Blown-In Attic Insulation Calculator

How it works

The R-value to add is the target less what is already in the attic: either the R-value on the old insulation's label, or its measured depth times its R per inch.

Blown insulation is bought by the bag, and every bag carries a coverage chart. Under the FTC's R-value rule (16 CFR 460.12), a loose-fill label must give, for R-13, 19, 22, 30, 38 and 49 and any other R-values listed, the initial installed thickness, the minimum settled thickness, the maximum net coverage per bag, the bags per 1,000 sq ft and the minimum weight per square foot. Enter the row you will install, at or above the R-value to add. Bags are the attic area divided by that row's coverage, rounded up; the label states that the coverage per bag must not be exceeded.

Charts differ by product. Greenfiber SANCTUARY cellulose (25 lb bag) reaches R-38 at 10.4 in settled with 19.0 sq ft per bag, 52.6 bags per 1,000 sq ft (attic card PM-6.3328 Rev K, 05/2026); Johns Manville Climate Pro blown fiberglass needs 13.5 in for R-38 at 59.0 sq ft per bag, 17.0 bags per 1,000 sq ft (data sheet BID-402, 09/2025). The settled depth for the R-value to add is shown at the product's R per inch (chart R-value ÷ chart thickness).

Formula

existing R  = label R-value, or depth × R per inch, or 0
R to add    = target R − existing R          (0 if already at or above the target)
depth       = R to add × chart thickness ÷ chart R
bags        = attic area ÷ chart coverage per bag, rounded up
total R     = existing R + chart R

Example

A 1,000 sq ft attic with R-11 batts, brought to R-49, needs R-38 more. Greenfiber SANCTUARY's R-38 row is 10.4 in settled at 19.0 sq ft per bag: 1,000 ÷ 19.0 = 52.6, so 53 bags, for R-49 in total.

Assumptions and limitations

• The area is the whole attic floor. Coverage defaults are the chart's net coverage with no adjustment for framing; some charts (Greenfiber's among them) also list a figure adjusted for joists, which you can enter instead.
• Existing insulation is counted at its label R-value or at its measured depth times the R per inch you enter; the per-inch defaults are typical and editable. Compressed, wet or damaged insulation performs below its label.
• Blown insulation settles; the chart's settled thickness is what delivers the R-value, and the label requires both the minimum bags per 1,000 sq ft and the minimum thickness. Coverage depends on the blowing machine and its settings named on the label.
• Air sealing, baffles at the eaves, clearances around recessed lights, chimneys and flues, and attic ventilation are outside this calculation.
• The result is an estimate. It is not a substitute for the insulation manufacturer's label and instructions, the applicable code or a licensed professional.

Frequently asked questions