Determine the square footage to be covered with insulation by measuring the height and width of the surface and multiplying those numbers. An 8-foot by 10-foot wall would be 80 square feet and would require two rolls of insulation at 40 square feet each.
Rolls of insulation come in mineral fibers such as fiberglass and rock wool, divided into pre-sized segments called batts. The rolls are available in different widths to fit standard wall stud and floor joist spacing. A 2-inch by 4-inch stud wall can hold R-13 or R-15 batts. A 2-inch by 6-inch stud wall can use R-19 or R-21 batts.