The framing you use to construct a greenhouse is not quite as important as the glazing material that you use. You can use wooden, aluminium or galvanized steel framing if you are particular about aesthetics. Wood is most durable in the long run. PVC is used in hoop houses for its ease of handling, low cost and flexibility.
For glazing, glass is the classic choice. You have durable tempered glass that is strong and suitable for use in greenhouses. Glass allows about 94% light transmission and is practically maintenance free. An additional advantage is that as you put it up in the form of panels, in case of damage you need to replace only the damaged panel. It is fragile to transport though.
Plastic films are a good choice if you want to construct a greenhouse on a tight budget and your design requires flexible and lightweight glazing material. These include polyethylene, PVC and copolymers. Polyethylene is the cheapest of the lot, but lasts about five times less in comparison, with an average life of a year. A new option that is gaining ground is UPVC which comes as prefabricated units ready to be bolted together.
Though you can use ordinary polyethylene, it would be better to purchase film especially designed for use in a greenhouse. These are sturdier; less prone to tearing and damage by the sun. They are coated with an anti-drip material that discourages condensation on the inside which could damage plants by causing fungal infections when water drips onto the plants.
Polycarbonate sheets as glazing are an excellent choice for their durability, life span, light transmission, and ease of transport. They are strong with high impact value, shatterproof, hail proof, have good load bearing capacity, are less flammable than plastic, and not as rigid as glass. Polycarbonate sheets for greenhouses come with a non-drip coating to prevent condensed moisture from dripping onto plants. There is only one catch: these sheets contract and expand with change in temperature, so installation must be done keeping this in mind.
Acrylic sheets are also used as glazing material though they are losing the battle to polycarbonates and plastic films. Acrylic sheets have a long life, but tend to yellow with age reducing light transmission. Compared with polycarbonates these are more flammable, are not hail proof and cost more. They are easily scratched and gather grime and dirt more easily. These also tend to contract or expand with change in temperature.
Fiberglass is made by coating glass with plastic resin to create FRP or fiberglass Reinforced Plastic. Though long lasting, the coating eventually erodes leaving the glass exposed to dirt and grime. It is possible to get the material recoated, but this requires a great deal of labor.
When you construct a greenhouse remember that your choice of glazing is a critical factor in the performance of your greenhouse. So, choose with care!
