So, been back and forth, playing with ideas... I think I have a vague plan now! I find when building I don't give myself too much of a plan so I can deviate as things progress haha. Shouldn't take long to get this one built though, I have some friends coming to help out until it's done!
Our first greenhouse is getting built next week starting around 1-18-14 ... I decided to go wood frame on it, skipping PVC altogether. The price point is pretty low for doing it all framed with pressure treated 2x4's and 5/4 decking. We can't afford the polycarbonate roof that I wanted right now, but by the time the film is ready to be replaced we should be able to replace the roof with something like that. The size we have to work with on the terrace we carved out of the hill is 14x36 so that's a pretty decent little greenhouse. Not huge, but big enough to do something in

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The design I have in mind is 2x4 framing on top of a 5/4 decking bottom plate, all pressure treated. This will allow us to put plastic on the inside and the outside for insulated air gap without too much trouble AND be really easy to go ahead and put the polycarbonate on when the film fails.
Right now I only have 1 more concern and that is what to do about the 5/4 decking for the bottom plate. It will not have direct DIRT contact as we will have a layer of our really nice weedproofing fabric down, but it will have contact with the ground... I don't want this to rot prematurely, so I'm considering putting the entire thing up on cement cap blocks so it'll be separated from the ground.
I'm wondering if anyone has ideas of what I can do to further prevent this deck board from rotting? I can't afford and can't even buy high grade pressure treated wood here, looks like the best is "grade 2" which clearly states on the tag "not for in ground use"... I'm thinking a routine spraying of thieves oil or something similar to keep it from getting mold? any ideas?