R-38 fiberglass batt insulation is approximately 6 1/4 inches thick, but loose cellulose requires over 12 inches of material to provide the same resistance value. R-values measure the thermal resistance based on the material and its thickness.
According to the Federal Trade Commission, all insulation must tell its R-value unless it's sold for use on pipes and ducts. This value allows consumers to compare prices regardless of the kind of material the company uses. While R-38 insulation is acceptable for most of California, south Texas and Florida, most homes in the United States require R-49 insulation in the attic. Many older homes do not have adequate insulation, due to older rules regarding the amount of required insulation and due to the settling of materials.