Batt Insulation Calculator
Calculates Kraft-faced or unfaced fiberglass and rockwool (mineral wool) thermal/acoustic batts for 2x4 walls (R-13/R-15), 2x6 walls (R-19/R-21), and ceilings/attics (R-30/R-38), accounting for wood stud displacement.
Formula
Net Cavity Area = Gross Area × 0.90 (stud displacement); Bags = Ceil((Net Area × (1 + Waste %)) ÷ Bag Sq Ft)
1. Stud Displacement Factor: 16" OC = 0.90; 24" OC = 0.93 2. Net Insulation Cavity Area = Gross Area × Stud Displacement Factor 3. Total Cavity Area with Waste = Net Cavity Area × (1 + Waste %) 4. Bags to Order = Ceil(Total Cavity Area with Waste ÷ Bag Sq Ft Coverage)
Inputs
- Total Wall / Ceiling Surface Area
- Insulation R-Value & Cavity Depth
- Framing On-Center Width
Worked example
Insulating 500 sq ft of 2x4 wood framed walls at 16" on-center spacing.
Outcome: Gross area is 500 sq ft. Deducting 10% for wood 2x4 studs gives 450 sq ft of open cavity. Adding 5% cut waste equals 472.5 sq ft. Dividing by 40 sq ft/bag yields 11.8 bags, rounding up to 12 full bags of R-13 Kraft-faced batts.
Which way should the paper vapor retarder face on fiberglass batt insulation?
In almost all American climate zones, the Kraft paper facing must face towards the warm conditioned interior living space (against the drywall) to prevent humid indoor air from condensing inside the cold wall cavity.