Can't find the 'structures' subforum or your post. And don't have experience with greenhouses or hoops. But I do garden in the foothills, at about 1800 feet, on the north side of Los Angeles.

What's the question?

Does a structure need to be airtight? Probably not, depending on what you are growing and orientation (south or north slope? exposed to wind?).

Sorry I missed you post for a while there. My question came from the fact that I have a bunch of dual pane sliding door panels and I was wondering if it is worth the bother to build a small greenhouse with them for starting vegetable seedlings in the winter and early spring. I think I am going to let them go "free-cycle" and focus on more lightweight stuff like a low tunnel/hot frame hybrid with heat cables for germinating.

Thanks for responding.
Russell