Calculate the number of batt insulation rolls or bags needed. Enter wall area, cavity depth, and R-value to determine fiberglass or rock wool batt quantities.
Batt insulation (fiberglass or rock wool) is the most common insulation type for walls, floors, and attics in residential construction. It comes in pre-cut or rolled blankets sized to fit standard stud cavities (16" or 24" on center). Proper sizing and quantity estimation prevents waste and ensures complete coverage.
This batt insulation calculator determines how many rolls or bags of batts you need based on the total area to be insulated and the coverage per package. Different R-values have different thicknesses and coverage, so the calculator helps you match the right product to your project.
Standard fiberglass batts come in R-11, R-13, R-15, R-19, R-21, R-30, and R-38 ratings. The width is either 15" (for 16" o.c. framing) or 23" (for 24" o.c. framing). Rock wool (mineral wool) batts offer the same R-values but with higher density and better fire resistance.
This data-driven approach helps contractors minimize rework, avoid delays caused by material shortages, and deliver projects on time and within the agreed budget.
Batt insulation is sold by the bag or roll with a specific square footage coverage. You need to divide total wall/ceiling area by the package coverage to get the right order quantity. This calculator does that math and helps you choose the correct product. Regular use of this calculation supports compliance with building codes and inspection requirements, helping projects proceed smoothly through the permitting and approval process.
Packages = ⌈Area ÷ Coverage per Package⌉ Coverage per Package = bags/rolls × batts per bag × batt area
Result: 12 bags
960 sq ft of wall area with 5% waste = 1,008 sq ft. R-13 fiberglass batts (15" wide, 93" long) cover 88.12 sq ft per bag. 1,008 ÷ 88.12 = 11.4 → 12 bags of R-13 unfaced fiberglass batts.
Batts come in standard widths for 16" o.c. framing (15" wide) and 24" o.c. framing (23" wide). Lengths vary: 93" (standard 8' wall), 105" (9' wall), and rolls (various lengths for attics). Always measure cavity depth to ensure the batt R-value matches the available space.
Friction-fit batts snugly into cavities without gaps, voids, or compression. Cut batts 1″ longer than the cavity. Use a straight edge and utility knife for clean cuts. Fill all cavities completely, including narrow spaces and areas around obstacles.
Compressing batts is the most common error — a R-19 batt compressed from 6.25″ to 3.5″ only performs at about R-13. Gaps and voids are equally harmful: even a 5% air gap in a wall cavity can reduce effective R-value by 50%.
Fiberglass batts are the most economical insulation option at $0.50–$1.00 per sq ft installed. Rock wool batts cost $0.75–$1.50 per sq ft. Spray foam insulation costs $1.50–$3.50 per sq ft for comparison.
2×4 walls (3.5" deep cavity) fit R-13 or R-15 batts. R-15 high-density batts provide better insulating value in the same cavity depth. R-13 is the most common and least expensive option.
2×6 walls (5.5" deep cavity) fit R-19 or R-21 batts. R-21 high-density batts maximize this cavity depth. R-19 is standard and widely available.
Kraft-faced batts include a built-in vapor retarder. Use them where a vapor barrier is needed (toward the heated side in cold climates). Use unfaced batts if applying a separate poly vapor barrier, or in mild climates where vapor barriers aren't required.
Coverage varies by R-value and width. R-13 (15" wide): about 88 sq ft per bag. R-19 (15" wide): about 75 sq ft per bag. R-30 (16" wide): about 88 sq ft per bag. Always check the specific product label.
Fiberglass is cheaper and lighter. Rock wool (mineral wool) is denser, more fire-resistant, better at sound blocking, and moisture-resistant. Rock wool costs 25–50% more but is preferred in many applications for its superior properties.
In climate zones 4–8 (cold climates), a vapor retarder is required on the warm-in-winter side of insulation. Kraft-faced batts satisfy this. In zones 1–3, vapor barriers are often not required or even discouraged to allow drying.
Yes, for attics you commonly stack unfaced batts perpendicular to each other to achieve higher R-values (e.g., two R-19 layers for R-38). Do not compress the batts — they need full thickness to achieve rated R-value.
Split the batt into a front and back piece. Place the back piece behind the box and the front piece around and over the box. This maintains R-value without gaps. Never compress batts behind boxes.