Crawlspace would be good but if it gets real cold in there in the winter that will be a problem. I had a grow in a basement and I built a small 6X6X6 room down there out of 2X4's and plywood and the used foil backed insulation to do up the walls. The foil side was for my reflective walls, and it insulated the room, and it sealed the room well. I used aluminum tape to seal all the joints in the insulation. Kept all the stink in too And my lights were plenty to keep it warm enough in the room, even with ventilation.