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Air vs water as insulation for my cold frame

  1. Nov 18, 2015 #1
    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?? o_O
    Thank you.
  2. jcsd
  3. Nov 18, 2015 #2
    Welcome to PF!

    On the question of insulation, air is a far better insulator that either water or ice. In point of fact, it is the standard to which all other insulating materials is compared. The U. S. Army Quartermaster Corp equates their standard unit of clothing insulation (1 clo) with the insulating value of a quarter-inch of still air. Fresh snow, because of its air content, is a moderately good insulator. Igloos are only temporary shelters, never permanent ones. They are abandoned when the snow in the structure gets too icy. In terms of your examples, egg cartons and hay sound good, bricks and rocks do not. Loose dirt is questionable--it depends upon the air content. The basic purposes of a cold frame are to reduce air circulation and too rapid thermal conduction. When you think of insulation, think of down and fiberfill, not wet socks and cold stone floors.
  4. Nov 18, 2015 #3


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    Staff: Mentor

    Welcome to PF!

    I'll take that last post a step further: the vast majority of insulation is air. Insulating products (like styrofoam) are just fancy containers of air, who's job is to contain air without interfering with its ability to be insulation and disrupt air's main downside as insulation: convection. So, egg crates are good (albeit a bit thin) as is [dry] hay.

    Or, you could go all-in and use actual 3.5" foam panels to make the whole thing. They sell them at most hardware stores.
  5. Nov 18, 2015 #4
    Sounds like you want to build a green house that captures the daytime sun, have it absorbed by the ground and use the flywheel effect to cost through the night! The top 3 to 4 feet of earth will allow easy transmission if you don't put a thermal barrier in the way. I would suggest you install 2 inches of closed cell isso board to create that barrier and hopefully have the thermal mass and the natural 50 degree earth temp in moderate climate zones.

    Sound good to the real scientists? Mike
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