Dirt has an r-value of between 0.25 and 1 per inch according to some quick googling, so 10ft would be maybe R-60.

Surprisingly, that's only equivalent to about 10" of polyiso rigid foam.

What this project is really taking advantage of is the super cheap thermal mass. Dirt has about a quarter of the specific heat of water, but it is, literally, dirt cheap, and much easier to keep in place than a liquid.

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ted_dunning 2 days ago | parent [-]

You can also have a temperature cycle of 600C with dirt. Water is limited to 50-70C so the dirt loses a factor of 4 in specific heat, but wins back a factor of 10 due to delta T.

The net is dirt wins by a factor of 2.5.

	▲	pfdietz 2 days ago \| parent [-]
		The thermal conductivity of rocks (and, I presume, soils) also typically declines as temperature increases, falling by a factor of 2 or 3 at 600 C.