There are several covering materials to choose from, each of which has its own advantages and disadvantages. Common covering materials are glass, polyethylene film, fiberglass reinforced panels, and double-layer structured panels.
Glass is the traditional greenhouse covering against which all others are judged. Good-quality glass is an attractive, very transparent, and formal (in appearance) covering material. It is very strong (tensile strength), but it is subject to shattering and can become brittle with age. Glass is also very expensive, and because of its weight requires sturdier framing support than is required with other covering materials.
Polyethylene film (PE) is a good choice for home-built greenhouses because less structural support is required and it costs much less than other materials. However, PE only lasts about 2 years. Clear PE is used for growing most plants, but white PE can be used to reduce light and heat for growing low-light plants or for propagation.
Fiberglass reinforced panels (FRPs) are rigid plastic panels made from acrylic or polycarbonate that come in large corrugated or flat sheets. FRPs are durable, retain heat better than glass does, and are lightweight (less structural support needed). Large panels are flexible enough to be applied to a Quonset greenhouse. Light transmission may be better than glass simply because less structural support is needed, which creates less shadow.
Acrylic or polycarbonate double-layer structured panels (DSPs) are made of two layers of plastic held apart by ribs spaced 1/2 to 1 inch apart. The double-layer construction increases structural strength and heat retention but decreases light transmission compared to single-layer materials.
