Greenhouse Structures and Design
Greenhouse Covering Materials
The greenhouse covering materials, or glazing as it is referred to in the industry, represents the greatest decision in selecting the design for the greenhouse. The different glazing materials have different characteristics, associated costs, and require different support structures. Therefore, selection of the glazing material needs to be made before selection of the structure. The National Greenhouse Manufactures Association (NGMA) lists three categories of materials for glazing commercial greenhouses. Type I glazing materials are thin plastic films, which include polyethylene, ethylene vinyl acetate (EVA), polyvinyl chloride (PVC), polyvinyl fluoride (PVF), and polyester films. Type II glazing materials are ridged plastic panels, which include fiberglass-reinforced plastic (FRP), acrylic, polycarbonate, polypropylene, PVC, and PETG (glycolmodified polyethylene terephthalate). Type III glazing materials are glass (e.g., annealed, tempered, and laminated).
Flexible plastic films, including polyethylene, polyester, and polyvinyl chloride (PVC), have been used for greenhouse coverings. Plastic film is currently the leading greenhouse covering for two reasons. First, film plastic greenhouses with permanent metal frames cost less than glass greenhouses. Even greater savings can be realized when film plastic is applied to less permanent frames, such as the cheaper Quonset greenhouses.
Polyethylene, sometimes also known as “poly-film,” has always been and still is the principal choice of film plastic for greenhouses in most of the world. The major advantage of polyethylene film plastics is cost, which is why new growers often choose a poly-house design. The greenhouse-structure itself, and the polyethylene plastic covering, are considerably less expensive to purchase and install. Polyethylene film is very light in weight.
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%. In addition, Tefzel film film is resistant to ultra-violet radiation, which is the primary cause of degradation of the physical properties of other covering materials.
Polyvinyl Chloride (PVC)
PVC 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.
The rolling-roof greenhouse system typically has bows mounted on lattice trusses spanning each bay (See Figure 1.17). Tube-type motors drive aluminum pipes that roll the covering up or down on each side of the bow. The covering is typically a nylon mesh film enclosed within two layers of plastic and provides from 15 to 25 percent shade when closed. Along with flat-roof systems, rolling-roof structures are often much lower in price compared to other designs.
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. The structured acrylic and polycarbonates are very similar in characteristics. They are double-layered and ribbed for strength, creating an air pocket reducing heat loss.
Fiberglass-reinforced Plastic (FRP) 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. Where FRP is used, corrugated sheets are preferred because of their greater strength compared to flat sheets. Flat panels are used occasionally for the end and sidewalls, where the load is not great. 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 is one of the most widely used glazing materials in greenhouses. This rigid material performs well in areas prone to heavy snow loads and wind loads, while being very lightweight and also easy to install. Light transmitted through polycarbonate, when compared to polyethylene film, has a higher percentage of direct radiation vs. diffused radiation. 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 ten to 15 year life span can be expected (depending on the manufacturer). Polycarbonate is available in flat and corrugated single or multi-layer sheets, separated by flutes.
Polymethyl Methacrylate (PMMA) or Acrylic
Acrylic has been used for many years and is considered to be the most suitable rigid transparent plastic for greenhouse glazing. Acrylic has 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.
Glasshouses for commercial production are the most commonly found greenhouse structures in cold parts of the Northern Hemisphere. They are usually built in very large compartments in order to lower cost per unit area, improve efficiency and reduce heat loss through the sidewalls. They usually have only roof ventilators, which may be discontinuous (e.g. Venlo type, one-side mounted windows) or continuous. Glass has the highest light transmission, lasts the longest (30-plus years) and is the most expensive. 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. Glass greenhouses tend to have a higher air infiltration rate, which leads to lower interior humidity. This is advantageous for disease prevention. Despite these advantages, there are several disadvantages to consider.
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