hey i finnally get to build a greenhouse/sunhouse. it will be 12 ft by 12 ft. Going from 8 ft in the back to 6 ft in the front. the back and at least one side will have plywood for a wood. the top and front will by a double layer of 4 mil plastic stuff. how many btu's do u think it will take to heat it. it can get to 0 degrees F. pretty often here. i want it to be a warm house as decribed in the Savage Garden. staying at least 50 degrees F.