Greenhouse Glazing
Rigid Plastics
Rigid plastic coverings include fiberglass-reinforced plastic (FRP) rigid panel, polycarbonates, and acrylics. Light transmission through rigid plastics is very good, although it usually decreases over time as the plastics age and turn yellow due to the amount of UV radiation contained in sunlight. The large sheets are much lighter than glass and require fewer support bars to attach them to the greenhouse frame. However, these rigid panels are not so easy to install on curved roofs.
Fiberglass-Reinforced Plastic Rigid Panel
Fiberglass-reinforced plastic (FRP) rigid panel has been used in greenhouse coverings since the 1950s but its popularity has declined in recent years. Fiberglass is available in flat and corrugated configurations (Figure 2.4). Corrugated panels are commonly used for greenhouse roofs, as its corrugated shape lends strength and rigidity to the panels. Flat panels are usually used for sidewalls, windows, and vents. Although FRP panels are classified as a rigid plastic, they are flexible enough to be bent in a curve to fit the framework of a Quonset type or arch type greenhouse.
Polycarbonate
Polycarbonate is one of the most widely used structured sheet material in greenhouses today (Figures 2.5 and 2.6). Polycarbonate products provide up to 85 percent light transmission with 100 percent light diffusion to eliminate hot spots and improve yield. Light transmitted through polycarbonate, when compared to polyethylene film, has a higher percentage of direct radiation versus diffused radiation. Like acrylic in heat retention properties, it allows about 90 percent of the light transmission of glass. Polycarbonate has slightly less light transmission compared to acrylic, but is considerably stronger and impact resistant, more flexible, and only flammable when an active flame is maintained in contact with the material. Although the initial cost of polycarbonate is high, a 10- to 15-year life span can be expected (depending on the manufacturer). It’s incredibly strong, with 200 times greater impact resistance than glass, 20 times greater impact resistance than fiberglass, and 10 times the strength of acrylic, yet its weight is just one-eighth that of glass.
Acrylic
Acrylic has been used for many years and is the most suitable rigid transparent plastic for greenhouse glazing (Figure 2.7). Acrylic has excellent clarity and light transmission (up to 90%), they are flame retardant, possess high impact resistance, they are UV stabilized, and have a textured surface which diffuses light thus preventing condensation drip. Light transmission and heat loss are equivalent to double-layer polycarbonate. It is lighter and tougher than glass and has excellent outdoor weathering properties.
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