Chapter 2

Greenhouse Glazing

Types of Greenhouse Glazing

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). A description of some of the more common types of glazing materials follows.

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. The major advantage of polyethylene film plastics is cost, which are considerably less expensive to purchase and install than glass. Polyethylene film is very light in weight. Thus the film does not require a structural support system.

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 (PVC) 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.

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 (See Figure 2.2). Fiberglass is available in flat and corrugated configurations. 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 (See Figure 2.3). Light transmitted through polycarbonate, when compared to polyethylene film, has a higher percentage of direct radiation versus diffused radiation. Similar to 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 ten to 15 year life span can be expected (depending on the manufacturer). Polycarbonate has high impact strength—about 200 times that of glass.

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 (See Figure 2.4). 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.

Plastic Material Additives

Some polyethylene film as well as rigid FRP, acrylic, and polycarbonate panels are available with can be formulated to control or utilize more effectively the heat and light energy from the sun, and also heat energy radiated from the soil. These forms of energy are part of the electromagnetic spectrum and differ only in their wavelength. By using different polymers and additives, it is possible for films to transmit, absorb or reflect different wavelengths preferentially. It is possible, therefore, to create a wide variety of microclimates by using different types of film, either as covers over crops or as a mulch on the soil surface.

Anti-Condensation Inhibitors

Water vapor condenses on the cold inner cover surface forming small droplets of liquid water. Condensation is particularly a problem in the morning when, due to a cold greenhouse covering and humid air inside, a thick layer of droplets often forms. This has negative consequences on light transmission. Moreover, condensation can fall onto the crop fostering development of fungal diseases. Anti-drip additives modify the surface tension of water, eliminate droplets and form instead a continuous thin layer of water so that it drains to the side rather than forming droplets that drip onto the plants.

Infrared (IR) Blocking Plastic Materials

Polyethylene is a poor barrier to radiant energy. Energy absorbed by objects inside a greenhouse during the day is reradiated at night, leading to rapid cooling inside a structure on cloudless nights. However, thermal polyethylene, sometimes referred to by manufacturers as IR film, is available, which contains an additive that reduces re-radiation of this IR energy.

UV-Blocking Materials

The ultraviolet rays of the sun break down polyethylene; after three or four years the plastic becomes quite brittle and subject to tear. The film clarity of the film also changes (turns opaque) and the amount of light and color spectrum is significantly restricted, which will affect the success of your plants growth and development. Most polyethylene films used in greenhouses are manufactured with a UV stabilizer to extend the life of the material.

Anti-Dust Inhibitors

Most polymers are poor electricity conductors, particularly prone to the accumulation of static electricity when two surfaces are rubbed against each other or when there is friction caused by the wind. As a consequence, most plastics attract dust.

Light Diffusion Materials

In areas with clear skies and high solar radiation, direct radiation can cause leaf burning in greenhouse crops on warm days. New plastic films have been developed to increase the percentage of diffuse radiation in the greenhouse.

Glass

Glass is the traditional covering material for greenhouses (See Figure 2.5). It is still favored by many growers, particularly those who consider greenhouses a long-term investment. Glass can be used on any style of greenhouse, but is commonly found on Venlo and widespan houses, which were developed with glass glazing in mind. Glass is the most expensive material to purchase and install, and the overall cost of building a glass structure will also be substantially higher due to the increasecl material content of the structure; however, amortization over its long-life span may make glass a more cost-effective cover than some others. The operating costs of a glass greenhouse are also higher than for double-poly or twin-wall polycarbonate greenhouses.

Advantages and Disadvatages of Glass

The main advantage of glass is its relatively long life span. Maintenance on glass is similar to other rigid materials and includes washing off dust to improve light transmission. Glass is still used, due in part to its superior light transmitting properties and less excessive relative humidity problems. Glass “breathes” (the glass laps between panes allow air to enter), whereas polyethylene, acrylic, and polycarbonate-structured sheet houses are airtight, which can result in excessive humidity and undesirable water drip on the plants if not properly controlled.

Types of Glass

Greenhouse glass is available in several types, including float, tempered, laminated, and frosted or “hammered” glass. The typical glass that is used for glass greenhouses is called float glass. It is a transparent glass with a high light admittance to ensure sufficient light in the greenhouse. As the safety awareness has grown, the application of float glass has decreased. Today, new greenhouses are often fitted with safety glass (e.g., tempered or laminated). Tempered glass is four to six times more shatter-resistant than float glass, and when it breaks it breaks into small-square pieces, making it unlikely to cause injury.

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