What is the difference between regular insulation (batts or blown fill) and foam boards?
Batt and blown fill insulations (fiberglass, cellulose, etc.) are a more economical way to achieve insulation in wall cavities than foam boards. A common batt insulation used in the 3½” thickness of the cavity in a 2’ x 4’ wall structure has an R-Value of either 11 or 13. The same thickness of XPS foam board would yield an R-Value of 17.5 to 18, but at a higher cost. However, the rigidity and consistency of the XPS foam board, as well as its resistance to water, make it an excellent product for the outside of the wall, where even in thicknesses of ½” it adds an insulating R-Value of 3.0.