Foam Density

Open-cell foams for insulation typically weigh 0.4 to 0.7 lb./cu. ft. Closed-cell insulation foams range from 1.7 to 2.6 lb./cu. ft. Roofing foams typically use 2.5 to 3.0+ lb./cu. ft. to better support traffic and loads.

The higher the density of the foam, the heavier and stronger it becomes. Some polyurethane foams are even molded into decorative interior moulding (around 30–40 lb./cu. ft.) and painted or stained for a simulated wood effect.

Density Is What Separates the Two Foams

Spray polyurethane foam comes in two families defined by density. Low-density open-cell foam is roughly half a pound per cubic foot, soft and flexible, with cells that are open to each other. Medium-density closed-cell foam is around two pounds per cubic foot, rigid and hard, with sealed cells. Almost every practical difference between the two products follows from that.

• Open-cell: roughly 0.5 lb/ft³, soft, vapour permeable
• Closed-cell: roughly 2 lb/ft³, rigid, vapour resistant
• Closed-cell delivers a higher R-value per inch
• Open-cell absorbs sound more effectively
• Closed-cell adds structural rigidity; open-cell does not

Choosing by Application, Not by Preference

Higher density is not automatically better, it is better for particular jobs. Below-grade walls, rim joists, metal buildings and any assembly where depth is limited call for closed-cell. Interior partitions, sound-sensitive rooms and many vented attic applications are well served by open-cell at lower cost. A contractor who only ever quotes one product is choosing for their convenience rather than your building.

What Density Means for Cost

Closed-cell costs more per square foot because there is more material in it and it delivers more R-value per inch. When comparing quotes, check the product density and the installed thickness rather than just the price, because those three numbers together determine what you are actually getting.