I hope he does something to prevent this stuff from burning. Stucco, cob ...or whatever. Fiberglass, cellulose and rock wool insulation are all available for free from demolitions and renovation projects. All could be gathered in one visit. Always check for asbestos and vermin poop contamination. Always wear a mask. These materials don't gass off like styrafoam does and they don't burn. Rock wool and cellulose are both sometimes applied wet with a blower to fill odd spaces. They are combined with a cement binder. Clay would work if wire were suspended a few inches out from the steel walls. Condensation would be an issue in most climates.
this is supposed to be a surprise, but it smells like a tiny ad:
Switching from electric heat to a rocket mass heater reduces your carbon footprint as much as parking 7 cars