How many inches is R49 blown fiberglass insulation?

R49 blown fiberglass insulation typically requires about 16 to 18 inches of depth when installed properly at the correct density, though the exact thickness varies by manufacturer and product density. For example, Owens Corning AtticCat and similar loose-fill fiberglass products achieve R49 at around 16.25 inches when installed at 0.42 pounds per cubic foot, while other lighter fiberglass products may need up to 18 or even 19 inches to reach the same R-value because they are fluffier and less dense. It is important to understand that blown fiberglass does not have the same R-value per inch as cellulose, fiberglass achieves about R-2.5 to R-2.8 per inch in blown form, so it needs more depth than cellulose at R-3.2 to R-3.8 per inch to reach R49. Installation should be measured with a ruler in multiple spots after fluffing, and you should not compress the fiberglass because compression reduces R-value dramatically. Always check the bag chart that lists installed thickness, coverage, and minimum bags per 1000 square feet to meet the stated R-value for warranty and code compliance.