cob foundation insulation

anybody know a good way to insulate a rubble trench with dry stacked urbanite? the structure will be round and about 150 sq ft. walls will be load bearing and about 18 in thick at the base. located in north central maryland so we do get a bit of a winter and a hot humid summer. the cob floor will have rigid foam insulation but i guess i cant use that for the foundation due to the shape of the building. can i lay a course of something between the interior and exterior urbanite like pumice or vermiculite or pieces of foam board? or can i put something vertically into the ground and over the interior and/or exterior urbanite and plaster over it?