Presuming that there is an even weight distribution across both the upper and lower surface there will be one pound on each square inch on both the upper and lower surface. The force will follow the direction of gravity so this assumes that both surfaces are parallel and perpendicular to gravity. For practical purposes as long as there is enough force to hold the sheeting in contact with the foam, at least as long as you don't exceed the crush strength of the materials, then you have enough.