R-30 insulation has varying thicknesses depending on the type of material and in what form it comes. The R-value of insulation refers to heat transference, or how quickly heat travels through the material.
Insulation is tested to determine its R-value, with a higher value indicating that heat moves more slowly through the material. Two different types of insulation with the same R-value should provide the same level of heat flow resistance even if the thicknesses are different.
Rigid foam insulation typically gives the best insulation in the smallest thickness, measuring up to R-6.5 per inch. A standard mid-performance roll of fiberglass blanket insulation measures up to R-3.8 per inch.