Which foam is hardest?

The hardest foam is typically high-density closed-cell foam like high-density polyurethane foam, high-density EVA foam, or rigid polystyrene foam with high ILD, with hardness measured by ILD or Shore hardness, similar to tough storage box material. Among soft foams for mattresses and upholstery, hardest is high-density, high-ILD polyurethane foam with ILD 50 plus and density 2.5 pcf or higher, used for firm support and orthopedic applications, feeling very firm with little sink. For rigid foams, hardest is extruded polystyrene foam or polyurethane rigid foam used for insulation and structural panels, which is very hard and not compressible, used for construction. For example, high-density EVA foam with Shore C hardness 60 plus is very hard, used for shoe soles and protective padding. For mattress, hardest foam is extra firm high-density foam with ILD 44 to 50, providing firm support for heavy individuals or those needing firm surface for back pain. So hardest foam is rigid closed-cell foam like extruded polystyrene or high-density EVA, and among flexible foams, hardest is high-density, high-ILD polyurethane foam with ILD over 50, providing firm, supportive, non-sinking feel, opposite of soft low-ILD memory foam.