If I were building a new greenhouse I would definitely insulate at least the north wall. According to Eliot Coleman ("Four Seasons Gardening," among others), each layer of cover raises the environment to the next zone up... I'm not sure that's strictly true, but it's probably close.
Interestingly, in my hoophouse, which is covered with a layer of 3-mil plastic, I have lettuce, chard, spinach, broccoli, turnips, kohlrabi, mache, chives and parsley, covered with Reemay over fiberglass hoops inside the HH, and even though we've had 2 nights in a row with a temp of 8F, and many nights that went down to about 20F, everything is still doing fine. (We also haven't had a heck of a lot of sunshine to warm things up recently) So, while I definitely didn't get some of the things started nearly early enough, I consider this to be a total success... my goal was simply to be able to harvest a few things for a salad by Christmas, and I'm feeling pretty confident now that I'll at least come close to that. Then there will be a couple of months off, and sometime in February I'll be able to start planting a few things again. (at least this way I
feel like I'm still gardening...
)