Is there any problem with installing 2" polystyrene on both sides of a poured concrete basement wall.
outside of wall is waterproofed. Inside is bare concrete. I had heard also that PL200 softens polystyrene, and that one has to use PL300 to attach boards to wall.
Considering the waterproofing on the exterior surface there should not a be a problem other than using the appropriate adhesive. The application would basically mimic the ICF (Insulated Concrete Form) systems that are currently in use to a very large extent in the building industry. There may be a challenge in fastening a wallboard finish product to the poly styrene. One might consider furring strips depending on the application.