Polycarbonate panels are made from a particular type of thermoplastic polymers. They are one of a number of alternatives to glass glazing used in commercial and residential greenhouses. The main advantages of polycarbonate panels over glass is lighter weight and lower cost.
The lower weight means your greenhouse can have framing that is not as strong as that required for glass, and the lighter framing, which can be of wood, aluminum or steel, will not block as much light from the plants inside.
Polycarbonate panels for greenhouse glazing are available from many manufacturers; some of the brand names include Polygal, Palgard, Lexan, Macrolux and Suntuf. All are available in double or triple wall thicknesses.
A double wall panel of 6 mil thick polycarbonate sheets will transmit around 85% of available light, compared to the 90% that glass transmits, and will have an R-value of approximately 1.6 (triple wall r-value is about 2.1) compared to glass’s 2 R-value.
Another benefit of these panels over glass is that if the greenhouse is located in an area with frequent hailstones, small animals, birds, golf balls, or baseballs, it will be less prone to breakage.
Polycarbonate panels have an impact resistance greater than glass; one manufacturer, Macrolux, claims an impact resistance 200 times greater than glass. This can also be a consideration if you have children in your family, since even in the rare occurrence of a broken polycarbonate greenhouse panel, it will not be dangerous like glass shards are.
When installing multi-wall polycarbonate panels on your greenhouse frame, be aware that the material has greater thermal shrinkage and expansion than glass. The panels should be stored in a cool, dry place, indoors, but with the temperature matching that of the outdoors as much as possible.
Allow an expansion gap of 1/8 inch per 3 feet of width or length between panels. The panels must always be installed with the corrugated ribs running vertically; this is to allow any water to drain off. Panels are sealed off at the ends of the sheets with plastic channels to prevent moisture and insect ingress after installation. The joints between adjacent panels are covered with a 1 X 2 inch cap piece, nailed to rafters or wall studs.
The standard panel size is 48 inches long by 72 inches width, but custom lengths are available. Panels should be installed on the vertical side and end walls first, in order to stiffen the frame, then the roof glazing panels can be installed. Any vent openings required will need to be cut in the polycarbonate prior to installing. All joints must be sealed with glazing tape and sealant or exterior caulking.
All in all, polycarbonate makes a good glazing for home greenhouses, although your final choice of glazing material should be determined by the overall purpose and design of the greenhouse. The smartest thing to do is to first figure out plants what you want to grow, determine their needs, and build your greenhouse design around them, not the other way around.