What is the difference between ½ pound foam and 2 pound foam?

The raw materials of both types of foam are nearly identical. Both 1/2-lb and 2-lb SPF are made from blended systems of polyol resins, catalysts, surfactants, fire retardants and blowing agents on the B-side, with polymeric MDI (methylene diphenyl diisocyanate) on the A-side. The difference between SPF types isin how these materials are formulated. The main differences between them are how dense they are, their R-values, cost, use as a vapour barrier, water permeability and tensile strength. Closed cell foam is the denser formulation of the material. The denser the physical properties, the greater the benefits. Both ½ lb and 2-lb SPFs have air barrier qualities that can help reduce noise from outside the building envelope (i.e. airplanes and car traffic). The ½ lb foam’s density offers additional sound absorbing qualities;however, neither foam is exceptionally effective at reducing vibrational impact noises.

½ lb SPF has a high open-cell content (greater than 50 percent) and liquid water can enter the foam. Conversely, 2-lb SPF has a high closed-cell content (greater than 90 percent) and resists water absorption. In a building assembly, the latter SPF offers added weather or rain barrier protection. Closed cell foam is a vapour barrier at installations of 50mm or more. Closed cell foam is rigid in strength, whereas open cell foam is spongy. Most open cell foams only perform at a maximum of R3½ per inch. Closed cell foams are better for outdoor use and in crawlspaces. As of January 1, 2010 both types of foam promote 0ozone depletion.