There are two general categories of SPF insulation materials; open-cell, low-density (a.k.a. ‘half-pound foam’) and closed-cell, medium-density (a.k.a. ‘two-pound foam’). Both foam categories provide excellent insulation and air sealing. Although both utilize almost identical chemical reactions, there are some inherent physical property differences. These differences often determine the choice of product for a particular project.
Open-cell Spray foam
Open-cell spray foam has an open-cell structure where the cells fill with air. The open-cell structure renders soft, flexible foam, with a density of about 0.5-0.8 pounds per cubic foot. Still, the air is the primary insulation medium in Open Cell Spray Foam, fiberglass, and cellulose. These insulations work by reducing the natural air movement within these materials. Thereby, reducing the ability of the material to conduct heat. The R-value per inch of open-cell foam typically ranges from R3.6 to R4.5 per inch.
Unlike fiberglass and cellulose, the fine cell structure of Open Cell Foam makes it air-impermeable at certain thicknesses. The air-impermeability of Open Cell Foam qualifies it as an air-barrier material. It dramatically reduces air leakage through the building envelope, significantly lowering the building’s heating and cooling costs. Open Cell Foam, like fiberglass and cellulose insulations, is moisture-permeable. It may require the installation of a vapor retarder in colder climates.
Closed-Cell Spray Foam
Closed-cell spray foam has a closed-cell structure which yields a rigid, hard foam, with a density of 1.8-2.3 pound per cubic foot, and previously demonstrated a structural enhancement in certain framed buildings. These smaller cells trap an insulating gas, called a blowing agent. This blowing agent has a lower thermal conductivity than still air and increases the R-value. Typical R-value per inch of closed-cell foam ranges from R5.8 to R6.9* per inch, which makes it a great choice in applications with limited clearance.
Like Open Cell Foam, Closed Cell Foam is also air impermeable at certain thicknesses and can qualify as an air-barrier material. The closed-cell structure of Closed Cell Foam makes it water-resistant and is the only spray foam that can be used where contact with water is likely (e.g., below-grade concrete walls, in contact with the ground, or on the exterior side of the building envelope). At a thickness of 1.5 inches, Closed Cell Foam has moisture permeance typically less than 1.0 perms and no additional vapor retarder is required for most applications.