Hi, new here, with bits & pieces of basic college physics rattling around in my brain from years ago, so explain things at that level, please. I am building a cold frame in order to try growing food through the winter in NJ, USA. It will get below freezing here at night but not every day. I want to attempt to slow the cold & frost from creeping into my cold frame through the soil, or rather, my heat from escaping. I can dig down up to 6" or so and so far the walls of my cold frame have a thin (1-2mm) wood wall down that far (the walls above ground are thicker). I was thinking of putting EITHER (and here's where you all can help me decide) plastic and/or Styrofoam egg cartons down in the dirt along the outside of my wall, covering them with a little dirt and maybe hay or bricks/rocks, creating insulation... OR plastic half gallon milk jugs filled with water, partially above ground and painted black so that they can warm up during the day. Some people put them inside the structure to collect heat that will last into the night, and I will do that, too. My concern here is the soil. So... which is a better insulator, air or water, considering the water may get quite warm yet freeze at times. Thinking of igloos, won't a frozen wall of ice also insulate? If the ice block doesn't get much below 32degrees itself I don't think it would freeze the dirt touching it in any reasonable time (considering the sun will come up eventually and start warming it again). Would milk jugs of air be better? Or piles of hay? Or milk jugs of hay?? Thank you.