Twin Size 4 Inch Firm Conventional Polyurethane Foam RV/Truck Mattress Bed Cushion USA Made One four inch firm slab of foam 33lb conventional polyurethane 4" layer of Firm Superior Support Foam for spinal alignment and transitional support. Our firm conventional polyurethane foam features a 33lb compression. This firm conventional polyurethane foam slab is a supportive cushion reducing pressure points. 30 NIGHT SLEEP TRIAL! 3 Year Manufacturer Limited Warranty! Not For Home or Mobile Home Use.