I have never used Dow Blue extruded polystyrene foam. I want to use it for part of my upperl level. This will be either 12 or 16 inches wide. I have one sheet of 1 inch and two sheets of 1/2 inch. I was wondering if it would work to make a fram of 1 by 1s with 1/4 inch plywood. then put the foam in that. Do you mount the raodbed right on top of the foam or do you need a sub roadbed.