I think 2" styrofoam insulation is the best way to go.
It is comfortable,warm, doesn't take on odour it's airtight and mold proof. the list go's on.They insulate foundation walls with it. I used styrofoam blocks and poured the concrete in the blocks instead of using plywood foundation forms when I built my house. Saved my on the cost of damproofing the founadtion and polying and insulating the frostwalls.I used it throughout my entire basement,attic, bathroom.Man I come up with the greatest ideas. This advice is free as well.