3.0 Greenhouse covering materials
Glass is the traditional greenhouse covering material against which all others are judged. Good-quality glass is an attractive, transparent and formal (in appearance) covering material. It is very strong (tensile strength), but is subject to shattering and can become brittle with age. Glass is also rather expensive and because of its weight requires sturdier framing support than is required with other covering materials. Originally, glass panes for greenhouses were 18 by 16 inches, but larger sizes are more common now—larger panes are actually less fragile than smaller panes. Many greenhouses are covered with double-strength float glass (1/8 inch thick) costing $0.85 to $2.00 per square foot. Large glass panes in many more expensive greenhouse kits are tempered glass (5/32 inch thick) costing $3.00 to $7.00 per square foot, depending on the pane size.
3.2 Rigid fiberglass
Fiberglass reinforced panels (FRPs) are rigid plastic panels made from acrylic or polycarbonate that come in large corrugated or flat sheets. FRPs are available in 24- to 57-inch widths and up to 24-foot lengths. 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. The prices of FRPs range from $1.00 to $1.25 per square foot, depending on the guaranteed life span of the material. Inexpensive materials are guaranteed for as little as five years whereas more expensive options for as long as 20 years.
3.3 Plastic film
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 film only lasts about two 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. PE manufactured for greenhouse application comes in widths from 10 to 50 feet, thicknesses from 1 to 8 millimeters, and costs $0.06 to $0.09 per square foot. Two layers of PE are frequently applied to greenhouses to reduce heating demand. Double-layer PE houses generally cost 30 to 40 percent less to heat than single-layered houses do. The two layers are kept air-inflated by a 100- to 150-cubic-foot-per-minute squirrel cage blower mounted to the inside PE layer. Purchase 4-millimeter PE for the inside and 4- or 6-millimeter PE for the outside. Use 6-millimeter PE for single-layer applications. PE can be installed on wood-frame greenhouses by nailing wood batten strips over the film into the foundation boards and rafters or arches. Since PE must be replaced frequently, investing in special fasteners makes the job easier. Fastening systems are available for single- or double-layer applications.
3.4 Rigid double-wall plastics
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. Panels are 4 feet wide and up to 39 feet long. DSPs made of polycarbonate cost $1.75 to $2.50 per square foot, and those made of acrylic cost $2.00 to $3.50 per square foot.