The choice of plastic for a greenhouse covering is one of the most significant decisions a grower makes, directly influencing the structure’s performance and longevity. Unlike standard plastic sheeting, specialized greenhouse film is designed to manage light, temperature, and moisture for optimal plant growth. Selecting the correct thickness, often measured in “mil,” is a fundamental step that balances durability with light transmission and cost. The integrity and energy efficiency of the entire growing environment depend on this initial material choice.
Defining the Mil Rating and Film Lifespan
The term “mil” is a unit of measurement used to specify the thickness of the plastic film, representing one-thousandth of an inch (0.001 inches). This measurement is a direct indicator of the film’s physical strength and protective capabilities. A higher mil rating signifies a thicker, more robust material with greater resistance to tearing, puncturing, and degradation from abrasion or wind stress.
The mil count directly correlates with the expected lifespan of the greenhouse cover. Thicker films are inherently more durable and can endure harsh weather conditions for a longer period before needing replacement. While a 4 mil film may be suitable for temporary, short-term applications, a standard 6 mil film is engineered to provide multi-year protection. Increasing the mil count also means a higher material cost and a slight reduction in the film’s clarity and light transmission capabilities.
Standard Thickness Recommendations for Greenhouses
For most full-season greenhouse applications, a 6 mil film is the recognized industry standard, offering an optimal balance of strength, light transparency, and cost-effectiveness. This thickness is typically warrantied for a lifespan of four years when appropriately stabilized against ultraviolet (UV) radiation. The 6 mil polyethylene is durable enough to withstand moderate wind and weather, making it the most popular choice for both commercial and hobbyist growers.
Thinner films, typically 3 to 4 mil, are primarily used for short-term structures like temporary row covers, low tunnels, or seasonal cold frames. These lighter materials are inexpensive and easy to handle, but they often only last for one to two growing seasons before becoming brittle and failing.
Conversely, thicker films in the 8 mil to 12 mil range are reserved for situations requiring maximum durability and insulation. The heavy-duty 8 mil and 10 mil options are often employed in regions with extreme weather, like high snow loads or prolonged high winds. They provide greater puncture resistance and may last five or more years with proper care. These thicker grades are also frequently specified for end walls or other rigid sections of the greenhouse where film replacement is difficult.
Essential Film Properties Beyond Mil
While thickness determines physical resilience, the performance of a greenhouse film is equally dependent on specialized chemical additives incorporated during manufacturing.
UV Stabilization
UV stabilization prevents the polyethylene film from rapidly breaking down when exposed to solar radiation. Without these stabilizers, the film would quickly become brittle, develop a chalky appearance, and fail within a single season.
Infrared (IR) Retention
Infrared (IR) retention, often referred to as a thermal additive, traps the long-wave infrared heat radiating from the ground and plants at night, significantly reducing nighttime heat loss. Using an IR-treated film can lead to energy savings of 15% to 20% in heated structures by maintaining a more stable internal temperature.
Anti-Condensate (AC) Feature
Modern films also include an anti-condensate (AC) or anti-drip feature, which is a wetting agent applied to the interior surface. This additive prevents water vapor from forming light-blocking, dripping droplets that can lead to plant disease and uneven light distribution. Instead of forming beads, the moisture is encouraged to sheet out into a thin, continuous layer that runs harmlessly down the sides of the structure, maximizing light transmission.
Choosing the Right Mil for Structure Type and Climate
The optimal mil choice is determined by the construction method and the specific environmental challenges of the location. For a single-layer greenhouse covering, it is prudent to select a robust thickness, which is why the 6 mil UV-stabilized film is the minimum recommendation to ensure a reasonable multi-year lifespan against the elements. A single layer must bear the full brunt of weather and requires the inherent durability of a thicker material.
In contrast, a double-layer inflated system utilizes two sheets of film separated by a continuous layer of air, which acts as a powerful insulator. In this configuration, two 6 mil films are often used, with the inner layer sometimes being a thinner, less expensive film, although using two 6 mil sheets is common. This dual-layer system dramatically increases the structure’s thermal efficiency.
Areas prone to high winds, heavy hail, or deep snow accumulation should lean toward the higher mil options, such as 8 mil or 10 mil, to prevent premature tearing or puncture failure. While 6 mil UV-stabilized film remains the general standard for most applications, the final selection must weigh the localized climate conditions and the structural design against the film’s cost and expected longevity.