R-values, like R-38, are used by the construction industry to describe the thermal resistance of insulation materials, like those used to insulate your attic. When you are looking at an R-38 insulation, you are actually reading a description of its resistance, rather than size or thickness of the material. Determining how thick R-38 insulation is will depend upon not only its R value, but also other factors such as size, use of material, and weight.
To determine the R-value of a material, a range of values are taken into account, including the fabric of the item, the density of the material, and its overall thickness. The R-38 fabric is usually a fiberglass or similar material, although it can also be made from cellulose, which is shredded paper that is tightly sealed into the area being insulated.
Thickness of R-38 Insulation
The thickness of your R-38 insulation will depend on the material you have, the brand, and how it's installed. The fiberglass comes in rolls, also known as batts, which vary in size. An R-38 fiberglass batt should be approximately 12 inches thick, while the thickness of cellulose insulation of a finished, fitted product will larger.