I had been planning on doing a whole concrete foundation to get insulation under the whole living space including the tire walls. After playing with the EnergyPlus Ground Temperature Calculator I discovered that even here with permafrost down around 5-15m, that with no insulation under the center a living space a slab or floor directly on the ground (mass) is always warmer than an insulated floor in a house with a room temperature around +20C. Wow. The insulation under the floor really functions to protect the floor around the edges. Most of the heat loss is not straight down, but out the edges of the slab or floor on the ground. Even just 1 meter around the edge was fairly good performance.
I realized that I was planning on a slab, but now only 1-2 m of the slab edge really needs insulation, not the center. I discovered that exterior horizontal insulation will do the trick. All we have to do is put insulation down to the first course of the ground of the first tire course and go out horizontally about 1 ft. underground. Here 2m is probably enough. It doesn't even have to be that thick. This is also called frost-protected shallow foundation. This way we can make an earthship in the coldest city in the world and use standard procedures to build o the ground, like doing tire walls directly on dirt. The external insulation acts like the slab or floor is larger, so the heat has much further to travel outside of the house. The house itself has insulation on the walls.
This would drastically save costs unnecessarily. The mortgage bankers won't like this if people can build decent houses even in cold places without the need to do expensive insulated slabs, and news will not be allowed to get out about this fabulous technique. We don't have to dig deep 2m deep trenches around the perimeter of the house, which would disturb earth to lay tires on.