission or insulation.
An advantage to polycarbonate structured sheets is their 10-year life span, which most polycarbonate manufacturers guarantee. Polycarbonate’s longer life expectance will result in a higher price per square foot than polyethylene film. Light transmitted through polycarbonate, when compared to polyethylene film, has a higher percentage of direct radiation vs. diffused radiation. Polycarbonate panels have low flammability rates, and very high impact resistance.
Acrylic panels are often bonded together to form a sandwich construction 0.6″ apart. With this high energy-efficient twin-wall design, panels can be purchased 4′ wide by as long as 12′ long. They have excellent clarity and light transmission, they are flame retardant, possess high impact resistance, they are UV stabilized, and have a textured surface which diffuses light thus preventing condensation drip. In terms appearance they are considered highly attractive. In retail garden centers they can be tinted solar bronze and shades of translucent white, which greatly helps to reduce the heat-load beneath them making for a much more enjoyable shopping experience (5)
Because the sheets are rigid and come in specific widths and lengths, installation on a greenhouse requires more components than polyethylene film. The sheets need to be supported horizontally from underneath at intervals that are specified by the polycarbonate manufacturers. Extrusions and point fasteners are required to attach the sheets to the support structure. Depending upon the layer configuration, splices are used between the sheets to form a continuous covering for the roof and / or walls. Expansion and contraction has to be considered as well.
References
- Counting on key components. 2001. Matt Hopkins, managing editor for Greenhouse Grower magazine.
- Greenhouse Covering Systems. Gene Giacomelli and William Roberts, Rutgers University Cook College, New Brunswick, New Jersey.
- The Commercial Greenhouse. 1998. James Boodley, Delmar Publishers, Albany, New York.
- The Greenhouse and Nursery Handbook: A Complete Guide to Growing and Selling Ornamental Plants. 2000. Francis Jozwik.
- Starting a Greenhouse Business: A Commercial Growers Guide. 1994. Alan Stevens. Kansas State University.
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