Blog /
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 N1101.7 or Table N1101.7) and the assembly you’re insulating (attic, wall, floor).
IRC Table N1102.1.3 minimum R-values (2021 IRC, residential):
| Climate Zone | Ceiling | Wood Frame Wall | Floor |
|---|---|---|---|
| 0-1 | R-30 | R-13 or R-10ci | R-13 |
| 2 | R-49 | R-13 or R-10ci | R-13 |
| 3 | R-49 | R-20, 13+10ci, or 15ci | R-19 |
| 4 (except Marine) | R-60 | R-30, 20+5ci, 13+10ci, or 20ci | R-19 |
| 5 and Marine 4 | R-60 | R-30, 20+5ci, 13+10ci, or 20ci | R-30 |
| 6 | R-60 | R-30, 20+5ci, 13+10ci, or 20ci | R-30 |
| 7-8 | R-60 | R-30, 20+5ci, 13+10ci, or 20ci | R-38 |
“ci” means continuous insulation, usually rigid foam across the studs. “20+5ci” means R-20 in the cavity plus R-5 continuous. Continuous insulation cuts the heat that leaks through the studs themselves. The table is the prescriptive path; the IRC also allows U-factor and performance paths.
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. And if you’re insulating an attic, you’ll need the roof area from pitch to figure out the attic floor footprint, which is the square footage you’ll be blowing insulation over.
Blown-In Attic Insulation
The most common DIY insulation job. Cellulose or fiberglass, blown into the attic with a rented machine.
Cellulose (Blown-In)
Every bag has a coverage chart: for each R-value it lists the bags per 1,000 sq ft, the installed thickness, and the settled thickness. The numbers change from brand to brand, so read the bag you’re buying.
Bags = attic area / 1,000 x bags per 1,000 sq ft (from the chart)
For a 1,200 sq ft attic, if the chart says 50 bags per 1,000 sq ft at R-49, that’s 1,200 / 1,000 x 50 = 60 bags.
Fiberglass (Blown-In)
Blown fiberglass is lighter and takes more depth than cellulose to reach the same R-value. Same method: use the chart on the bag.
Settling
Blown insulation settles, cellulose more than fiberglass. The R-value is based on the settled thickness, so blow to the chart’s installed thickness and expect it to drop to the settled number. The IRC requires depth markers in the attic for blown insulation (N1101.10.1.1).
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. The stud spacing and cavity count come directly from your framing layout — the stud count calculator gives you the bay count you need for batt ordering.
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. Bag coverage is printed on the package; an Owens Corning kraft-faced R-13 15” x 93” bag covers 106.56 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 106.56 sq ft per bag: 35.1 bags. Buy 36.
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): about R-4 per inch. Cheaper. 4x8 sheets.
- Polyiso (foil-faced): about R-6 per inch. The highest of the three, though it loses some performance in cold weather.
Sheet count: Wall Area / 32 sq ft per sheet. Add 10% waste. Once the insulation is in, the drywall goes over it — both estimates use the same wall area numbers, so running them together saves time.
For 2-inch XPS on a basement wall, 120 linear feet x 8 feet tall: 960 sq ft / 32 = 30 sheets, plus 10% = 33.
Spray Foam
Closed-cell spray foam runs roughly R-6 to R-7 per inch, open-cell roughly R-3.5 to R-4; the data sheet has the exact number. Spray foam is priced by the board foot (1 sq ft x 1 inch thick).
For 3,744 sq ft of wall with 3 inches of closed-cell foam: 3,744 x 3 = 11,232 board feet. Get the per-board-foot price from your installer.
Foam plastic inside a house generally needs a thermal barrier such as 1/2” drywall over it (IRC R316.4).
Running the Numbers
Trussly’s insulation calculator handles batts, blown-in, spray foam, and rigid board. Enter the area and the coverage from the bag (batt defaults come from Owens Corning packaging), and it gives bags, board feet, or sheets, plus material cost at your prices. It cites IRC Table N1102.1.3 so you can check the minimum for your climate zone.