Hi Sarah, I can't belive nobody replied to your message yet. I have been thinking about building a greenhouse for years.
PVC might be too brittle in the cold... depends where you're doing it.
The easiest (most cost effective) thing I could think of was used windows (The old wood sash kind, not the flat aluminum storms). They would probably work good for walls, but not for the roof (remember it's NOT safety glass). Corrugated Fiberglass or some type of greenhouse Lexan or Plexiglass for the roof. Right now I'm working out a prototype. I just found a source for windows (from a local remodeler, he was taking them to the landfill) and he dropped off 36 windows...... So it looks like I better get hopping with that design
I'm thinking of a wooden frame construction (based on a shed I put together once upon a time...)
Then I might try a channel (H) stud to hook the windows in vertically and figure out a way to seal them. Copper flashing would be a nice touch to seal the horizontal seams, if it's not too pricey.... Anyway, I'm no expert, but it seems like it'd work to me.... I'd like to hear (or read) what you think. Good Luck! Don't give up. Keep me posted. L8R