Under-insulated attics are one of the most common sources of wasted heating and cooling energy in older homes. Our attic insulation calculator estimates how much more insulation you need — in both inches and bags — to hit a target R-value.
The math
Additional R needed = Target R-value − Current R-value
Inches to add = Additional R needed ÷ R-value per inch (depends on insulation type)
Bags needed = (Attic area × Inches to add) ÷ Bag coverage per inchExample
A 1,000 sq ft attic currently at R-10, targeting R-49 (a common recommendation for colder climates) using blown fiberglass (R-2.2 per inch, ~40 sq ft of coverage per bag at 1-inch depth): needs R-39 more insulation, 17.7 inches of added depth, and approximately 444 bags.
What R-value means
R-value measures a material’s resistance to heat flow — higher R-value means better insulating performance. Building codes typically recommend attic R-values between R-38 and R-60 depending on climate zone, with colder regions needing higher values to keep heating costs down.
Why insulation type changes the numbers so much
Blown cellulose (about R-3.7 per inch) is denser and insulates better per inch than blown fiberglass (about R-2.2 per inch) — so reaching the same total R-value takes noticeably less depth of cellulose than fiberglass, which affects both material volume and bag count.
Why the bag estimate is approximate
Bag coverage (square feet covered per bag at a given depth) varies by manufacturer, product line, and even blowing-machine settings. This calculator uses commonly cited industry-average coverage figures as a starting estimate — always check the coverage chart printed on your specific insulation product’s bag before purchasing, since it’s the authoritative source for that exact product.