Another tip... most of the cold air comes into the greenhouse at ground level. If you build up a low wall of wood or straw bales or other insulation (outside or in) it will help. My greenhouse stays about 10 degrees warmer than outdoors.
A friend spreads a layer of chicken manure and covers it with mulch (dirt floor of course). The manure generates sufficient heat for his wifes' tropical plants. I daresay she doesn't putter in there though. lol
