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Open-cell foam is soft - like a cushion or the packaging material molded inside a plastic bag to fit a fragile object being shipped. The cell walls, or surfaces of the bubbles, are broken and air fills all of the spaces in the material. ... Closed-cell foam has varying degrees of hardness, depending its density. A normal, closed-cell insulation or flotation polyurethane is between 2 and 3 pounds per cubic foot. It is strong enough to walk on without major distortion.