Insulation Calculator
Enter your attic area and target R-value to estimate the required depth and how many bags of insulation to buy.
Last updated: September 5, 2026
Frequently asked questions
How many bags of insulation do I need for R-49 in an 800 sq ft attic?
At 2.5 R-value per inch, R-49 needs about 19.6 inches of depth - roughly 46 bags at a 30 cu ft yield per bag, including a 5% waste allowance.
Why does R-value per inch matter?
Different insulation types cover different R-value per inch - fiberglass and cellulose have different coverage charts, so using the wrong figure will throw off your bag count.
What R-value do I need for my attic?
Recommended attic R-values vary by climate zone, generally ranging from R-30 in warmer regions to R-49 or higher in colder ones - check your local building code or the U.S. Department of Energy's zone map for the recommended value in your area.
Does insulation R-value stack if I add a second layer?
Yes - R-values are additive, so adding a layer of R-19 insulation on top of existing R-13 insulation gives a combined R-32. This is a common way to boost attic insulation to current recommended levels without removing the older layer first.
How many bags do I need for R-38 in a 1000 sq ft attic using cellulose at 3.4 R-value per inch?
R-38 at 3.4 R-value per inch requires about 11.2 inches of depth. Over 1000 sq ft that's about 931 cubic feet, or roughly 33 bags at a 30 cu ft yield per bag, including a 5% waste allowance.
Does loose-fill insulation stay at the depth I install it at?
No - loose-fill (blown-in) insulation settles over time, losing some of its installed depth and R-value. Manufacturers publish a higher 'initial installed thickness' than the long-term 'settled thickness' specifically to compensate, so install to the initial thickness figure on the product's coverage chart, not the settled one.
Does this calculator work for batt insulation too, or just blown-in?
This calculator is built for blown-in/loose-fill insulation, which is sold by bag yield in cubic feet. Batt insulation is sold by R-value and square footage per package instead, so for batts compare your attic's square footage directly to the batt's coverage rating rather than using bag yield.