What if I used an earthen plaster on the interior walls, so the earthbags could breathe that way?
I've seen a method of earthbag construction where the earthen plaster is applied at the same time as the earthbags are laid (like, a few people are laying higher courses while other people start throwing mud on the lower courses. The idea is that even though the bags have to cure, leaving the other side of the bags unplastered for a few months allows the moisture to escape that way, so it doesn't matter that the plaster's applied 'too soon' on the first side.
By the same logic, even if the earthbags couldn't breathe against the spray-foam insulation, if they could breathe against the interior earth plaster, wouldn't that be fine? Obviously we would cure them before applying at least one of the layers.
My current thinking is that I like the look, cost, DIY-ability and thermal mass of earthbags, but am worried about their lack of insulative properties. I'm pricing up bulk pumice, but I suspect it'd be too expensive to use to fill the bags. (Haven't priced up scoria yet, but again, I suspect where I live it wouldn't be financially feasible; likewise vermiculite, and I don't think we could even get rice hulls here at all.)
So if I could do earthbag walls and spray insulation over the outside, and then cover that (quickly, so it wouldn't be exposed to heat and light for too long) in plaster/stucco, possible with a stone facade for the bottom few feet... well, that could work well. Assuming it does, uh, work at all.