Blown-In Attic Insulation: How Deep, How Many Bags, and What About Settling?

Depth for a Target R-Value

Depth = target R ÷ R per inch. For R-38 with fiberglass at 2.5 per inch, you need 15.2 inches. For R-49, 14 inches of cellulose, or 19.6 inches of fiberglass. The bag label includes a coverage chart showing the number of bags needed per 1,000 square feet for each R-value and the minimum thickness. Use the chart instead of guessing.

Recommended attic R-values depend on climate zone, and recommendations from energy agencies run from about R-30 in mild climates to R-60 in the coldest. Check the current guidance for your region.

Volume and Bag Count

Volume in cubic feet = attic area × depth in feet. A 1,000-square-foot attic filled to 10.9 inches needs 1,000 × 0.908 = 908 cubic feet of installed insulation. The loose-fill material is compressed in the bag and expands when blown, so manufacturers state coverage per bag. Settling reduces the depth by 10% to 20% over time, especially for cellulose, so install to the initial thickness shown on the bag chart, which is higher than the settled thickness.

Allowing for 15% settling, you might blow in 1,040 cubic feet to be sure the settled depth meets the target. Dividing by the coverage per bag from the manufacturer gives the bag count; the typical result is in the range of 20 to 30 bags per 1,000 square feet for R-38 with cellulose, but verify with the label.

Do the Prep Work First

Air sealing comes before insulation. Seal gaps around plumbing stacks, wiring penetrations, chimney chases, top plates, and recessed light fixtures. Warm air leaking into the attic carries moisture and heat that insulation alone does not stop. Use caulk, spray foam, or sealant rated for the situation, and keep insulation away from heat-producing flues according to clearances.

Install baffles at the eaves to keep soffit vents open and maintain airflow from the eaves up toward the ridge. Without ventilation, moisture can build up and cause mold or roof damage. Add depth markers every few hundred square feet so you and the inspector can verify thickness.

Blowing It In

Many home centers rent a blowing machine when you buy insulation. It takes two people: one feeding the bags, one in the attic directing the hose. Work from the farthest corner back to the hatch, spreading it evenly, and avoid covering electrical junction boxes without marking them. Use a dust mask, eye protection, and long sleeves. Insulate and weatherstrip the attic hatch last, since it is a common leak.

Compare the cost of DIY with a professional quote. Professionals can seal and insulate faster and may offer warranties.

Safety and Special Cases

Keep insulation clear of recessed lights unless they are rated IC (insulation contact). Older homes may have vermiculite insulation, which can contain asbestos; do not disturb it, and call a professional for testing. Check for knob-and-tube wiring, which must not be buried. Resolve roof leaks and moisture problems before insulating.