Light penetration is initially as good as glass but can drop off, considerably over time with poor grades of fiberglass. These covers have two layers of rigid plastic separated by webs. The double-layer material retains more heat, so energy savings of 30 percent are common. Film plastic coverings are available in several grades of quality and several different materials. Generally, these are replaced more frequently than other covers.
Structural costs are very low because the frame can be lighter and plastic film is inexpensive.
A utility grade of PE that will last about a year is available at local hardware stores. Commercial greenhouse grade PE has ultraviolet inhibitors in it to protect against ultraviolet rays and it lasts 12 to 18 months. Copolymers last two to three years. The heating requirement of a greenhouse depends on the desired temperature for the plants grown, the location and construction of the greenhouse and the total outside exposed area of the structure. The heating system must be adequate to maintain the desired day or night temperature.