R-38 cellulose is 10.77 inches thick; R-38 fiberglass is 12 inches thick. Fiberglass weighs a lot less than cellulose, which isn't a factor in an attic but could be a Consider a garage, such as. Cellulose and fiberglass have several similar attributes, but at the conclusion of the day, it comes all the way down to a homeowner’s choice. When you�