Insulation Calculator Formula

Understand the math behind the insulation calculator. Each variable explained with a worked example.

Use the Insulation Calculator

Formulas Used

Batts Needed

batts_needed = ceil(area / batt_coverage)

Bags (Blown-In, approx)

bags_blown = ceil(area / (r_value <= 19 ? 40 : 25))

Approximate for blown-in cellulose or fiberglass

Coverage per Batt

coverage_per_batt = batt_coverage

Variables

VariableDescriptionDefault
areaTotal Area to Insulate(sq ft)500
stud_spacingStud/Joist Spacing16
batt_lengthBatt Length(inches)93
r_valueR-Value Needed13
batt_width_ftDerived value= (stud_spacing - 1) / 12calculated
batt_length_ftDerived value= batt_length / 12calculated
batt_coverageDerived value= batt_width_ft * batt_length_ftcalculated

How It Works

How to Calculate Insulation

Formula

Batts Needed = Total Area / Coverage per Batt

Batt width fits between studs: 15 inches for 16" OC studs, 23 inches for 24" OC studs. Batt length is typically 93 inches (8 ft walls) or 105 inches (9 ft walls). Choose R-value based on application:

  • R-13: Standard 2×4 exterior walls
  • R-19: 2×6 exterior walls
  • R-30 to R-38: Attic floors
  • R-49: Attic floors in very cold climates

    • Worked Example

    500 sq ft of wall area with 16-inch OC studs, 93-inch batts, R-13.

    area = 500stud_spacing = 16batt_length = 93r_value = 13
    1. 01Batt width = (16 - 1) / 12 = 1.25 ft
    2. 02Batt length = 93 / 12 = 7.75 ft
    3. 03Coverage per batt = 1.25 × 7.75 = 9.69 sq ft
    4. 04Batts needed = 500 / 9.69 = 51.6 → 52 batts

    Ready to run the numbers?

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