Batt Insulation Calculator
Find out exactly how many rolls of batt insulation you need before buying. Enter your insulation area, target R-value, and cost per square foot to get an instant roll count and material cost estimate — whether you're insulating walls, attic, or floors with fiberglass or mineral wool batts.
How to Calculate Batt Insulation Rolls Needed
Calculating batt insulation rolls is a simple three-step process. Step 1: measure your total insulation area. For walls, multiply perimeter × height then subtract doors and windows. For attic floors, measure length × width of the attic. For floors over crawl spaces, measure the floor area below the conditioned space.
Step 2: add 10% waste factor. Cuts around pipes, wires, and corners waste 5–15% of insulation. Using 10% as a standard buffer prevents running short mid-job.
Step 3: divide by coverage per roll. Check the package for exact coverage. Typical values: R-13 batt (15-in wide) = 40 sq ft/roll. R-19 batt (23-in wide) = 48 sq ft/roll. R-30 batt = 30 sq ft/roll. Round up to whole rolls.
Example: 800 sq ft attic needing R-38 coverage. An R-38 batt roll covers 25 sq ft. With 10% waste: 800 × 1.10 = 880 sq ft needed. 880 ÷ 25 = 35.2 → buy 36 rolls.
Batt Insulation R-Value Reference Chart
Choosing the right R-value for each location in your home matters more than the brand. Here's a practical reference by application:
- 2×4 exterior walls: R-13 (fits perfectly) or R-15 (slight compression, acceptable)
- 2×6 exterior walls: R-19 (standard) or R-21 (requires 6.5-in cavity or compression)
- Attic floors (zones 1-3): R-30 to R-38
- Attic floors (zones 4-8): R-38 to R-60
- Floors over unconditioned space: R-19 to R-30
- Basement walls (interior): R-11 to R-15
Fiberglass batts compress easily — never compress insulation to fit a smaller cavity. Compression reduces R-value proportionally. Compressed R-19 in a 3.5-inch space performs closer to R-11.
Fiberglass vs Mineral Wool Batt Insulation
Fiberglass batts are the most widely installed insulation type in the US. They're light, widely available, easy to cut, and cost $0.30–$0.60/sq ft for materials. Downsides: they're irritating to handle, can sag over time in walls if improperly installed, and lose effectiveness when wet or air moves through them.
Mineral wool batts (Rockwool Comfortbatt, Owens Corning Thermafiber) cost 30–40% more but offer compelling advantages: they're water-resistant (won't absorb moisture and lose R-value), fire-resistant up to 2000°F, and significantly better for sound control (STC rating 45+ vs 35+ for fiberglass). For exterior walls in mixed climates or when noise control matters, mineral wool is worth the premium.
Both product types are batted pre-cut to fit 16-inch and 24-inch on-center stud spacing. Always friction-fit batts without gaps — unfaced batts in interior walls don't need a vapor barrier in most climates. Faced batts (with kraft paper) should face the warm side.
Frequently Asked Questions
How do I calculate how many rolls of batt insulation I need?
Measure the total area to insulate (length × width or perimeter × height). Subtract doors and windows. Add 10% waste. Divide by the coverage area per roll listed on the package. Standard R-13 batts (15-in wide, 93-in long) cover about 40 sq ft per roll. R-19 batts (23-in wide) cover about 40–45 sq ft per roll.
What is the difference between R-13 and R-19 batt insulation?
R-13 batts are 3.5 inches thick and designed for 2×4 stud walls (3.5-inch cavities). R-19 batts are 6.25 inches thick for 2×6 walls. R-19 provides significantly better thermal performance: a 2×6 wall with R-19 loses about 32% less heat than a 2×4 wall with R-13. The cost difference is typically $0.10–$0.20 per sq ft more for R-19.
How much does batt insulation cost?
Fiberglass batt insulation costs $0.30–$0.60 per sq ft for materials. R-13 (2×4 walls) runs $0.30–$0.45/sq ft; R-19 (2×6 walls) runs $0.40–$0.60/sq ft; R-30 (attic) costs $0.50–$0.80/sq ft. Professional installation adds $0.50–$1.00/sq ft. Mineral wool batts cost 30–40% more than fiberglass but offer better fire and moisture resistance.
Can you double up batt insulation for more R-value?
Yes. Running two layers perpendicular is common in attics — R-19 batts between joists plus R-19 batts running perpendicular on top = R-38. In walls, you can't double up in standard stud cavities without adding framing depth. Options: use a thicker stud (2×8 instead of 2×6), add a second stud wall, or add rigid foam board on the exterior.
What is the coverage area per roll of batt insulation?
Coverage per roll varies by product. Standard 15-in wide R-13 roll: 40 sq ft. Standard 23-in wide R-19 roll: 40–48 sq ft. R-30 15-in roll: 30–40 sq ft. R-38 15-in roll: 25–30 sq ft. Always check the package label — coverage varies by brand. Buy 10% more than calculated to account for cuts and waste.