Selecting the appropriate plastic film for a greenhouse structure is crucial, as it directly influences the structure’s performance and lifespan. The thickness of the plastic covering is the primary factor determining its physical durability, insulation value, and resistance to environmental stress. Choosing the right thickness ensures a stable growing environment, optimizes light transmission, and manages long-term maintenance costs.
Defining Greenhouse Plastic Thickness
The standard unit of measurement for greenhouse plastic thickness is the “mil.” One mil is precisely defined as one one-thousandth of an inch (0.001 inches or 0.0254 millimeters). It is important to note that “mil” is not an abbreviation for millimeter; a 6-mil plastic is significantly thinner than a 6-millimeter sheet.
Typical plastic films are quite thin, ranging from 4 mil to 8 mil for most common applications. The industry relies on this precise measurement to differentiate between films that may appear similar but offer distinct levels of performance. The most readily available thickness options for consumers are generally 4 mil, 6 mil, and 8 mil, with 6 mil being the most popular choice for year-round structures.
Thickness and Material Lifespan
The physical thickness of the plastic film has a direct correlation with its mechanical strength, which dictates its resistance to physical wear and tear. A thicker film provides more material mass to resist abrasion, punctures, and tears caused by installation, contact with the frame, or wind-whipped debris. For example, a 4 mil film is more prone to damage than an 8 mil film, which offers significant tear and puncture resistance.
While physical strength is related to thickness, a film’s longevity is also heavily dependent on the inclusion of Ultraviolet (UV) inhibitors. These chemical additives prevent the sun’s radiation from breaking down the polyethylene polymers, which would otherwise cause the plastic to become brittle and fail prematurely. Under standard conditions, a 4 mil film is typically rated for a single growing season or about one year of use. The industry-standard 6 mil film with UV protection can last three to four years, and the thickest common option, 8 mil plastic, can often extend the replacement cycle to five years or more.
The trade-off between initial cost and labor is a significant factor when choosing thickness. Although a thicker film costs more upfront, its extended lifespan reduces the labor and material expenses associated with frequent replacement. Investing in a longer-rated film minimizes disruption to the growing cycle and lowers the long-term cost of ownership. The increase in thickness from 4 mil to 8 mil minimally affects light transmission, reducing it by only one or two percent, which is acceptable for the gain in durability and insulation.
Matching Thickness to Structure and Environment
The ideal plastic thickness balances the type of structure and the environmental stresses it will endure.
4 Mil Plastic
For temporary structures, such as season-extension low tunnels or cold frames intended for use for only one year, 4 mil plastic is often recommended. This option is lightweight and affordable, making it suitable for low-budget projects or areas with mild weather and minimal exposure to strong winds.
6 Mil Plastic
The 6 mil film is widely considered the industry standard and the best all-around choice for a permanent, year-round greenhouse. It offers a good balance between cost-efficiency, insulation, and durability, making it suitable for most moderate climates. This thickness is often required for compliance with agricultural programs and is strong enough to withstand typical weather patterns while still allowing excellent light transmission.
8 Mil Plastic and Above
In regions experiencing high wind loads, significant snowfall, or extreme temperature fluctuations, an 8 mil film is a better investment. The increased thickness provides superior strength to handle heavy snow loads without risk of collapse and offers better insulation to maintain a stable internal temperature. Commercial growers requiring maximum durability and minimal maintenance also frequently opt for 8 mil plastic. For the most severe climates, commercial options extend to 9 to 12 mil, often used in a double-layer setup with an air inflation system to maximize thermal retention and snow load capacity.