Glazing Materials

Figure 1: Glass glazed greenhouse. Source: Scott Sanford

Greenhouse glazing materials provide a barrier between the outside environment and the one being created within. Glazing provides protection from wind, rain, snow and hopefully hail, while allowing sunlight through. Regardless of the glazing material used – glass, plastic film or rigid plastic panels – all single-layer glazing will have about the same thermal resistance, or U-value, of 1.0 to 1.2 Btu/hour-square foot-°F. Double glazing will reduce heat loss and save about 35 percent over single glazing, with U-values of 0.6 to 0.7 Btu/hr-ft2-ºF.

Glass 

The first modern greenhouses were built in Italy in the sixteenth century and used glass glazing. Glass is the time-tested material for greenhouse coverings (Figure 1). It transmits more light than other types of glazing materials, is non-flammable and has low thermal resistance. However, a glass house typically has lots of joints where air leakage can occur, and it requires a strong structure to hold it up. Glass has potentially the greatest longevity, although glass can be broken by hail. Glass greenhouses are usually the most costly to heat because they normally have only a single layer of glazing. Using energy-saving technologies such as night curtains can help reduce heating costs.

 

Figure 2: Polycarbonate glazed greenhouse. Source: Scott Sanford

Figure 2: Polycarbonate glazed greenhouse. Source: Scott Sanford

Polycarbonate 

Polycarbonate glazing is available with single and multiple walls. Twin-wall construction is the most popular and provides good thermal resistance (Figure 2). Polycarbonate is typically available in four to six foot wide sheets of various lengths. Sheets are available in thicknesses of six, eight, ten, and sixteen mm. Polycarbonate can be cold-formed to fit large radius curved surfaces. Light transmission varies from 78 to 82 percent for twin-wall types. Most polycarbonate for greenhouses comes with a coating on the underside (inside surface) to reduce condensation adhesion which blocks light transmission. It has a life of 10 to 15 years and typically fails because years of UV rays cause it to yellow, which reduces light transmission. Polycarbonate is classified as self-extinguishing for flammability and it has very high impact resistance, which makes it a preferred glazing material in areas prone to hail.

 

Figure 3: Plastic film glazing on freestanding greenhouse.  Source: Scott Sanford

Figure 3: Plastic film glazing on freestanding greenhouse. Source: Scott Sanford

Acrylic 

Acrylic glazing has similar construction and advantages to polycarbonate, but has better UV resistance, which helps it last an average of 20 years. It also has high impact strength. It has a higher price tag and scratches more easily than polycarbonate, but it has 2-3 percent higher light transmission, which means faster plant growth.

 

Plastic Films

Plastic films can be made from polyethylene (PE), polyvinyl chloride (PVC) or polyester and have the advantage of being low cost and light weight, and requiring fewer joints, which reduces air infiltration losses (Figure 3). The main disadvantage is the life expectancy of only three to four years. Plastic films can be used as a single layer or a double layer with air forced between the layers to create a thermal barrier. Plastic films are poor at reflecting long-wave radiation compared with polycarbonate or glass.  Manufacturers have developed films that will reflect 50 percent of the long-wave radiation without affecting light transmittance. These films, typically referred to as IR films, reduce heat loss and are often combined with anti-condensate treatments. If using with a double film glazing system, the IR-rated film should be installed on the inside of the greenhouse and the standard film on the outside. IR-rated films will pay for themselves in ttwo or three months of a heating season in northern climates.