Although open-cell foam costs less than closed-cell foam, it has a lower R-value per inch, so a thicker layer is required. If the framing members are deep enough to accommodate your required R-value, open-cell foam may end up costing less.
Half-pound foam, also known as open-cell foam, has a density of about 0.5 lb. per cubic foot and an R-value of 3.5 or 3.6 per inch.
Open-cell foam is relatively vapor-permeable. Three inches of open-cell foam have a permeance of 16 perms.
Some of the low-density foams are made in part from bio-based raw materials — for example, soybean oils — in place of a portion of the petrochemicals. Open-cell foams use water or carbon dioxide as the blowing agent.
Compared with closed-cell polyurethane, open-cell products use significantly less material, making them attractive from a resource-use standpoint. However, open-cell foams have a lower R-value per inch than closed-cell foams.
Open-cell foam often requires an interior vapor retarder. When used to create a cathedralized attic in a cold climate (climate zones 5 and higher), open-cell foam should always include a vapor retarder (for example, a layer of gypsum wallboard finished with vapor-retarder paint). Recent research has shown that vapor-retarder paint is ineffective when sprayed directly onto cured foam insulation, so cold-climate builders who don’t plan to cover the spray foam with a layer of drywall should stick with closed-cell spray foam.
Open-cell foam is riskier than closed-cell foam when it is installed on the underside of roof sheathing. Evidence is accumulating that roof sheathing can get wet when open-cell foam is sprayed directly against the underside of roof sheathing. For more information on this problem, see Open-Cell Spray Foam and Damp Roof Sheathing.
Closed-cell foam isn’t cheap, but it provides a much higher R-value per inch than open-cell foam. Because of its density and glue-like tenacity, it also adds structural strength to a wall, ceiling, or roof assembly.
Two-pound foam, also known as closed-cell foam, has a density of about 2 lb. per cubic foot and an R-value of 6 to 6.5 per inch. Two-pound foam is significantly more expensive than half-pound foam.
Closed-cell foam is a vapor retarder. Two and a half inches of closed-cell foam have a permeance of 0.8 perm.
The blowing agents in most types of closed-cell spray polyurethane foam are hydrofluorocarbons (HFCs) with a high global warming potential. Because the global warming potential of these damaging blowing agents is 1,430 times more potent than carbon dioxide, many green builders avoid the use of closed-cell spray foam.