The choice of a plastic covering for a greenhouse is a fundamental decision that directly impacts plant health, energy consumption, and the structure’s longevity. This covering acts as the climate control barrier, allowing light in while managing temperature and humidity to create an optimized growing environment. Selecting the wrong material can lead to increased heating costs, poor light quality, and premature failure. Understanding the differences between the major plastic categories and their technical specifications is paramount for maximizing horticultural success.
Flexible Films vs. Rigid Panels
Greenhouse coverings are separated into flexible films and rigid panels. Flexible films, primarily made from Polyethylene (PE), are the most common and cost-effective option for many growers. PE is lightweight and easy to handle, making it ideal for large structures like hoop houses where the material must conform to curved frames. Installation is simple, requiring minimal structural support, but this material is the least durable and has the shortest lifespan.
Rigid panels, typically constructed from Polycarbonate (PC) or sometimes Acrylic, offer a more robust solution. Polycarbonate sheets are known for their extreme impact resistance, often cited as being hundreds of times stronger than glass. These panels provide superior structural integrity against high winds and heavy snow loads, making them suitable for permanent greenhouses. Polycarbonate is frequently manufactured in a twin-wall or multi-wall structure, which significantly improves its insulating properties compared to a single sheet of film.
Understanding the Key Technical Properties
The performance of any plastic covering is defined by several technical properties that dictate the environment inside the greenhouse.
Light Transmission and Diffusion
One of the most important factors is the quality of light transmitted to the plants. Clear transmission allows light to pass straight through, which can create intense hot spots that may scorch sensitive foliage. Light diffusion is often preferred because it scatters the incoming light, distributing it more uniformly across the growing space. This scattering action reduces shadows and ensures light penetrates deeper into the plant canopy, optimizing photosynthesis in the lower leaves. Many modern films and panels incorporate additives to enhance this diffusion.
UV Stabilization
Plastic materials require protection from the sun’s energy, which is achieved through UV stabilization. Without stabilizers, polyethylene film quickly degrades, becoming brittle and yellowing within a season or two. These additives are incorporated into the plastic to absorb harmful radiation, extending the functional lifespan of the covering to several years. UV blocking is a separate feature that can be added to control the amount of UV light reaching the plants for specific growth or pest control needs.
Thermal Retention
Thermal retention is measured by a material’s R-value, indicating its resistance to heat flow. Rigid multi-wall polycarbonate excels here because the trapped air layers create an insulating barrier. Flexible films can also achieve good thermal performance by using an air-inflated double layer of film, creating a similar insulating air pocket. Infrared (IR) additives in films help to absorb and reflect heat back into the greenhouse at night, which can significantly reduce heating costs.
Anti-Condensate Coatings
Anti-condensate or anti-drip coatings are applied to the interior surface of the plastic. This specialized coating prevents the formation of large water droplets, which would otherwise drip onto plants and increase the risk of disease. Instead, the coating causes the water vapor to sheet or run down the side walls. This maintains higher light transmission and a healthier, drier environment.
Practical Decision-Making: Thickness, Durability, and Cost
Film Thickness
The thickness of flexible film is measured in “mils,” with one mil equaling one-thousandth of an inch. The standard thickness for most commercial-grade polyethylene film is 6 mil, which provides a good balance of cost and durability. Thinner films, such as 4 mil, are typically reserved for temporary season extension or low tunnels due to their limited strength. For applications in harsh weather or where greater puncture resistance is needed, films up to 8 or 12 mil are available.
Durability and Lifespan
The expected replacement cycle is a major factor in the decision-making process. High-quality, UV-stabilized polyethylene film is typically rated for a lifespan of four years before its transmission and structural properties degrade significantly. In contrast, rigid polycarbonate panels often come with a warranty of ten years or more against yellowing and material failure.
Cost Comparison
The initial investment cost presents a clear difference between the two material types. Polyethylene film is significantly more affordable for the initial purchase and installation. However, the higher upfront cost of polycarbonate is offset by its superior durability and insulation. Over time, the energy savings from polycarbonate’s better R-value and the reduced frequency of replacement can make it the more economical choice for a long-term, year-round operation.