One of our most interesting projects this winter is watching the effectiveness of the "figloo" or insulated box that I built around the eight greenhouse fig trees. It is a very simple design... 35 bales of hay laid on the ground pushed tightly together to form a rectangular "well" forming walls around the fig trees. On top of that I placed 10 sheets of inch thick 4x8 insulation boards, their ends resting 6 inches onto each wall, with a prop holding up the center so the insulation doesn't sag in the center (you can see the 2x4 board in this picture). Rocks hold down the edges of the insulation boards to keep a pretty tight seal against the bales. In the picture above I've got both ends of the insulation "roof" propped up to inspect underneath. Normally these are down to keep the "earth heat" in. Earth heat being the main driver of heat gain inside the figloo. Similar to a basement under a house, the earth always stays warm (around 50 degrees F) due to the thermal mass of the planet. As long as the soil of the interior of the figloo doesn't freeze solid. The ground will continue to radiate "earth heat" into the figloo chamber. If you consider on this day that exterior to the greenhouse there was a -30 degree F wind chill (yes NEGATIVE 30 degrees), and inside with the figs it's a balmy 31 degrees F, the figloo is holding a 60 degree difference between outside and inside!