Modern agriculture relies on controlled environments, making the greenhouse covering material a major factor in crop success. While glass was once the standard, contemporary designs frequently utilize specialized plastics due to their lower installation cost, improved safety, and greater flexibility. Selecting the appropriate covering is important, as the material directly influences the internal climate, plant light exposure, and the structure’s longevity. The choice involves balancing the initial investment against long-term performance in light transmission and heat retention. The best material is determined by the specific climate, the crops being grown, and the required operational lifespan.
Comparing the Primary Plastic Materials
Polyethylene (PE) film is the most widely adopted and economical option for structures like high tunnels and hoop houses. This flexible material is sold in large rolls and requires replacement every one to six years, depending on the quality of its ultraviolet (UV) stabilization treatment. Its low initial price and ease of installation make it popular for hobbyists and commercial operations seeking a cost-effective solution. Since PE is prone to degradation from solar radiation, UV inhibitors are necessary to achieve a moderate lifespan.
Polycarbonate (PC) panels represent a significant step up in both durability and initial investment. These rigid, often multi-walled sheets are the covering of choice for permanent structures where lifespan is prioritized. A typical PC installation can last fifteen to twenty years before requiring replacement. The higher upfront expense is offset by its superior structural stability and inherent resistance to impacts, including heavy snow loads and wind damage.
Polyvinyl Chloride (PVC) film was historically a common choice, valued for its relatively good thermal properties compared to early polyethylene. However, modern PVC films have largely been supplanted by advanced PE due to several drawbacks. PVC tends to yellow and become brittle over time, significantly reducing light transmission and requiring replacement after a lifespan shorter than ten years. Its use has diminished as manufacturers focus on developing more resilient polyethylene products.
Key Selection Factors: Light Quality and Heat Retention
The success of any greenhouse depends on maximizing Photosynthetically Active Radiation (PAR), the specific light spectrum (400 to 700 nanometers) used by plants for photosynthesis. While clear materials maximize the amount of light transmitted, light quality is influenced by diffusion. Materials that diffuse light scatter it more evenly across the plant canopy, which is beneficial because it reduces the risk of leaf scorching by eliminating hot spots.
Diffusion allows light to penetrate deeper into the lower parts of the plant canopy, leading to more uniform growth and higher yields compared to direct sunlight. Multi-wall polycarbonate and specialized diffused polyethylene films achieve this scattering effect, ensuring plants receive light from multiple angles. The goal is achieving the optimal balance between total light received and the evenness of its distribution for the specific crop.
Minimizing energy costs, particularly in cooler climates, depends on the material’s ability to retain heat, quantified using the R-value (a measure of thermal resistance). Higher R-values indicate better insulation and reduced heat loss, especially during nighttime hours. Heat retention is significantly improved by the material’s capacity to block long-wave infrared (IR) radiation, which is the heat radiating naturally from the ground and plants.
Single-layer polyethylene has a low R-value, allowing considerable heat to escape unless specialized additives are included. Multi-wall polycarbonate naturally incorporates insulating air pockets within its structure, resulting in a substantially higher inherent R-value. For example, 16mm five-wall polycarbonate can achieve an R-value of 3.2, offering superior thermal performance compared to a single layer of PE film (R-value near 0.87). This performance means that while polycarbonate is more expensive upfront, it delivers long-term savings in heating expenses, making it a better choice for year-round operations in cold regions.
Practical Considerations: Thickness, Layers, and Specialized Additives
The physical thickness of flexible film coverings is measured in mil (one mil equals one-thousandth of an inch). A thicker film, commonly 6-mil, offers increased puncture resistance and a longer physical lifespan, making it the industry standard for most commercial applications. While 4-mil film is suitable for temporary or seasonal use, the 6-mil thickness provides the necessary durability to withstand wind and handling.
A simple way to improve the thermal performance of flexible films is through an inflated double-layer system. This technique involves installing two separate sheets of film and continuously blowing a cushion of air between them using a small fan. The resulting air gap provides substantial insulation, raising the R-value from approximately 0.87 for a single layer to 1.7 for an inflated double layer. This improvement can reduce heat loss by around 40%, making standard PE film more competitive with rigid polycarbonate panels for heat retention.
Manufacturers integrate specialized chemical components into the plastic to enhance performance and longevity. UV stabilizers are necessary for flexible films, as they absorb damaging ultraviolet radiation and prevent the plastic from becoming brittle or degrading prematurely. Anti-condensate or anti-drip agents are applied to the inner surface to prevent moisture from forming large droplets that reduce light transmission and potentially spread disease. Thermal or IR additives can also be incorporated into polyethylene to help trap long-wave radiation, significantly improving the material’s ability to hold heat overnight and offering fuel savings.