The material used to cover a greenhouse is the most important factor determining its internal environment, regulating light exposure, heat retention, and moisture levels. Selecting the proper glazing requires identifying the option that aligns with specific needs, growing goals, and local climate conditions. The ideal covering must balance initial cost and ease of installation with long-term durability and thermal performance.
Flexible Film Coverings
Polyethylene (PE) film is the most common flexible covering, offering the lowest initial investment and simple installation over various frame types. Standard single-layer PE film, often 6-mil thick, provides a low R-value around 0.85, meaning it offers minimal resistance to heat loss. This makes it a poor choice for year-round growing in colder climates, as heating costs become prohibitive.
Modern PE films, however, incorporate UV inhibitors to prevent premature degradation, extending their lifespan from one or two years to between three and five years. Growers often use a double layer of film, inflated with a small blower system, to create an insulating air pocket. This technique can raise the thermal resistance, increasing the R-value to approximately 1.5 or 1.7, significantly reducing heat loss compared to a single layer.
Specialized versions of flexible film also include infrared (IR) additives, which help trap heat radiating from within the structure during the night. While films are lightweight and easy to replace, their relatively short lifespan requires frequent maintenance and replacement, adding to the long-term cost of operation. They are best suited for seasonal operations or for growers who prioritize a low upfront budget.
Rigid Plastic Panels
Rigid plastic panels offer a substantial upgrade in both durability and thermal efficiency compared to flexible films, making them the preferred modern solution for commercial and serious hobby growers. The two main types are Polycarbonate (PC) and Acrylic, each offering different performance characteristics. Polycarbonate is exceptionally impact-resistant, up to 250 times stronger than glass, providing superior protection against hail and physical damage.
Polycarbonate achieves superior insulation through multi-wall designs, such as twin-wall or five-wall configurations, which incorporate internal air channels that function as thermal breaks. A 16mm five-wall polycarbonate sheet can reach an R-value of 3.2, which is highly effective at retaining heat and reducing energy consumption. This configuration also naturally diffuses light, scattering it more uniformly over the plant canopy and eliminating harsh shadows that can cause localized burning.
Acrylic panels are favored for their exceptional clarity, often transmitting up to 92% of available light, which is closer to the optical properties of glass. While acrylic is more impact-resistant than traditional glass, it is more prone to cracking and shattering than polycarbonate, especially under severe impact. Acrylic also tends to maintain its optical clarity longer than some older polycarbonate formulas, as it is less prone to yellowing over time.
Glass and Permanent Cladding
Glass remains the premium choice for permanent structures, offering maximum clarity and aesthetic appeal. Its primary advantage is its unrivaled light transmission and inability to degrade or yellow over time, giving it an extremely long lifespan of 20 years or more. However, single-pane glass provides poor thermal insulation, with an R-value of only about 0.95, leading to significant heat loss.
To address this lack of thermal performance, glass greenhouses often require double-glazing, which can raise the R-value to 2.0 or higher, though this significantly increases the already high material cost and weight. Glass is also the heaviest covering material, demanding a robust and expensive structural frame, and it carries the highest risk of breakage from impact.
Fiberglass, often used in corrugated panels, provides an alternative set of benefits. Fiberglass is highly durable and excels at light diffusion, scattering sunlight evenly across the growing area to prevent hot spots and shadows. While it is generally more affordable than glass or high-end polycarbonate, its light transmission can be lower, and it can become brittle or lose clarity over decades of exposure.
Matching Material Choice to Climate and Budget
Choosing the appropriate covering material requires prioritizing specific growing needs against the constraints of budget and climate. For a grower focused on minimizing initial investment or operating a temporary, seasonal structure, the low cost and easy installation of UV-treated polyethylene film are highly advantageous. The trade-off is the need for film replacement every few years and poor insulation unless a double-layer blower system is used.
For year-round cultivation in regions with cold winters or high energy costs, materials with a high R-value are essential to conserve heat. Multi-wall polycarbonate, particularly the thicker, multi-chambered varieties, offers the best balance of insulation, impact resistance, and longevity for energy-efficient growing. This choice sacrifices some light clarity for superior thermal performance.
Growers prioritizing maximum light quality, aesthetic appearance, and a permanent structure should look toward glass. While glass demands the highest initial investment, especially when double-glazed for insulation, its long-term stability and maximum light transmission are unmatched for crops that require an abundance of direct sunlight.