I'm in Alaska hoping to successfully create an earthbag floor/foundation for a couple room additions to our yurt (to build on using cordwood construction).  What materials would be best to keep the floor as cozy as possible?  Should I lay several inches of foam board down somewhere in the stack?  build an insulated wood floor on top of the uppermost earthbag?  Incorporate some type of insulation into the earth bag?  

Some concerns are: insulation degradation over time, humidity and moisture control in winter/ventilation, warmth, compatibility in being build next to a yurt whose platform is on pier blocks.  I'm building on ground level on top of gravel, so frost heaving isn't really a major concern I don't think.  What are your thoughts?  Thank you in advance!

For the floor I think a sandwich would give the most isolation from ground cold coming up through the floor.
Since the gravel is down you might want a layer of sand for protection of the insulation board.
The layers would be: 1 gravel, 2 sand, 3 vapor barrier (visquene), 4 insulation board, 5 subfloor bags, 6 insulation board, 7 floor surface.
The vapor barrier would need to come up the foundation inner wall surface to prevent any capillary action I would think.
Unless you want to have a step down for the addition, you will need to build up the foundation so the floors are level with the yurt floor.
If you are going for level floor, you probably could just lay the first 4 layers then build a raised floor with lots of insulation under it (between and under the floor joists).