Although glass is the traditional material for enclosing greenhouses, plastic is now often used because glass is expensive, heating a plastic-enclosed greenhouse costs less, and plastic doesn't break. The types of greenhouse plastic are classified in various ways.
Film-plastic coverings for greenhouses are generally lightweight and inexpensive, and they let in almost as much light as glass. They also must be frequently replaced. The most common types of greenhouse plastic films are made of polythene (PE), polyvinyl chloride (PVC) and copolymers.
Polythene plastic is available in either utility grade (lasts about a year) or commercial grade. The commercial grade contains ultraviolet inhibitors to protect against ultraviolet (UV) rays and lasts up to 18 months. Polyvinyl chloride costs two to five times as much as polythene but lasts up to five years. However, PVC requires regular cleaning to prevent the build-up of dust from the air.
Copolymers last two to three years. They contain additives that block and reflect radiated heat back into the greenhouse (in the way glass does), which helps reduce heating costs.
Fibreglass-reinforced plastic lets in the same amount of sunlight as glass. However, fibreglass-reinforced plastic has become less popular because it degrades easily from ultraviolet (UV) light, dust and pollution, and it is flammable, potentially increasing insurance rates.
Double-wall plastic includes rigid double-layer plastic sheets of acrylic or polycarbonate. Acrylic and polycarbonate types of greenhouse coverings have almost completely replaced the use of fibreglass-reinforced plastic because they are lightweight and easy to install and provide excellent heat insulation and good light transmission. However, they are flammable. They both have a long-life (at least 10 years). Acrylic does not yellow, while polycarbonate normally does.
Different types of polycarbonate are available. Corrugated polycarbonate has almost the clarity and light properties of glass, is relatively inexpensive, lightweight and easily installed. Multi-wall polycarbonate is stronger, resisting the impact of hail, and insulates almost as well as insulated glass.
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