Although there are many types of spray foam for different applications, they generally fit into two main categories: open and closed cell spray foam.
Open cell foam is soft to the touch when cured. This is because the tiny cells of the foam are not completely closed which allow air and moisture to enter. These open cells cause the foam to be permeable and lighter in weight and weaker than its counterpart while still delivering a high R-value per inch of thickness.
Closed cell foam cells are completely sealed and impermeable to air and water infiltration. This type of foam is denser and weighs more. When cured, it is hard to the touch and strong enough to walk on and provide structural strength to building materials. Where moisture is a problem, such as crawl spaces and basements, this is the preferred formulation of foam because it retards vapor movement. Closed cell has a higher R-value when compared with open cell due to its density.
In practical use, open cell insulation is usually used in interior walls or where there is no concern about moisture infiltration. It is also used for exterior wall cavities. Another use for open cell is under stairways.
The cost of each material can differ widely. Closed cell spray foam insulation requires more material due to its density and costs more per inch of thickness. Even though it provides more insulating value, it is still more expensive per R-value than open cell. When evaluating the different options for your project and moisture is a factor, then closed cell foam would be preferred. Similarly, in areas where there is a limited depth in which to install foam, a higher R-value can be achieved with closed cell foam vs. open cell foam even though the cost will be higher.