Greenhouse Glazing
Plastic Films
Flexible plastic films, including polyethylene, ethylene tetrafluoroethylene (Tefzel), polyvinyl chloride (PVC), and polyester, have been used for greenhouse coverings (See Figure 2.1). Plastic film is currently the leading greenhouse covering for two reasons. First, film plastic greenhouses with permanent metal frames cost less than glass greenhouses.
Polyethylene Film
Polyethylene, sometimes also known as polythene or poly, has always been and still is the principal choice of film plastic for greenhouses in most of the world (Figure 2.1). The major advantage of polyethylene film plastics is cost, which are considerably less expensive to purchase and install than glass. Plastic glazing generally provides more diffuse light than glass. Diffuse light can potentially reach deeper into the plant canopy than direct light, since diffuse light is scattered more uniformly throughout the greenhouse.
Multi-Layered Co-Extrusion Film
Most greenhouse polyethylene film is manufactured as a coextrusion of three or more layers with different polymers and additives. Multi-layered co-extrusion enables the desired properties of different polymers to be combined and can be used to compensate for their negative effects. In this way, a final polymer can be made with all the desired properties at an acceptable price for some markets. For example, the co-extrusion of ethylene vinyl acetate (EVA) as a middle layer with polyethylene as the two outside layers can overcome the weaknesses of both materials.
Film Polymers and Additives
Most polyethylene film is manufactured as a coextrusion of three layers with different polymers and additives. Each of them contributes to the quality of the film and enhances its performance.
Tefzel Film
The most recent addition of greenhouse film plastic covering is Tefzel T2 film (ethylene tetrafluoroethylene). The light transmission is 95 percent and is greater than that of any other greenhouse covering material. A double layer has a light transmission of 90 percent.
Polyvinyl Chloride Film
Polyvinyl chloride has a number of properties that make it desirable as a covering. PVC has excellent resistance to wear. Oxidation has little effect on PVC, but heat and light break down PVC film in 2 to 3 years. PVC film reduces the transmission of long wavelength infrared radiation, resulting in less heat loss at night using PVC as a covering than using polyethylene.
Polyester Film
Although the cost of polyester film is higher than that of polyethylene film, polyester film is known for its durability and longer life expectancy. Other advantages include a level of light transmittance equal to that of glass and freedom from static electrical charges, which collect dust.
Double-Layering
Single-layer polyethylene-covered greenhouses lose more heat at night or in winter than a glass-covered house since polyethylene allows passage of heat energy from the soil and plants inside the greenhouse much more readily than glass. However, double layer poly-covered greenhouses retain more heat than glass because the greenhouses are more airtight and less infrared radiation escapes. Double-layering consists of two layers of film plastic with air inflation between the layers to provide added insulation. The layers are kept about 3 to 6 inches (7.5–15.2cm) apart by a small fan that pumps air between the layers, sustaining a slight positive pressure (Figure 2.2). Normal installation involves two layers (4–6 mils for the outer, and 2–4 mils for the inner), which provides a total thickness of 6 to 10 mils.
Life Span of Plastic Film
The greenhouse cover is likely to last longer in mild climates than in climates with extreme cold, heat, rain, snow, or hail. Heavy precipitation is more likely to weigh your greenhouse cover down, causing it to stretch. Extreme changes in temperature can cause your plastic to shrink and grow. Climates with heavy dirt and dust can cause your greenhouse to accumulate dirt, and areas with heavy trees or jutting branches pose a risk to your greenhouse. While high winds can cause scratches from blowing debris. Understanding how your greenhouse frame materials will interact with the greenhouse plastic sheeting can help you extend the life of your cover.
Poly Locking System
The poly locking system for the greenhouse is a component that must be thoroughly researched before installation (Figure 2.3). Poly films are fastened to the house at the edges by special poly locking extrusions. Many brands of extrusions for locking plastic films are available. Most rely on friction via a clamping mechanism to hold the edges of the poly in place.
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