Step 2: Insulated earthbag foundation
Build the insulated earthbag foundation using bags or tubes filled with scoria or pumice. Other fill materials could be used, but they’re still considered experimental. Scoria (lava rock) works perfectly for making insulated earthbag foundations, because it’s insulating, rot proof, fireproof, and doesn’t attract insects or pests. Scoria is also lightweight and easy to work with. It’s almost like handling bags of popcorn. Buy screened ½” scoria for best results. Place the bags or tubes directly on the gravel-filled trench. Check the radius to each bag so the foundation comes out perfectly round.
Use standard earthbag techniques: barbed wire between courses, tamp and level each course (only light tamping required for scoria), angle bag ends toward center and so on.