I am building an earthen floor RMH. What are my options for insulating the mass from Mother Earth on the cheap? Foam? Can it handle the heat? Sawdust?? Air space? Is heat loss to Mother so great that she steals the heat continuously or would she eventually warm up to be a helpful heat sink and return heat to me? Perimeter Insulation? Infloor exhaust pipes combined cross sectional total should have same cross sectional areas as heat riser ect? I am thinking of more than one exhaust pipe. I am located in the windy wet PNW coast of Canada. Oh boy! the questions keep coming!!
It's true that the capacity of the Earth to take away heat is in all practical considerations infinite. But on the plus side, heat rises, so when you run hot exhaust gases through in-floor pipes, you have a classical Roman hypocaust or what the Koreans call ondol. I don't think they got too worried about heat going down and not up.
As long as your design does not have a thermal connection to something like a seam of permafrost, you should be OK.