I want to comment about the ground temp comment. I'm not sure where you live, but around here the ground will freeze several feet, more if it is run over by vehicles. That is how they make ice roads in the winter. We have to build here to account for frost lines in excess of 2-3 feet. While we can receive some heat from the earth herself during the winter, it is not enough to keep a building from freezing. We need solar gain to keep the building warm. However, with that said, if we build a root cellar deep enough, it will keep our food from freezing during the winter and acts as a natural refrigerator during the summer. Thermal mass can do some amazing things.