Insulation thickness to reach the R-30 value depends on the insulation material, but fiberglass batts of R-30 insulation are 9 1/2 inches thick. Rigid foam insulation provides the same R-factor with approximately one-half the thickness.
Blown-in insulation conforms to irregular spaces and fills areas around pipes easily, but blown-in fiberglass insulation should be 12.3 inches thick. Blown-in cellulose requires 8.4 inches to provide the same amount of insulation.
In walls, the available space limits the amount of protection insulation provides. Standard 4-inch walls limit insulation to R-13 or R-15 batts, while 6-inch walls allow for installation of R-19 or R-21 products.