Insulthane 100 has an R-value of 3.23 per inch, the maximum. thickness of 5 Â½ inches yields an R-value of about 17.8. The ICC-ES report for Insulstar spray foam from NCFI (ESR-. 1615, issued February 1, 2005) lists a maximum thickness of. only 2 inches: âInsulstar Spray Foam Insulation is a spray-.