The global model earthship has that very nice development of putting insulation behind the tires about 4 feet or so. This is very much needed in a cold climate like Mongolia where it can be -40F or below in winter. The rammed earth in the tires has thermal mass, but heat can escape beyond the tires. The frost depth in Mongolia is something like 3 meters deep. A water line has to be that deep to avoid freezing in the coldest times in winter. In Mongolia rivers are ice blocks in winter until early April. I saw some people, like Dan Richfield's Earthship Blog , put the insulation down to the flat level. I would put the insulation deeper than the flat level at the floor.
I wondered if straw bale could be substituted for expensive insulation. The obvious risk is that the straw gets wet and doesn't work as insulation and rots. While reading, somebody said that straw covered with clay, then 1cm coating of cement after a few stacked bales. If we did a thick wall of straw, coated and waterproofed with plastic over it all, that would have to be sufficient. Then I would do insulation on the top cap that runs back to the roof and completely cover the straw bale wall.
On the global model the roof is north facing. That means the water is coming back to the gutter into cisterns right near the straw bale insulation wall, so it must work. If the cisterns overflow, they should drain with a pipe to the front of the earthship, not into the cracks of the straw bale.
The savings on insulation should be substantial of a few thousand dollars. If we feel that the straw bales somehow failed, it is possible to dig up most of the wall and replace it anyway, since it's behind the house in the berm. So I feel confident on using this approach.
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