Hi Dionysus. Sounds like your GH is exactly half the size of mine and same height. My winters have been down to 10F so far (5 years). I insulate with bubble wrap. I have 2 1500 Watt heaters, and can barely maintain 60F with baffling on all vents in winter. Check out my picture site of my GH. You'll see the baffling in some of the last few pictures.
Temporary Greenhouse Photo Gallery by Admiral Schnitz at pbase.com