There are lots of options for building insulated earthbag houses. At this time, scoria and pumice are my favorite. These materials are fireproof, rot proof, easy to work with, don’t attract pests, lightweight, etc. Kelly Hart pioneered the use of scoria bags in his dome home and carriage house in Colorado, and in my opinion this method deserves much more attention.
My Instructable above describes numerous ways for building insulated earthbag homes, and we keep learning more good options:
Foam glass gravel
Expanded clay granules
Foamy geopolymer with perlite
Rice Hull Earthbag House (make sure to keep the rice hulls dry or they will rot)
Solar Pit House (uses recycled polystyrene)