R-30 insulation can be different thicknesses depending on the type of insulation used. Insulating fibreglass panels and rolls with an R-30 value are usually eight to nine and a half inches thick.
High-density R-30 insulation can be as thin as 8 inches, while standard density R-30 fibreglass insulation can be up to 12 inches thick.
The R-value of insulation is always tested, and the higher the value, the better it is at keeping heat out. With higher resistance values, heat moves through the insulation much more slowly. Insulation of R-30 or more is best for attics and rooms with high ceilings. Most attic spaces are big enough to fit R-30 insulation, which can be different thicknesses depending on the insulation type and where it comes from.
A good rule of thumb for figuring out how much insulation you already have in your home is to measure how thick it is. If the width is less than 11 inches, the property may need more insulation to get the resistance value to R-30 or higher.