Unique properties of high light transmission and allowing in solar heat energy, but trapping heat re-emitted from soil, structure and plants makes glass the best covering for greenhouses in most cases.
However,
glass is expensive, heavy, and brittle. It can also cause
injuries. Safety glass
can be used, but this is especially expensive and heavy. As
a compromise it is often used for doors and panels where limbs
could come into sudden contact with the glass. Plastic materials
can offer an alternative.
Double-layered polycarbonate is costly and transmits slightly less light and does not trap re-emitted warmth. It breaks down in sunlight over the years and is more prone to scratches, but makes a rigid, well-insulated, safe, and vandal proof covering, that gives almost as good results as glass.
Single layered sheets of polycarbonate or acrylic sheets may flex, loosen and blow away in gales. They are poor insulators, although relatively inexpensive.
Robust, smart, polythene covered polytunnels (above) are sold for garden use, and can be covered with films treated in ways that make them nearly as efficient as glass. Tunnels offer an economical alternative to unheated greenhouses.