How many inches of blown-in insulation you need in your attic depends on the material type and the R-value target for your climate zone, because inches translate differently to R-value. The Department of Energy recommends attics be insulated to R38 to R60 in most of the United States, with R49 being the standard target for new construction in zones 3 through 8. For blown-in fiberglass like Johns Manville Climate Pro or Owens Corning AttiCat, which has an R-value of about R2.5 per inch, you need about 12 inches for R30, about 15 to 16 inches for R38, about 19 to 20 inches for R49, and about 23 to 24 inches for R60. For blown-in cellulose like GreenFiber or Applegate, which is denser at about R3.5 to R3.7 per inch, you need fewer inches. About 8.5 inches for R30, about 10.5 to 11 inches for R38, about 13 to 14 inches for R49, and about 16 to 17 inches for R60. These depths assume the insulation is fluffed to its proper density and not compressed. If you already have existing insulation, you add on top. For example, if you have 6 inches of old fiberglass at about R15 and you want R49, you need about 10 inches of new cellulose on top. Always keep soffit vents clear and maintain 3 inches clearance around non-IC-rated can lights.
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