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Insulation Calculator: R-Value, Bags, and Batts
What You Need to Know First
The IRC assigns minimum R-values by climate zone. Before you calculate quantities, you need to know two things: your climate zone (based on your county, from IRC Figure N1101.2 or the DOE’s zone map) and the assembly you’re insulating (attic, wall, floor).
IRC Table N1102.1.2 minimum R-values (2021 code, residential):
| Climate Zone | Ceiling | Wood Frame Wall | Floor |
|---|---|---|---|
| 1 | R-30 | R-13 | R-13 |
| 2 | R-38 | R-13 | R-13 |
| 3 | R-38 | R-20 or 13+5ci | R-19 |
| 4 (except Marine) | R-49 | R-20 or 13+5ci | R-19 |
| 5 | R-49 | R-20 or 13+5ci | R-30 |
| 6 | R-49 | R-20+5ci or 13+10ci | R-30 |
| 7-8 | R-49 | R-20+5ci or 13+10ci | R-38 |
“ci” means continuous insulation (rigid foam on the exterior of the sheathing). The “+5ci” options let you use less cavity insulation if you add a layer of rigid foam outside. That approach is gaining popularity because it reduces thermal bridging through studs.
Your local jurisdiction may have adopted a newer code with different numbers, or may have local amendments. Check with your building department before buying material.
Blown-In Attic Insulation
The most common DIY insulation job. Cellulose or fiberglass, blown into the attic with a rented machine.
Cellulose (Blown-In)
Coverage per bag depends on the settled depth and R-value target. A standard 25-pound bag of cellulose at the most common installed depths:
| R-Value | Installed Depth | Bags per 1,000 sq ft |
|---|---|---|
| R-13 | 3.7” | 16 |
| R-19 | 5.2” | 22 |
| R-30 | 8.1” | 33 |
| R-38 | 10.3” | 41 |
| R-49 | 13.2” | 53 |
| R-60 | 16.2” | 65 |
For a 1,200 sq ft attic at R-49: 1,200 / 1,000 x 53 = 63.6 bags. Buy 64.
At $12-14 per bag from a big box store (some offer free blower rental with 20+ bags): roughly $800-900 in material. That’s one of the cheapest home upgrades per energy dollar saved.
Fiberglass (Blown-In)
Fiberglass is lighter and doesn’t absorb moisture, but it takes more depth to hit the same R-value. R-49 in fiberglass: about 17 inches settled. Bags cover less square footage per bag. You’ll use more bags but they’re similar in price per bag.
Settling
Blown insulation settles over time. Cellulose settles 15-20% in the first year. If you blow 13.2 inches today for R-49, it’ll be 10.5-11.2 inches next year, which is closer to R-40. Most manufacturers account for settling in their coverage charts (the “installed” numbers above include anticipated settling). But if you’re using a chart that lists “initial” depth, over-blow by 20% to compensate.
Fiberglass settles less, about 2-4%. It’s already low-density, so it doesn’t compress much under its own weight.
Wall Insulation (Batts)
Batt insulation comes in precut widths to fit standard stud bays.
- 2x4 walls at 16” OC: R-13 or R-15 batts, 15” wide, 93” long (for 8-foot walls)
- 2x4 walls at 24” OC: same R-value, 23” wide
- 2x6 walls at 16” OC: R-19 or R-21 batts, 15” wide
- 2x6 walls at 24” OC: R-19 or R-21, 23” wide
A 2x4 cavity is actually 3.5 inches deep. R-13 fiberglass fills that completely. You can’t physically fit R-19 in a 2x4 wall without compressing it, and compressed insulation loses R-value. The right answer for more wall insulation in 2x4 construction is continuous exterior insulation (rigid foam or mineral wool board outside the sheathing), not thicker cavity batts.
Batt Count
Count the stud bays, not the square footage. A 20-foot wall at 16” OC has 15 stud bays (240 inches / 16 inches = 15 bays). Each bay gets one batt. A bag of R-13 15” batts (unfaced) typically contains 12 batts, covering about 120 sq ft.
For a 1,500 sq ft house with roughly 520 linear feet of exterior wall at 8 feet tall:
Wall area: 4,160 sq ft. Subtract 10% for windows and doors: 3,744 sq ft.
At 120 sq ft per bag: 31.2 bags. Round to 32.
At $50-60 per bag for R-13 unfaced: $1,600-1,920.
Crawlspace / Floor Insulation
Fiberglass batts between floor joists. R-19 or R-30 depending on climate zone.
Floor joists at 16” OC: use 15” wide batts. Support them with spring-wire insulation supports (tiger teeth), not staples to the joists. Batts hanging from staples eventually sag and fall.
For a 1,200 sq ft crawlspace: 1,200 / coverage per bag (roughly 50-60 sq ft per bag of R-30 floor batts) = 20-24 bags.
The alternative to insulating the floor is insulating the crawlspace walls and treating the crawlspace as a semi-conditioned space. Rigid foam on the crawlspace walls (R-10 continuous) with a sealed vapor barrier on the floor. This approach is becoming preferred in humid climates because it avoids moisture problems where cold floor batts meet warm humid air and create condensation.
Rigid Foam Board
For continuous exterior insulation and basement walls.
- XPS (Extruded polystyrene, pink/blue board): R-5 per inch. 4x8 sheets.
- EPS (Expanded polystyrene, white beadboard): R-3.8 per inch. Cheaper. 4x8 sheets.
- Polyiso (foil-faced): R-6.5 per inch. Best R-value per inch but performance drops below 50 degrees F.
Sheet count: Wall Area / 32 sq ft per sheet. Add 10% waste.
For 2-inch XPS on a basement wall, 120 linear feet x 8 feet tall: 960 sq ft / 32 = 30 sheets. At $35-45 per sheet for 2”: $1,050-1,350.
Spray Foam
Closed-cell spray foam: R-6.5 per inch. Open-cell: R-3.7 per inch. Spray foam is priced by the board foot (1 sq ft x 1 inch thick).
A 2x6 wall cavity filled with 5.5 inches of closed-cell spray foam: R-36. Cost: $1.50-2.50 per board foot. For a 1,500 sq ft house with 3,744 sq ft of wall area x 5.5 inches = 20,592 board feet. At $2/board foot: $41,184. This is why spray foam is typically used for rim joists, band joists, and targeted air sealing rather than full-house cavity fill.
Running the Numbers
SiteCalc has an insulation calculator that takes your square footage, R-value target, and insulation type, then tells you bag count for blown-in or batt count for fiberglass. It references the IRC climate zone table so you can check code minimums before ordering. The show formula feature displays the coverage math per bag, and you can export the material list for the supply run.