Air vs water as insulation for my cold frame

In summary, the conversation discusses the best way to insulate a cold frame for winter gardening. Suggestions include using plastic or Styrofoam egg cartons along the outside walls, or using water-filled milk jugs to collect heat during the day. The experts suggest that air is the best insulator, and recommend using dry hay or foam panels for insulation. One person suggests using closed cell isso board to create a thermal barrier and utilize the natural temperature of the Earth.
  • #1
willskde
1
0
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.
 
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  • #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.
 
  • #3
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.
 
  • #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
 

Related to Air vs water as insulation for my cold frame

1. Which is a better insulator for a cold frame, air or water?

Both air and water have insulating properties, but air is generally considered a better insulator for a cold frame. This is because air is less dense than water, meaning that it is less likely to transfer heat. Additionally, air has a lower thermal conductivity, meaning it can maintain a consistent temperature for longer periods of time.

2. How does air act as an insulator in a cold frame?

Air acts as an insulator in a cold frame by creating a barrier between the inside and outside temperatures. When the cold frame is closed, the air inside becomes trapped and creates a layer of insulation. This layer of air helps to slow down the transfer of heat, keeping the inside of the cold frame warmer than the outside environment.

3. Can water be used as an insulator in a cold frame?

Yes, water can be used as an insulator in a cold frame. Water has a high specific heat capacity, meaning it can absorb and retain a large amount of heat. This makes it an effective insulator for maintaining a consistent temperature in a cold frame. However, water is not as effective as air due to its higher density and thermal conductivity.

4. Does the type of material used for the cold frame affect the insulation properties of air and water?

Yes, the type of material used for the cold frame can affect the insulation properties of air and water. For example, a material with a high thermal conductivity, such as metal, will transfer heat more easily than a material with a lower thermal conductivity, such as wood. This means that a wooden cold frame will provide better insulation than a metal one, regardless of whether air or water is used as the insulating material.

5. Is it better to use a combination of air and water as insulation in a cold frame?

The effectiveness of using a combination of air and water as insulation in a cold frame depends on the specific circumstances. In general, air is a more effective insulator than water, so using a combination may not significantly improve insulation. However, in situations where the outside temperature is extremely cold, using both air and water can provide an extra layer of insulation and help to maintain a higher temperature inside the cold frame.

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