In article ,
Mike Miller wrote:
I've seen the thermal conductivities for dry sand (.35W/m*K, IIRC)
and soil (0.16), so I can imagine regolith to be a good insulator.

Lunar soil conductivity is estimated at 0.015-0.03 W/m-K at 1m depth.
Note that this is *very* well compacted, packed harder than could easily
be achieved with power machinery. The much fluffier top 1-2cm is around
0.0015 W/m-K. Regolith that was excavated and then loosely piled on top
of, say, a shelter would presumably be somewhere in between.

However, solid rock can have an order of magnitude higher...
So, would a habitat buried in a "big" (mountain-sized or more) cold
rock (200K, 1.5W/m*K) need supplementary cooling like a habitat
buried under lunar regolith?

My guess would be yes. If it's a substantial habitat, the rock in its
vicinity won't stay cold for very long.

Also, bear in mind that said rock will almost certainly be covered with
regolith. You don't generally find large masses of exposed rock on the
lunar surface.
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