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Greenhouse coverings come in a variety of materials, including glass, rigid plastic (fiberglass, polycarbonate, or acrylic), and plastic films. The appropriate choice depends on your climate zone and budget. Regions with a colder climate will require the covering to provide increased insulation and low heat transfer measured by the R-value and U-value, respectively. The R-value measures how well the material insulates. The higher the R-value, the more insulation the material provides. The U-value quantifies heat transfer and describes how much heat is lost or gained. Materials with a lower U-value will be more energy efficient. Approximately 75% of plastic used for covering greenhouses in the U.S. is air-inflated double- layer polyethylene plastic. 6ml polyethylene plastic covering is inexpensive and has an R-value of 1.4 and a U-value of 0.5 (high insulation capacity and energy efficient). A single layer fiberglass covering is moderately expensive and has an R-value of 0.83 and a U-value of 1.2 (moderate insulation capacity and not energy efficient) (Table 11). Choosing the right material for your climate zone is critical to reduce heating costs during winter. Energy cost is the second greatest production expense, just behind labor.
Table 11: Comparison of greenhouse glazing materials. MaterialLifeR valueU valueAdvantagesDisadvantagesCostGlassSingle layer, tempered(Until it breaks)0.951.13Strong, attractive, good seal, 90% light penetrationBreaks, difficult to install, heating$$$- $$$$Rigid plastics Fiberglass6-15years0.831.20Lightweight, strong, light penetrationOpaque, degrade/yellow over time-6yrs, needs resin recoat$ - $$Rigid single-wall$ - $$Rigid double-wall acrylic20 yr1.4-1.90.75-1.0Transparent, 30% energy savings with double layer, BendsExtra layer reduces light pentration to about 80%$$ - $$$$Rigid double or triple wall polycarbonate with UV coating10-15 yr1.4-1.9Triple 2.50.53-0.70Same as acrylic, may bend more easily than acrylicSame light as acrylic, but will yellow without UV coating, about 80%$$ - $$$$Plastic films (rolls) Single layer, 6 ml1-4 yr.851.20Good light penetration (90% 1-layer) inexpensiveNeeds to be replaced frequently, condensation$Double layer, 6 ml (polyethylene)1.40.5-1.0Reduces condensation, increases warmthReduces light 10& with each layer (80% total)$
Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.