@sinbad 2/2 Having tried using expanded polystyrene, that contaminates everything around it. Wherever rodents can get to it it's spread widely, even under ground. Rodents WILL eventually get to everything. Touching a board of polystyrene while working with it breaks off chunks.

Better to build good drainage, wide roof overhangs and isolate the foundation with sand. Probably costs much less and no worry about spreading microplastics at your home.

Plastics are designed to fail and contaminate.