Greenhouse framing takes second billing to the material used as siding, but an unsuitable frame material can mean and unsteady frame that will never really be square. Aside from the headache that will give you with assembly, it will also result in drafts and openings for pests and critters.

Many times you don't get a choice of framing, but here are some considerations while you are shopping around for a greenhouse.

Current framing materials include: wood, aluminum, galvanized steel and PVC.

Wood

Wood, like glass, is beautiful but impractical, unless what you’re really looking for is a sunroom. Wood scores well on insulation and ease of assembly, but greenhouses are wet, damp spaced and wood warps and rots. If you must have wood, opt for redwood, cedar or specially treated wood and apply a seal.

Aluminum

Aluminum is low maintenance: it doesn’t rust or break down from exposure to the elements. But it’s not very strong and supports must either be large or doubled up. But it provides a good rigid form for glass or polycarbonate and can be painted or anodized in any color.

Galvanized Steel

Galvanized steel provides durability at a low cost. There’s less physical framing needed, because of it’s strength, so there are less shadows projected into the greenhouse. However most steel frames are designed to be used with polyethylene film. They’re popular with commercial growers and I’m not sure why they don’t make steel frames that can handle the more rigid polycarbonate panels.

A big negative for galvanized steel is that the galvanizing will eventually wear and the steel will rust.

PVC Plastic

PVC Plastic is low cost, lightweight (portable) and easy to assemble. It’s not as rigid as metal or wood, but the industry is addressing that with metal supports used in conjunction with the PVC. Plastic is also offers limited heat loss. Most hobby greenhouses now come with PVC frames.
The biggest negative for PVC is that sunlight can deteriorate it. But a UV protected frames should last at least 20 years. Another drawback is that the frame is large, to compensate for it’s lack of strength, and can cast shadows.