Insulation Calculator

How the Insulation Calculator Works

Enter the area to insulate and the insulation product. The calculator returns total square footage, the number of batts or rolls (or 4×8 rigid foam sheets) to buy, the R-value achieved, and a material cost estimate. Framing spacing matters: batts are manufactured to fit 14½ in or 22½ in clear cavities, and rolls come in specific widths, so counts change with 16 in vs 24 in on-center framing.

R-Value and Quantity Formulas

• R-value = R-per-inch × installed thickness (fiberglass ≈ R-3.1–3.4/in, rockwool ≈ R-3.7–4.2/in, rigid foam ≈ R-5/in)
• Batts/sq ft of coverage: R-13 batt = 8 ft × 15¼ in ≈ 10.2 sq ft per piece (16 in oc)
• Rolls: standard roll = 40 ft long × 15 in or 23 in wide; R-38/R-49 come mostly as batts or rolls 25–80 ft
• Buy = area × (1 + waste%) ÷ coverage per package
• Compressed batts lose R-value: squeezing an R-19 into a 3½ in cavity yields only ~R-13

Worked Example

A 30 ft × 24 ft attic ceiling (720 sq ft) insulated with R-38 fiberglass: 720 ÷ coverage per R-38 roll (23 in × 40 ft = 76.7 sq ft) = 9.4 → with 10% waste, buy 11 rolls. Achieved R-value = R-38 at full 12 in thickness. At $0.75/sq ft, material cost ≈ $594.

Frequently Asked Questions

What R-value do I need? Typical code minimums: R-13–15 exterior 2x4 walls, R-19–21 for 2x6 walls, R-30–38 floors over unconditioned space, R-49–60 attics in most US climate zones (IECC climate zones 2–8).

Can I stack two layers of insulation? Yes — R-values add. R-19 plus R-30 yields R-49. For attics, a second perpendicular layer over batts also reduces thermal bridging through the joists.

Faced or unfaced batts? Faced batts include a vapor retarder (use on exterior walls in cold climates); unfaced are for layering over existing insulation.

How much waste should I add? 5–10% for open attics and regular framing; 12–15% for walls with many outlets, windows, and odd cavities.

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