MATERIALFOREMAN · SHEET IN-001 · ENVELOPE WORKS
CALC-N-001 · DRAWING NO. IN-001 · REV A

Insulation calculator.

Drafted to scale · cited sources · honest numbers

Select the insulation type and target R-value. The calculator returns depth, bag count (blown), board feet (spray foam), or coverage area (batt), with a material-cost range. Cellulose shows both installed and settled depth. Fires IRC R806.5 and R316 code warnings.

◈ DRAFTING PANEL · INSULATION TAKEOFF · N.T.S. SHEET IN-001 · REV A
Redline · scope notice Material takeoff only. Does not evaluate vapor barriers, air sealing, ventilation baffles, or flash-and-batt minimum closed-cell depths by climate zone. Blown and spray quantities assume open cavity with no obstructions. Batt output is coverage area only; check manufacturer packaging for pieces per bag.
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Note 01 · Field Procedure

How to measure for insulation

  1. For attics: measure the exterior footprint of the house (length x width). Subtract overhangs if the attic floor does not extend past the exterior wall line. The attic floor area is the insulation area.
  2. For walls: measure each wall height and length. Subtract window and door openings. The net wall area is the insulation area. Standard stud cavities are 3.5 in. deep (2x4) or 5.5 in. deep (2x6).
  3. Check existing insulation. Push a ruler through the material to the ceiling deck. Subtract existing R-value from the target. Blowing over existing batt insulation is common in attic upgrades.
  4. Determine whether the attic is vented or unvented. Vented attics have soffit and ridge vents. Unvented (hot roof) assemblies require air-impermeable insulation per IRC R806.5.
Note 02 · Equation

The formula

depth  =  Rtarget  ÷  R/in
R_targettarget R-value (from IECC by climate zone)
R/inR-value per inch for the insulation type
depthinches of insulation needed
Cellulose: depth shown is installed (pre-settlement). Settled depth = installed x 0.82. Bag count uses installed depth.
Note 03 · Sanity Ranges

Insulation by project

T Use case Notes
600 sfSmall attic / cape codCellulose R-49: ~26 bags. Half-day job with a blower rental.
1,200 sfStandard ranch atticCellulose R-49: ~52 bags. The most common DIY blown-in job.
2,000 sfLarge attic / colonialCellulose R-49: ~86 bags. Budget a full day plus helper.
200 sfWall cavity sectionR-13 batt: 200 sf material. Match batt width to stud spacing (15 in. or 23 in.).
800 sfCrawlspace floorR-25 batt or spray foam. Crawlspace insulation often requires vapor barrier on top.
Note 04 · References

Sources

Authorities cited on this sheet
  1. IECC 2021: International Energy Conservation Code · ICC. Defines minimum R-values by climate zone and location (attic, wall, floor). Table R402.1.2 is the residential insulation schedule. Basis for the zone selector and target R-value defaults.
  2. IRC R806.5: Unvented Attic and Unvented Enclosed Rafter Assemblies · IRC 2021 Chapter 8. Prohibits air-permeable insulation in unvented attic assemblies. Requires air-impermeable insulation (spray foam) or a flash-and-batt hybrid. Enforced as a hard warning in this calculator.
  3. GreenFiber Blown-In Cellulose Insulation · 30 lb bag coverage: ~40 sqft at 10 in. depth. Basis for the cellulose bag count and installed-depth math.
  4. Cellulose Insulation Manufacturers Association (CIMA) · Trade association for loose-fill cellulose. Publishes the ~18% settlement factor from installed to settled depth that drives the dual-depth display and the FTC Rule 460 labeling requirement.
  5. Insulation Institute (NAIMA) · North American Insulation Manufacturers Association. R-value per inch reference tables for fiberglass blown (2.2-2.7), batt (3.4-3.7), and general fiberglass guidance.
Note 05 · Field Manual

What the sheet count does not tell you

A 1,000-square-foot attic, worked out

Take a 1,000 square foot ranch attic insulated to R-49, the IECC target for most of the country. Blown fiberglass adds R-2.5 per inch, so hitting R-49 takes 19.6 inches of loose fill, which works out to 25 bags. That matches Owens Corning's own AttiCat coverage chart at R-49, the cross-check the constant is built on. At national-baseline pricing the fiberglass runs about 625 to 875 dollars. Cellulose is denser per R: at R-3.5 per inch it reaches R-49 in less depth, but it settles, so the calculator shows two numbers. The same attic in cellulose is 43 bags blown to 17.1 inches installed, settling to 14 inches, for about 516 to 645 dollars.

Installed depth versus settled depth

Cellulose is the one insulation where the depth you blow in is not the depth you end up with. Loose cellulose settles about 18 percent in the first year as it compacts, so the calculator shows two numbers: installed depth, which is what you blow in and what drives the bag count, and settled depth, which is what delivers the R-value after a year. FTC Rule 460 requires the settled thickness on the bag for exactly this reason. The mistake is blowing to the settled number: do that and the attic loses R-value over its first winter and never recovers it. Blow to the installed depth, let it settle, and the R-value lands where it should. Blown fiberglass settles far less, so it shows a single depth.

R-value targets and the unvented-attic trap

The target R-value comes from your climate zone. IECC 2021 Table R402.1.2 sets attic insulation at R-30 in the warmest zone, R-49 across most of the country, and R-60 in the cold north. Walls run R-13 to R-21, crawlspaces R-13 to R-30. The calculator loads the target from the zone, but local amendments can raise it, so confirm before ordering. The trap that gets people is the unvented attic. IRC R806.5 prohibits air-permeable insulation, which means blown cellulose, blown fiberglass, and batts, in an unvented (hot-roof) assembly. That space needs air-impermeable spray foam, or a flash-and-batt hybrid with a minimum closed-cell depth by zone. The calculator fires a hard warning when you select a blown type for an unvented attic, because filling a hot roof with cellulose is a moisture failure waiting to happen.

Note 06 · Common Questions

Common questions

How much insulation do I need for an attic?
Insulate to your climate zone's R-value, which is R-49 for most of the country and R-60 in the cold north. A 1,000 square foot attic at R-49 takes about 25 bags of blown fiberglass at 19.6 inches deep, or 43 bags of cellulose blown to 17.1 inches. The calculator returns depth and bag count for the type you pick.
How many inches of insulation is R-49?
It depends on the material. Blown fiberglass at R-2.5 per inch needs about 19.6 inches. Cellulose at R-3.5 per inch needs about 14 inches settled, blown a couple inches deeper to allow for settling. Batts hit R-49 in roughly 14 inches. Closed-cell spray foam, at R-6.7 per inch, gets there in about 7.5 inches.
What is the difference between installed and settled depth for cellulose?
Installed depth is what you blow in; settled depth is what remains after about 18 percent first-year compaction. The R-value comes from the settled depth, but the bag count comes from the installed depth, so the calculator shows both. Blow to the installed number, not the settled one, or the attic loses R-value over its first winter. FTC Rule 460 requires the settled thickness on the bag.
Can I use blown insulation in an unvented attic?
No. IRC R806.5 prohibits air-permeable insulation, blown cellulose, blown fiberglass, and batts, in an unvented (hot-roof) attic. That assembly needs air-impermeable spray foam or a flash-and-batt hybrid with a minimum closed-cell depth by zone. A vented attic with soffit and ridge vents takes any type. The calculator flags the conflict when you pick a blown type for an unvented attic.
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REV A · 2026-06-07
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