Spray foam insulation, because of its high recycled content and superior insulating qualities over loose fitting batt insulation, is being used with increasing frequency. It is even more common when a project is seeking to meet objectives outlined by sustainable building programs, such as LEED.
Spray foam insulation comes in two different flavors: Open cell, and closed cell. These two different types of spray foam insulation are composed of different materials which react and expand differently in air, resulting in the different cell structure. In addition to being a better insulator, both types seal penetrations air-tight. This is helpful acoustically as it keeps unwanted sound from being transmitted through the wall system. However, these two types of insulation have many differences as well. Open cell foam is more expansive and can fill a ten-inch cavity in one application; closed cell foam is less expansive and is limited to two to three inches in thickness per spray treatment requiring multiple applications to fill larger cavities. The difference in expansion results in closed cell insulation being denser (2 pounds per cubic foot) than open cell (0.5 PCF). The higher density results in a higher R-value per inch of thickness.
When cured, closed cell insulation becomes rigid and does not allow sound to interact with the cell void in the [closed cell] insulation, and in the process absorb sound energy. Open cell insulation on the other hand, when cured remains flexible. The open-cell molecular structure allows sound energy to interact with the cell structure of the insulation and by physics of friction lose its energy to heat. This loss of sound energy results in an increase in sound transmission loss of the wall system when compared to an equivalent wall system that is either un-insulated or insulated with closed cell spray foam insulation.
Ultimately, the cost of spray foam insulation is higher than more conventional forms of insulation. However, it has many benefits and, when the correct kind is specified, could help your next project be a success.