Glass, Acrylic, Fiberglass, Polycarbonate & Film Plastic
What glazing material should I use to cover my greenhouse? First, a material that will let in the maximum amount of light while allowing the least amount of heat to escape. It must be strong enough to withstand hail & heavy snow loads. Besides glass there are a wide range of plastic glazing in the form of film sheeting and rigid panels. Discuss your options with a reputable greenhouse supplier, make sure the plastic is for greenhouse use and UV coated. Lexan is the popular brand name for polycarbonate, Plexiglass is a brand name for acrylics.
Glass: The traditional greenhouse covering, glass is the preferred material for permanence. It lasts indefinitely although it does become brittle with age. Since glass is breakable it is more difficult and dangerous to install. It also requires a much sturdier frame than plastic covered greenhouses. Glass has undergone many improvements in recent years. Among the most important is increased strength (double and triple strength ratings) to resist breaking. Larger panes are also available because fewer structural members are needed the more light can enter the greenhouse. Frosted and hammered panels can be used break up the light rays for more even distribution. Other advances have cut down on heat loss, double-walled tempered glass reduces it by about one third. Low-emissive or low-E coating is another option, it reduces heat loss without a corresponding loss of light. In addition to being energy efficient low-e glass reduces condensation, partially blocks ultraviolet rays and makes the inside glass warmer to the touch.
Film Plastic: Film plastic is one sixth to one tenth of the cost per square foot of a glass covered structure. Although less permanent than glass, film plastic can be heated as efficiently as glass. Previously film plastic had a life span of about three months in summer and nine months in winter. Newer clear types that resist yellowing last three to five years and allow up to 89% light transmission. Sold under many trade names they are available in many thickness from 2 mil to 15 mil. The thicker the film the more expensive it is. Weather resistant polyethylene film plastic 4 mil thick is perhaps the least expensive film plastic that makes a satisfactory covering for a hobby greenhouse. In choosing plastic film make sure you buy one that has UV protection. You can also buy a plastic film with an infrared inhibitor, it cuts heat loss inside the greenhouse by up to 20% on a cloudless night. Another way to reduce heat loss is to double- layer the plastic creating an air pocket for insulation. This space can be 3/4 to 4 inches thick. Although there is some light transmission loss heating costs are reduced by one third. Another innovation in film covering is an anti condensate additive that allows moisture buildup to run down the sides of the film instead of onto the plants. By removing the condensation drops that block the sun's rays, this new type of film plastic allows more light into the greenhouse. In addition, it helps stop disease infection by keeping contaminated moisture from dripping on the plants.
Rigid Plastics: These greenhouse coverings, which include fiberglass, acrylic, and polycarbonate come in corrugated and flat forms. Shatterproof, they resist hailstone damage to varying degrees polycarbonate being the strongest. Some types of rigid plastics get dirty and do not quite let the same amount of light in as glass does. However a good wash with detergent and water to remove dirt or smudges is all that's needed to clean the panels up. All rigid plastics retain heat well, for example, fiberglass retains heat 4.4 x more efficiently than glass and 70.8 x more efficient than polyethylene film. Plastic panels have fewer lap joints through which heat can escape. Corrugation in some types makes a very tight fit at lap joints, thus saving heat. The total amount of light transmitted through fiberglass rigid panels is roughly equal to that transmitted through glass. Fiberglass actually has the advantage over glass because it transmits less heat. During the summer a fiberglass covered greenhouse requires less cooling than a glass greenhouse of the same size. Fiberglass with PVC is a durable, relatively lightweight material that resists damage from weather, ultraviolet radiation, and acid rain. Recent improvements in plastics include the introduction of structured sheets. Available in both acrylic and polycarbonate, these are double skinned (for insulation) or corrugated. Acrylic transmits light better but polycarbonate is stronger and more resistant to impact and fire. Both materials can be used on curved areas. When purchasing polycarbonate panels make sure they are UV coated to guard against premature yellowing.
From left: The Rutherford Family Conservatory at the University of Guelph consists of a glass greenhouse, built in 1930, containing a variety of exotic plants. The building incorporates limestone blocks from the foundation of original barns on campus to accent the various plants. Greenhouse and garden restoration was funded by alumni. Middle: Polyethylene commercial greenhouses. Right: Commercial greenhouse using acrylic panels
Many factors need to be considered when selecting a glazing material or greenhouse covering. The life of the material, its strength, its weight, initial cost, light transmittance, thermal conductance, maintenance issues and flammability are all very important.
Cost: All aspects of cost need to be considered. These include the initial cost of the glazing material, structural support costs, life span of the glazing and thermal conductance of the glazing. A glazing material that has a high initial cost when compared to other glazing materials may be more economically attractive if it has a long lifespan or has a low thermal conductivity.
Life span: A short life span means frequent replacement. Therefore, the initial cost of the glazing may be low as compared to other glazings, but after the glazing is replaced several times, it may become less economically attractive than one with a higher initial cost and a long life span.
Strength: The stronger the greenhouse glazing the more resistant it is to breakage from debris or weather events such as high winds and hail. Therefore, the higher the strength, the lower the probability of breakage and the resulting costs associated with replacing the glazing.
Weight: The heavier the glazing material, the higher the dead load on the structure. To account for the increased dead load, a stronger support structure is required. This results in increased costs and may result in a reduction in greenhouse light levels due an increase in obstructions by the support structure.
Light transmittance: The higher the light transmittance of a glazing, the higher the amount of sunlight that can penetrate the glazing and enter the greenhouse. In northern climates and in the winter, light is often the limiting factor for photosynthesis. Therefore, maximizing the amount of natural sunlight entering the greenhouse is desirable. Sometimes, such as in summer or in southern or equitorial locations, the amount of light entering the greenhouse is above optimal levels. In these situations, a shadecloth or shading compound may be used to temporarily reduce the amount of light entering the greenhouse. When light levels drop below optimal, the shading material is removed. Light transmittance of a glazing is not constant. As a glazing ages, it tends to have a reduction in its light transmittance due to scratching from dust and debris and aging or "yellowing" of the glazing material due to U.V. exposure.
Thermal conductance: This is the rate at which heat energy moves through a glazing material and is expressed as Btu loss/ ft2/hr/(oFinside - oFoutside). Generally, a low thermal conductance is desired in order to minimize heating costs.
Scratch resistance: Dust, soil particles and other debris can scratch the glazing. Scratching reduces the light transmittance of the glazing and can therefore result in reduced light levels inside of the greenhouse.
Visit Greenhouse Kits for more information on our kits and shipping costs. We also have Greenhouse Plans for a 16' freestanding cedar greenhouse that makes a great DIY project. Whether you are buying a kit or building from a plan there are many different Frame Styles to chooose from. For specifications and how to apply, cut and clean panels visit Lexan Corrugated and our Lexan Thermoclear polycarbonate information pages. Besides being beautiful to look at we use Western Red Cedar for our greenhouse kits because of it's resistance to rot and insects. We have free eco friendly Solar greenhouse plans, simple Lean-to plans for a greenhouse that attaches to your home and Gambrel or barn shaped greenhouse plans. Read about lighting, ventilation, pests and diseases as well as free greenhouse potting bench plans at Gardening Tips. Our customer Photo Gallery and greenhouse YouTube Videos round out our site.
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