The thickness of R-30 insulation varies, depending on the type of insulation used. Fiberglass panels and rolls with an R-30 insulation value have a typical thickness of eight to nine and a half inches.
High-density R-30 insulation can be as narrow as eight inches while standard density fiberglass insulation with an R-30 rating can be as thick as 12 inches.
Insulation is always tested to determine its R-value, and the higher the value the higher its thermal resistance. With higher resistance values, heat travels much slower through the insulation. Attics and rooms with high ceilings will benefit most from R-30 insulation or above. Generally, attic cavities have enough space to accommodate R-30 insulation which varies in thickness between supplier and insulation type.
A good rule of thumb when determining the value of existing insulation within your home is to measure the thickness. If the width is less than 11 inches, the property may benefit from additional insulation to bring the resistance value to R-30 or more.