With the windows closed, I stay at least 10 degrees warmer than the outside (I put a thermometer inside just out of curiosity). I agree with TeamWISCO, just keeping the wind off you makes a huge difference.
My Dad uses a propane heater in his stand, no insulation. He uses 1 lb propane cylinders, and he has to turn the heat off at least part of the day, or it gets too hot.
Good luck with your stand. Have it ready install in the early spring, and the deer will be well used to it come November.