How many inches of fiberglass to achieve R49 depends on the form of fiberglass and its installed density, because loose-fill and batt fiberglass have slightly different R-values per inch. For blown loose-fill fiberglass, which is the most common way to achieve high attic R-values, you typically need 16 to 18 inches to reach R49. For example, Owens Corning AtticCat requires about 16.25 inches at 0.42 pounds per cubic foot to reach R49, while lighter products like John Manville Climate Pro may require about 17 to 18 inches. For fiberglass batts, which are denser, R49 is not available as a single batt for a 2x4 or 2x6 wall, it is achieved by stacking batts, such as an R-30 batt at 8.25 inches plus an R-19 batt at 6.25 inches, totaling about 14.5 inches in an attic floor. High-density R-30 batts alone are 8.25 inches thick and only R-30, so you would need additional layers. It is critical to measure depth in multiple places with a ruler and also verify bag count per 1000 square feet per manufacturer charts, because fiberglass that is fluffed too much may look thick but fail to meet density and R-value requirements.
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