How to use this calculator
- Measure the area. Length × height for walls, length × width for attics. Subtract major openings (doors, windows).
- Pick R-value. R-13 for walls (2×4 framing), R-19 for walls (2×6), R-30 for attics in mild climates, R-49+ in cold climates.
- Read bag count. The calculator returns bags or rolls plus per-bag coverage. Add 5–10% extra for irregular bays.
- Optional cost estimate. Enter price per bag for a total estimate. Default $45/bag is mid-2026 retail for R-13.
Formula
Bags = Area ÷ Coverage per Bag (88 ft² for R-13, less for higher R)
Worked example
A 40 × 8 ft basement wall (320 ft²) at R-13: 4 bags of fiberglass batt (88 ft² coverage each). At $45/bag = $180 in insulation. R-19 batt for the same wall: 5 bags × $60 = $300.
Common project sizes
Quick reference for the most common insulation calculator use cases. Use these as a sanity check on your calculator inputs.
| Project | Dimensions | Result |
|---|---|---|
| Basement wall (40 × 8 ft, R-13) | 320 ft² | 4 bags · ~$180 |
| Attic (1,500 ft², R-49) | 1,500 ft² | 31 bags batt · ~$1,400 |
| 2,000 ft² home walls (R-13) | ~1,800 ft² | 21 bags · ~$945 |
| Garage (24 × 24 × 8 ft, R-13 walls) | 768 ft² | 9 bags · ~$405 |
2026 cost reference
Typical retail price range in the United States for insulation. Local pricing varies by region, supplier, and grade — confirm with two or three quotes before ordering.
Per square foot (installed)
$0.50 – $3.50
Fiberglass batt R-13: $0.50–$1.00/ft² installed. Blown-in cellulose R-49: $1.00–$1.80/ft². Closed-cell spray foam: $1.50–$3.50/ft². Mineral wool: $1.20–$2.20/ft². Add 30–50% for retrofit (existing walls).
Tips for accurate results
- R-value scales by zone: IECC climate zone 5 (most of the U.S. Midwest/Northeast) requires R-49 in attics, R-19 to R-21 in walls.
- Blown-in cellulose is 25–35% cheaper per R-value than fiberglass batts and fills cavities better. Requires rental of a blower.
- Spray foam (open or closed cell) costs 3–5× fiberglass but air-seals at the same time. Worth it on rim joists and band joists.
- Always check fire codes — exposed insulation in basements needs a thermal barrier (½-inch drywall).
- For attics, blown-in is faster and cheaper than batts. For walls in new construction, batts are easier.
When to consult a pro
Fiberglass batt installation is a workable DIY project — physically uncomfortable but mechanically simple. Air sealing (caulking gaps before insulation) is the high-ROI step most DIY jobs skip. Hire a pro for: spray foam, attic blow-in over 1,500 ft², any retrofit through walls (drilling holes, blowing cellulose), and assemblies with vapor barrier requirements.