Polyethylene Films: A Budget-Friendly, Short-Term Solution 

You can use polyethylene for a variety of applications from ground-to-ground greenhouse and hoophouses to gutter-connected Quonset styles.

Overall, it’s best to view poly as a short-term or less permanent glazing option due to its shorter lifespan. Certain structures, such as the Prospiant Grand Teton and Zephyrs, have a design that accommodates growers who want to start with a poly covering and then replace it with a rigid covering later. 

Here’s an overview of everything you ever wanted to know about polyethylene film. 

Properties/Characteristics:

Pliable, lightweight, sturdy, transparent to translucent, generally chemical resistant (but is susceptible to brittleness when burning sulfur), low moisture absorption, recyclable, higher insulative values

Note that you can customize polyethylene films somewhat to meet your various needs by selecting them for additives included during processing that improve their performance.  

Lifespan:

3-5 years (single layer). Greenhouse-grade polyethylene comes in various film thicknesses. The thicker the film, the better its longevity and strength.  

Longevity depends also on the application and whether you live in an area prone to severe winds, heavy hailstorms, or heavy snow and ice accumulation. You can obtain some added insurance with woven poly or nylon-reinforced poly, which has a high tensile strength that contributes to its durability for better resistance to weather damage. However, the expense to purchase reinforced poly is generally more than standard thicknesses such as 6mm. 

 

Cost:

Polyethylene glazing costs the least of the main covering options. Its affordability, in part, accounts for its popularity among growers. 

 Check out this example comparison found in the Ball Redbook (Jacobson, 2021, p. 30), but keep in mind this is an approximation. Glazing costs vary based on the type of glazing (e.g., thickness, opacity, etc.), additives, brand, availability, etc.   

Materials and Installation Costs* for Double Poly Greenhouse vs. Glass Greenhouse 

Double-poly Greenhouse	$0.50/sq. ft.	$5.40/sq. m
Glass Greenhouse	$2.50-$3.50+/sq. ft.**	$27-$32+/sq. m**

*Costs are an approximation for example purposes only   **The type and weight of glass can add to the cost of a project. 

R-Value:

R-value in simple terms refers to a greenhouse covering’s insulating quality or how well a greenhouse covering does at keeping the inside of the greenhouse warm in cold weather. If you want to get more complex, it is a measure of thermal resistance per unit area.  

The higher the R-value, the better the covering performs as an insulator that reduces heat loss to the outside environment. Paying attention to R-values can save you energy costs in the long run.  

However, keep in mind that the R-value of the greenhouse can come from its curtain, too. Not every covering is applicable in every case, so your choice should be driven by the type of crop(s) you want to grow and the climate at the greenhouse’s location. 

The R-value of polyethylene coverings changes depending on whether its two layers being inflated or just a single layer. Generally, single-sheet poly has an R-value of 0.85, while double poly has an R-value of 1.25 (Fowler et. all, 2021). 

Polyethylene glazing with infrared radiation (IR) additives reflects IR radiation. They allow heat to leave the greenhouse in the summertime and prevent heat from escaping during the winter, which means it takes less energy to heat the house during the night.  

Growers who need to prevent a portion of IR radiation from entering the greenhouse would want the IR plastic on the exterior layer, and those who want to prevent it from leaving the greenhouse need the material as the inside layer. It’s most common to see the IR coating used on the inside layer.  

When inflated, poly coverings tend to have some of the higher R-values among the various greenhouse covering options. However, this depends on the quality of the installation and how well the greenhouse perimeter is sealed.  

When installed properly, poly films allow for less air infiltration because they are one continuous, solid sheet running the entire length of the greenhouse. This leaves less opportunity for gaps in the covering that allow cool air to enter the house. 

Light Transmission:

Light transmission is the portion of light moving through a greenhouse covering not absorbed by the covering or reflected out. Dust accumulation, air pollution, condensation build-up, structure parts, and plastic deterioration over time all impede the light transmission of polyethylene coverings.  

Photosynthetically active radiation (PAR) light transmission values also vary depending on the additives included during manufacturing. 

TABLE: Typical PAR Light Transmission Ranges for Polyethylene (Bartock, 2013). 

UV-Stabilized	88%-91%
IR-AC film	82%-87%
IR-AC with diffusion	77%-88%

 

The diffusion of polyethylene impacts light transmission. Additives in the film give growers the advantage of controlled light diffusion to reduce shadowing in the greenhouse and to allow light to access the lower crop canopy. There are also clear and white poly products that offer different opacities. 

 One final note on light transmission — the higher insulative values of polyethylene coverings typically come at the cost of lower light transmission values. This can throw you into a constant balancing act between the cost of heating and the cost of supplemental lighting when it comes to covering choices. 

Ultra-Violet Filtering:

Ultra-violet (UV) stabilizers, or rather chemicals embedded within the polymer of the film, boost the durability of poly coverings and extend their lifespan while helping to prevent discoloration and brittleness. They also shield plants and people from harmful UV rays. 

Despite their protective effects, UV poly treatment still allows ample light to reach the crop canopy. Keep in mind, however, that in some plants, exposure to wavelengths of UV light such as UV-A or UV-B can bolster their defenses against pests and diseases and improve their flavor and aroma (Loconsole & Santamaria, 2021). 

 Beneficial insects, such as parasitoids and pollinators, as well as insect pests also experience the beneficial and detrimental effects of UV light transmission. The same holds true for plant pathogens (Runkle, 2020). 

Thermal expansion/contraction:

Polyethylene has high thermal expansion and contraction. This simply means it expands in warmer temperatures and contracts in colder temperatures.  

 You need to time the installation of your covering accordingly to avoid problems with sagging, leaking, and cracking. This holds true for the quality of your installation job as well because improper stretching of the covering increases its vulnerability to sagging or stretching. 

Carefully consider weather conditions before you install poly; they play a critical role. The less wind, the better. Even light to strong breezes can present a problem. Temperatures need to reach above 35°F for poly installation to avoid contraction. If they climb to 85°F or more, avoid installation or you will get too much stretch in the covering.   

References:

Bartok, J. J. (2013). Plastic Greenhouse Film Update. University of Massachusetts Amherst. Retrieved September 11, 2023, from https://ag.umass.edu/greenhouse-floriculture/fact-sheets/plastic-greenhouse-film-update#:~:text=Reduced%20nighttime%20heat%20loss%20%28IR%29%20%E2%80%93%20this%20is,as%20the%20inner%20layer%20to%20retain%20nighttime%20heat. 

Fowler, P., Bucklin, R., Baird, C., Chapman, F., & Watson, C. (2021, March 4). Comparison of Energy Needed to Heat Greenhouses and Insulated Frame Buildings Used in Aquaculture. AskIFAS. Retrieved September 8, 2023, from https://edis.ifas.ufl.edu/publication/AA212  

Jacobson, P. (2021). Polyethylene Film. In C. Beytes (Ed.), Ball Redbook 19th Edition: Volume 1 – Structures, Equipment, and Technology. (p. 30-33). Ball Publishing.

Loconsole, D., & Santamaria, P. (2021). UV Lighting in Horticulture: A Sustainable Tool for Improving Production Quality and Food Safety. Horticulturae, 7(1), 9.