One of the main components of any type of greenhouse construction — be it for home or commercial use — is the covering, also called glazing. There are four main types of glazing: Plastic Films, Polycarbonate, Glass, and Fiberglass. What you select depends ultimately on budget, climate and what you plan to grow in the greenhouse. Each has its advantages and disadvantages.
Plastic Film Greenhouses
According to a 1993 study by researchers Gene A. Giacomelli and William J. Roberts of Rutgers University, plastic (polyethylene) film, is the most popular glazing material, mainly due to its low initial cost. It is fairly inexpensive to install, requiring a relatively limited amount of other structural materials, especially when used in a Quonset-style construction. Additionally, if you install a double layer of polyethylene plastic film with an air space in between, you can reduce your heating costs by up to 40 percent versus polycarbonate, glass or fiberglass.
Other advantages to plastic film are that it is a great diffuser of sunlight, allowing for intense light at the canopy level of the greenhouse. It also produces few shadows because fewer braces and trusses are typically used in constructing a polyethylene greenhouse. It is a great choice for first-time greenhouse growers.
One of the biggest drawbacks to using plastic film is it has a fairly short lifespan. Over time, sunlight and air pollution greatly weakens polyethylene and makes it brittle, causing it to shred and tear. UV inhibitors, applied to the plastic, prolong age somewhat, but you are still looking at replacing this type of glazing every two to three years. So, while in the short term plastic film is cost effective, in the long run it becomes more expensive than other coverings.
A more rigid plastic than polyethylene, polycarbonate is the newest type of greenhouse covering available on the market and comes in flat layer, twin-wall, or corrugated single-layer thicknesses. It also comes in bronze-colored panels for a diffused light effect that might be desired in a retail setting. Polycarbonate is flexible enough to be used in cheaper Quonset-style construction, and the double layered sheets in particular offer great heat insulation. Polycarbonate sheeting is extremely durable, able to withstand hailstorms and bird accidents much better than glass. It is not as prone to UV or pollution damage as polyethylene, so it can last up to 10 years without needing replacement.
As for disadvantages, polycarbonate sheeting (depending on quality) can be prone to clouding, or yellowing, over time, resulting in light loss. The double layered polycarbonate is also prone to algae growth, which can easily be remedied by taping the ends shut during the construction phase of your greenhouse project. And while polycarbonate is much more durable than both glass and polyethylene, it is far costlier than both. The expensive price-tag makes polycarbonate prohibitive to many gardeners.
According to researchers Giacomelli and Roberts of Rutgers, glass is the ideal material for greenhouse glazing because it provides the best light transmission out of all the various coverings and has a higher comparable thermal rating. Also, the two researchers note, the light distribution can be improved even more by etching small patterns into the glass itself. When compared to other coverings, glass also lasts a long time, is non-combustible and is resistant to both air pollution and UV radiation.
The biggest drawback to using glass as a greenhouse glazing is cost. While the panels are cheaper than polycarbonate, glass is heavier and needs more structure to support it and therefore construction costs are initially a lot higher. Out of all coverings, glass is also the most susceptible to catastrophe, i.e., hailstorms, thrown objects and bird accidents. Cracked panels have to be replaced fairly often.
Fiberglass is another material frequently used in greenhouse construction. It is cheaper than glass and polycarbonate, but usually more expensive than polyethylene. Because it is rigid and very durable, it does not need the structural backing of a glass greenhouse and that lessens the price somewhat, too. Fiberglass also holds up reasonably well to catastrophic weather damage and for that reason has been used extensively in places like Florida where hail is a big problem.
Disadvantages to using fiberglass include its vulnerability to sun exposure. When exposed to long periods of sunlight, the fibers in the material tend to swell and reduce light transmission. Fiberglass may last as little as five years before the UV breakdown becomes significant. Fiberglass is also quite combustible—equal, say to wood—and flames will continue to ignite, even if you remove the ignition source. Many growers consider this safety issue a good reason not to use fiberglass.
Want to learn more about greenhouse coverings?
See these helpful resources:
Texas Greenhouse Management Handbook from Texas Cooperative Extension
Starting a Greenhouse Business [PDF] from Kansas State University
Greenhouse Covering Systems by Gene A. Giacomelli and William J. Roberts: Rutgers University [PDF]