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).
How we calculate this
Assumptions baked in
Bag count = ceil(area ÷ coverage_per_bag), where coverage varies by R-value: R-13 at 88 ft²/bag (3.5" batts), R-19 at 67 ft² (6.25"), R-30 at 49 ft² (10"), R-38 at 32 ft² (13"). These are Owens Corning EcoTouch published coverage figures for 16" o.c. stud spacing. Blown-in fiberglass and cellulose use different coverage tables (settled vs. installed depth) — the calculator returns the BATTS figure; for blown-in, multiply by 1.2 (≈20% extra for settling).
Accuracy and margin of error
±5% for standard 16" o.c. stud cavities. 24" o.c. cavities need 23" batts — different SKU, not just a count change. Floors over unheated crawl spaces and cathedral ceilings have their own R-value tables; consult IRC Table N1102.1.2 for your climate zone before ordering.
Edge cases this calculator does not handle
Wall cavities cannot exceed R-20 with batts (3.5" stud + best R/inch); for R-21+ walls you need 2×6 framing or rigid foam on the exterior. Spray foam (open-cell or closed-cell) is calculated by board-foot (ft² × inch of thickness), not bag. The calculator does NOT handle spray foam — use a separate quote from an installer.
Cited sources for this page
The figures and rules above are anchored to the following normative references. We link the underlying claim to its standard — not as generic SEO trust signals, but so you can audit any number on this page against a primary source.
Minimum insulation R-values for residential construction by climate zone are published in IRC Section N1102.1.2 and the IECC.
Fiberglass batt coverage figures of 88 ft²/bag (R-13) and 49 ft²/bag (R-30) are the manufacturer published values for nominal 16" o.c. framing.
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.