Polycarbonates: The Versatile, High Performer

Think of polycarbonate as the lightweight tough guy of the rigid-plastic glazing category with 20 times greater impact resistance than fiberglass and 10 times the strength of acrylic (Churchill and Hasler, 2021, p. 34). It exhibits 200 times the impact resistance of glass while weighing about 1/8 less (Churchill and Hasler, 2021, p. 34). 

Polycarbonate panels come in various types, including multi-wall and corrugated, with single-wall and twin-wall panels the most used for greenhouse applications. Characteristics such as strength, light transmittance, pliability, and more vary depending on the thickness, type, and span between the enclosed flutes (i.e., cross-sectional supports) of polycarbonate panels.   

Throughout the year, light transmission levels in the greenhouse vary as the angle of light hitting the greenhouse roof changes due to the motion of the sun as the Earth orbits around it each year. Under ideal conditions, corrugated polycarbonate, particularly clear corrugated polycarbonate, transmits light fairly well. It is not quite as good as glass or acrylic, but still comes close. 

Like other coverings, manufacturers incorporate different resins and pigments into polycarbonate glazings to enhance their performance and achieve different effects. Adjustments to light transmission levels can allow for either diffused or direct light to come in or varying degrees of haze.  

Considered a “self-extinguishing” thermoplastic, polycarbonate does not support a flame, making it a good choice for greenhouse safety. In a sense, polycarbonates are environmentally friendly. Because you can melt it down to make other items, it is considered recyclable. However, it is not biodegradable. 

Properties/Characteristics: Good fire performance, lightweight, high-impact resistance, excellent transparency, heat resistance, recyclable (not biodegradable), UV resistant, flexibility, high strength-to-weight ratio, durable 

Lifespan

Polycarbonate handles easily and bends into different shapes thanks to its flexibility. Twin-wall polycarbonate panels can last for more than 20 years with proper care and cleaning, although most warranties cover around 10 to 15 years, with 10 years for yellowing. 

Cost

Polycarbonate glazing tends to cost more than polyethylene films but less than glass and acrylic coverings. Costs for high-performance polycarbonate coverings, not including installation materials or labor expenses, run from $1.20 to $2.60 per square foot (Churchill and Hasler, 2021, p. 36). 

R-Value

R-values for polycarbonate reach from 1.54 for 6mm twin-wall to 3.2 for a 16mm five-wall. Corrugated single-layer polycarbonates have a lower R-value than twin-wall polycarbonates with a higher insulative value. Generally, multi-wall polycarbonates have the best R-values and corrugates the worst.   

The air pockets or gaps between polycarbonate panels (e.g., flat twin-wall polycarbonate) add insulative value. The larger the gap size, the better the insulation. The label indicates the gap size. For example, 4mm twin-wall polycarbonate indicates a 4 mm gap between the panels. Commonly used standards in the greenhouse industry include 8 mm and 16 mm sizes.   

Light Transmission

Light transmission up to 90% for corrugated polycarbonate and from 76% to 80% for multi-wall polycarbonate approaches that of glass, which makes it a popular choice for growers, especially with its lighter weight and lower cost. Available options for the panels include clear, diffused, opaque, and every degree in between, letting all levels of light transmission come through. 

Ultra-Violet Filtering

One interesting feature of polycarbonate greenhouse glazing is it largely reflects UV transmission.  

Polycarbonate film also has UV protection either integrated into the film through co-extrusion or via UV coating to slow down the deterioration and discoloration caused by prolonged exposure to the sun. Polycarbonate sheets intended for open-roof structures have two-sided UV protection. These UV-protecting treatments prolong the covering’s lifespan, which ranges from 10 to 15 years. 

Thermal Expansion/Contraction

Polycarbonate coverings exhibit a high degree of thermal expansion and contraction. Thus, they need a special anchoring or locking system to secure them to the greenhouse frame and mitigate thermal movement.  

Corrugated panels are installed with an overlap of themselves and the rivets. Multi-wall is held and secured with extrusions, either aluminum or polycarbonate, with seams/joints placed every 47½ inches or 71½ inches on center. Some extrusions are gasketed, some are not. 

With polycarbonate pre-drilling is not a requirement. You can secure it in the extrusion and point fasten it with a special screw and galvanized washer. 

The timing of the installation is also important. Under extreme cold temperatures polycarbonate can chip when being cut with a saw. When cutting polycarbonate, a fine-tooth blade hand trim saw or Skill saw works best for cutting, as well as using an air compressor to blow out the cutting fragments and dirt and dust before installation. 

Uses

Greenhouse siding, inside walls (single wall) where insulation and low maintenance are very important

Reference

Churchill, T. and Hasler, P. (2021). Rigid-Plastic Glazing. In C. Beytes (Ed.), Ball Redbook 19th Edition: Volume 1 – Structures, Equipment, and Technology. (p. 34, 36). Ball Publishing.