Batt Insulation Calculator

About the Batt Insulation Calculator

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.

When This Page Helps

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.

How to Use the Inputs

1. Enter the total area to insulate in square feet.
2. Select the batt R-value that matches your code requirement.
3. Select the stud spacing (16" or 24" on center).
4. Set the coverage per package (check the product label).
5. Review the number of packages needed.

Example Calculation

Tips & Best Practices

• Do not compress batts to fit — compression reduces R-value significantly.
• Use unfaced batts for interior walls and where vapor barriers are applied separately.
• Kraft-faced batts have a built-in vapor retarder — face goes toward the heated side in cold climates.
• Cut batts 1" longer than the cavity for a friction fit — gaps reduce effectiveness.
• Split batts around wires and pipes rather than compressing them behind obstacles.
• Wear long sleeves, gloves, and a dust mask when handling fiberglass batts.

Batt Insulation Sizing

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.

Installation Best Practices

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.

Common Mistakes to Avoid

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%.

Cost Comparison

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.

Frequently Asked Questions

• What R-value batt for 2×4 walls?

  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.

• What R-value batt for 2×6 walls?

  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.

• Faced or unfaced batts?

  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.

• How many square feet per bag of insulation?

  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 or rock wool batts?

  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.

• Do batts need a vapor barrier?

  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.

• Can I stack two layers of batts?

  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.

• How do I insulate around electrical boxes?

  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.