Howdy Kevin and everyone else,
This is an interesting conversation and I'd like to add my thoughts because I'm getting ready to create a greenhouse on the land I take care of. However, as an aside and still germane to this prospect I'd like to broach two points. I have always appreciated Mollison's teachings; they are always design experimentation, to see what works and share that, as in were all in this together, here are my results. This is what, as I see it, Permaculture Institutes do. Secondly, since there are many different people and situations we can and should build what we want, what suits our goals, climate, personality, income etc. best. To that end, people being creative, build many different styles of greenhouses, buildings, cultures etc. For me, usually I think this is rather a good thing.
The greenhouse I will create, with a little help from my friends, no scratch that, a lot of help from my friends, will be 24' by 12' on a hillside slope to the South infilled with strawbales on the sides and back on grade. Then we're going to cover it with 6mil clear plastic. We'll see how much this simple, inexpensive greenhouse will perform. A last point I would like to note is, changing one's climate several zone's, usually means more expense and complex inputs, however I have seen a basic "heat pump" idea that seems to me on the face of it gets you there, "Nebraska retiree uses earths's heat to grow oranges in snow" on YouTube; It's worth the time. Last point, I like small gravel on the floor of greenhouses. Best of luck with your project.