There is no single "best" greenhouse covering. It all depends on the design and purpose, and the size of the greenhouse. Large scale commercial greenhouses frequently have rigid panels, either of glass or heavy-duty plexiglass or translucent plastic. They are cost-effective for long-term use, durable and may have louvers, ridge vents or integral cooling fans to help with air circulation and maintenance of constant temperature. Such venting can be controlled either manually or automatically, and the most sophisticated systems have sensors to monitor operation. When large-scale crops are involved, it is imperative that growing conditions are optimized and effectively controlled. When choosing the material for greenhouse covering, a primary consideration will be budget, of course, but over the long-term, a higher initial expenditure may be a long-term bargain. Modern technology and manufacturing techniques have brought vast improvements in the quality of the flexible plastic film that is often specified as greenhouse covering. Some has embedded fiber for strength and UV protection. Two or more layers with an air cushion between layers, will boost the insulating effect.
Hobbyists and homeowners who simply want a small greenhouse for occasional or seasonal use can save money with kit greenhouses, and may be able to construct a shed-style greenhouse or slope-roof lean-to frame with readily-available treated lumber and economical 6 mil poly. It is adequate for small-scale use, and should perform reliably for two or three seasons before needing replacement. Serious gardeners, small commercial farmers and large-scale producers, however, invariably consider longer durability and strength as important as cost, and will opt for higher-quality covering choices that offer better performance and higher ROI. When designing a greenhouse, first consider your needs and then develop a set of specifications to ensure that the finished greenhouse will perform as expected. Your choice of materials and pricing will depend on the purpose of the greenhouse.
