Section 2: Common Greenhouse Glazing Materials
Glass
Many types of glass are available including floated glass, insulated glass, low-iron glass and safety glass. Different thicknesses are also available. Typically, standard single layer glass used for greenhouses has a light transmittance of 88% to 94% when used as a single layer and about 77% as a double layer. Double strength glass has a light transmittance of approximately 88% and insulated glass as transmittance of approximately 78%. Low-iron glass will have the highest light transmittance levels. Glass-glazed greenhouses have relatively high air infiltration rates due to spaces between glass panels. Therefore, glass tends to have a higher thermal conductance than many other glazings. Also, because of the higher air exchange rate, glass glazed greenhouses typically have lower relative humidity levels than greenhouses glazed with many other types of glazings (depending on how well mounted the glazing is and the "tightness" of the greenhouse). Glass is resistant to heat, U.V. light, and abrasion but has a relatively low impact resistance. Glass is expensive to purchase and install and requires special supports to hold the glass panels in place and support their weight. However, glass has a long life span, usually exceeding 25 years. Most commercial greenhouses no longer use glass as a glazing because of the high weight and cost. However, safety glass is often used in botanical centers and conservatories.
Polyethylene film
Polyethylene film is a common greenhouse glazing that is particularly adaptable to quonset structures because of its flexibility. It is low in cost, light-weight, and easy to install. Typically standard polyethylene film has a light transmittance of 85% to 87% for a single layer of film and 74% to 77% for a double layer. Thermal conductance of polyethylene film is relatively high but varies between specific brands of polyethylene (i.e. Tufflite, Standard UV, Tufflite Dripless, Fog Bloc, Sun Saver, Dura-Therm ) and single versus double layers.
Additives may be included in the film to increase life-span, reduce condensation or reduce heat loss. These additives may be sprayed on or included in the film through a process known as coextrusion. During the process of coextrusion, three layers of polyethylene are laid down to form a single sheet of polyethylene film. Each layer may have materials included that alter the properties of the film.
Polyethylene is short-lived in comparison to other glazings. Without additives, polyethylene will last only two to three years before needing to be replaced. This is because it is very susceptible to degradation by U.V. light. However, if additives are included during the coextrusion process that make the material more resistant to U.V. light, polyethylene glazings may have a life span of three to five years.
As mentioned above, polyethylene film has a high thermal conductance. However, some brands of polyethylene films have an I.R. (infrared) inhibitor added to the inside layer of the film which reduces heat loss through the glazing.
Another problem with polyethylene glazing is that of condensation and dripping. Because of the difference between inside and outside air temperatures, water vapor tends to condense on the surface of polyethylene film inside of the greenhouse. Because the film is very hydrophobic, the water tends to bead and collect on the surface until large enough drops are formed that then fall from the glazing onto the plant materials below. This dripping of water from the glazing onto the plants can result in increased disease incidence. An additive may be sprayed onto the film (i.e. SunClear) or incorporated into the film that essentially acts as a wetting agent. This prevents the beading of water and allows smaller droplets to form that run down the glazing and to the floor.
Usually a 6 mil (0.15 mm thick) film is used for greenhouses if a single layer is being used. If a double layer is being used, 6 mil is used on the outside and 4 mil (0.10 mm thick) is used on the inside. In a double polyethylene glazing system, a small squirrel cage fan is used to force air between the layers. This provides a "dead" air space that serves as insulation and decreases thermal conductance.
Fiberglass Reinforced Polyester
Fiberglass reinforced polyester (FRP) panels (i.e. Excelite and Lascolite) are relatively strong, light weight, and low in cost. The panels are rigid and usually corrugated. New single panels have a light transmittance of up to 90% while double panels have a light transmittance of 60% to 80%. Panels can be easily attached to metal or wooden frames with screws and rivets. However, FRP is highly susceptible to U.V. degradation. Exposure to U.V. light causes yellowing (after only 1 or 2 years for untreated panels) of the panels and a reduction in the light transmittance. New types of FRP are treated with a U.V. inhibitor to minimize yellowing and increase life span. Whereas traditional panels had a life span of only about 2 to 3 years, treated panels can have a life span of 10 years or longer. Another serious problem with FRP panels was that they were highly flammable. Some new FRP panels are treated with a flame retardant. However, FRP panels are no longer commonly used as a greenhouse glazing for commercial greenhouses but are sometimes used for homeowner or hobby greenhouses.
Acrylic
Acrylic panels may come in various forms. They may be single panels (i.e. Plexiglass) or bi-wall panels (i.e. Exolite). The thickness of the actual material, the thickness of the overall panel (and thus the airspace), and the distance between the flutes (the supporting cross sections within the panels) may all be varied. These changes in the panel affect strength, flexibility, thermal conductance, light transmittance, weight and cost.
Typical acrylic bi-wall panels have a light transmittance of 87% - 93%. Acrylic panels are relatively strong, rigid, and lightweight. Panels are resistant to U.V. degradation and experience little reduction in light transmittance for 10 years, but the typical effective life span of acrylic bi-wall panels is 20 to 25 years. Panels may be treated with materials to make the panels more resistant to U.V. and to reduce condensation inside of the greenhouse. Acrylic panels are easily scratched, are flammable (less so than FRP), and have a high degree of thermal expansion and contraction, and therefore, require special anchors on the greenhouse frame. The initial cost of the panels is high compared to polyethylene and FRP but the high light transmittance and long life span make them a popular choice as greenhouse glazings.
Polycarbonate
Just as for acrylic panels, polycarbonate panels may come in various forms. They may be single panels (i.e. Dynaglass, Lexan Corrugated and Macrolux Corrugated), bi-wall panels (i.e. Macrolux, Polygal, Lexan Dripgard and Lexan Thermoclear), tri-wall panels and panels with crisscrossed supports. Single panel and bi-wall panels are most commonly used as greenhouse glazings. The thickness of the actual material, the thickness of the overall panel (and thus the airspace), and the distance between the flutes (the supporting cross sections within the panels) may all be varied. These changes in the panel affect strength, flexibility, thermal conductance, light transmittance, weight and cost.
Typical polycarbonate bi-wall panels have a light transmittance of 83% whereas single-wall panels have a light transmittance of about 94%. Panels are relatively strong, rigid, and lightweight. Panels are resistant to U.V. degradation and experience little reduction in light transmittance for 10 years. Panels may be treated with materials to make the panels more resistant to U.V. and to reduce condensation inside of the greenhouse and thus the typical effective life span of polycarbonate bi-wall panels is 20 to 25 years. Polycarbonate panels are easily scratched but are far less flammable than acrylic or FRP. As with acrylic, polycarbonate panels have a high degree of thermal expansion and contraction and therefore required special anchors on the greenhouse frame.
