Insulation Calculator
Calculate how much insulation you need for any location — attic, walls, floors, basement, or crawlspace. Covers blown-in fiberglass and cellulose, fiberglass and mineral wool batts, spray foam (open and closed-cell), and rigid foam boards. Checks your results against IECC 2021 minimum R-values for your climate zone. All calculations sourced from manufacturer coverage charts (FTC R-value Rule), ASTM standards, and ASHRAE design values.
Project Configuration
📐 What to measure: Measure the floor plan footprint: Length × Width of the space below the attic.
Blown-In Specifications
Waste Factor
Typical: 5–10% flat attic · 10–15% cathedral/walls · 15–20% spray foam
Recommended Tools & Materials
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Stanley FATMAX 25 ft. Classic Tape Measure (33-725)
$20–$26
Industry-standard 25ft tape measure with BladeArmor coating, 14ft standout, and 16"/19.2" stud center markings in a cushion-grip high-impact case.
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3M Safety Glasses, Virtua CCS, ANSI Z87, Anti-Fog, Clear Lens
$8–$12
ANSI Z87.1 certified anti-fog polycarbonate safety glasses with removable foam gasket, wraparound design absorbing 99.9% UV, and CCS earplug system.
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Owens Corning ProCat Fiberglass Insulation (33.5 lb bag)
$28–$34
ASTM C764-compliant blown-in fiberglass. No settlement — achieves labeled R-value at installed thickness. FTC-mandated coverage chart on each bag.
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Owens Corning R-38 Fiberglass Batt Insulation (16" o.c.)
$40–$55
R-38 unfaced fiberglass batt for open attic floors. ASTM C665 Type I. 12" thick, 16" wide. Tested at 75°F mean temperature per ASTM C518.
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Protective Insulation Suit — Tyvek Coveralls (25-pack)
$55–$75
Disposable coverall rated for fiberglass, mineral wool, and cellulose installation. Full-body protection against skin and lung irritation during blown-in or batt work.
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3M Respirator N95 — 20-Pack (8210)
$22–$28
NIOSH-approved N95 filtering facepiece. Required when installing blown-in fiberglass or cellulose. Also recommended for spray foam trimming and batt cutting.
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Great Stuff Gaps & Cracks Insulating Foam Sealant — 12 oz
$7–$10
Polyurethane foam sealant for sealing penetrations, rim joists, and gaps before or after insulation. Air sealing is as important as R-value — required at all framing penetrations.
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Industry Standards Referenced
ASTM C518-21
ASTM C665-24
ASTM C739-21
ASTM C578-22
ASTM C1289-22
ASTM C1029-21
IECC 2021 Table R402.1.3
FTC 16 CFR Part 460
IRC R702.7 (2021)
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Want to Learn More?
IECC 2021 requirements by climate zone, material cost comparisons, compression formulas, and vapor retarder rules for fiberglass, cellulose, spray foam, and rigid foam.
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How to Use This Calculator
- Select location: Choose ceiling/attic, cathedral ceiling, walls, floor, basement, crawlspace, rim joist, or slab edge.
- Select material: Choose blown-in fiberglass or cellulose, fiberglass or mineral wool batts, spray foam, or rigid foam board.
- Enter your climate zone (1–8): Used for IECC 2021 code compliance check and polyiso derating.
- Enter area: Square footage of the space you are insulating (linear feet for slab edge).
- Set material options: Target R-value, thickness, stud spacing, board type, or foam type depending on the material selected.
- Adjust waste factor: 5–10% for flat attics; 10–15% for walls; 15–20% for spray foam and irregular surfaces.
- Click Calculate: See bags, sheets, or board feet needed; R-value achieved; and IECC 2021 code compliance.
Understanding Whole-Wall vs. Cavity R-Value
The cavity R-value (R-13, R-21, etc.) is NOT what your wall actually achieves. Wood studs conduct heat at R-1.25/inch — creating thermal bridges. A 2×4 wall with R-13 batts at 16" o.c. achieves only R-12.1 whole-wall (parallel path method, ASHRAE). The whole-wall R-value is what matters for energy code compliance and energy calculations. This calculator shows both values for wall assemblies.
Frequently Asked Questions
How do I find my IECC climate zone?
Climate zones are assigned at the county level by IECC Table C301.1. Zone 1–2 covers Hawaii and the deep South (Florida tip, Gulf Coast). Zone 3 is the warm South (N. Florida, N. Texas, coastal California). Zone 4 covers the Mid-Atlantic, Tennessee, and NC. Zone 5 is the Great Lakes and N. Mid-Atlantic. Zone 6 is New England and northern Great Plains. Zones 7–8 are very cold northern states and Alaska. Look up your county at the DOE Building America Solution Center or ENERGY STAR Climate Zone Finder.
What is the difference between cavity R-value and whole-wall R-value?
Cavity R-value (R-13, R-21, etc.) is the rated R-value of the insulation alone in the stud cavity. Whole-wall R-value accounts for thermal bridging through wood studs, plates, and headers — which conduct heat at only R-1.25/inch. At 16" o.c., framing occupies 25% of the wall area. A 2×4 wall with R-13 batts achieves only R-12.1 whole-wall. A 2×6 wall with R-19 batts achieves approximately R-13.7 whole-wall. Always use whole-wall R-values for energy calculations.
Should I use R-19 or R-21 in a 2×6 wall?
Use R-21 (high-density, 5.5" thick). R-19 is 6.25" thick — it must be compressed into the 5.5" 2×6 cavity, reducing the R-value to approximately R-18 per NAIMA compression formulas and manufacturer data. R-21 is specifically designed to fill a 2×6 cavity at full depth without compression, delivering the full rated R-value. R-20 (medium-density) is also a good option at R-20 with no compression loss.
How many bags of blown-in cellulose do I need for a 1,500 sq ft attic at R-49?
Using GreenFiber INS510LD (30 lb bags): At R-49, the chart shows 67.3 bags per 1,000 sq ft. For 1,500 sq ft: 67.3 × 1.5 = 100.95 bags, plus 10% waste = 111 bags. The coverage charts are already based on settled thickness per FTC requirements, so you will achieve R-49 at that bag count. Install to the labeled depth marker (14.71" installed, settles to 13.24").
What is the minimum closed-cell spray foam thickness for a vapor retarder?
Per ASTM E96 testing (BASF Walltite US data): 1" = 1.39 perms (Class III), 1.5" = ~1.0 perm (borderline Class II), 2" = 0.70 perm (Class II vapor retarder per IRC R702.7). For a definitive Class II vapor retarder — required on the interior of frame walls in Zones 5–8 — you need at least 2" of closed-cell spray foam. Do NOT add polyethylene sheeting on top of closed-cell foam; that creates a double vapor barrier that traps moisture.
Is polyiso better than XPS in cold climates?
In warm climates (Zones 1–4): Polyiso LTTR R-5.7/inch outperforms XPS at R-5.0/inch. In cold climates (Zones 5–8): Polyiso loses 30–50% of its labeled R-value as blowing agents condense below 40°F. At Zone 6 conditions, polyiso may deliver only R-4.0–4.7/inch while XPS holds R-5.0/inch (though XPS also drifts slightly over decades). NRCA recommends R-4.7/inch design value for Zone 6+ polyiso. For cold-climate continuous insulation, XPS or EPS are more predictable performers.
Can I insulate my attic with fiberglass batts instead of blown-in?
Blown-in is strongly preferred for attics. Batts require careful installation around obstructions and at truss members, and air gaps around batts eliminate most of their insulating value. Blown-in covers all surfaces uniformly including around rafters and blocking, and achieves higher density with fewer air gaps. For horizontal open attics, blown-in typically costs less and performs better. Batts are better suited for cathedral ceilings and enclosed rafter bays where blown-in cannot be easily applied.
Do I need a vapor barrier with spray foam insulation?
Closed-cell spray foam IS its own vapor retarder — do not add polyethylene sheeting over it. Adding a Class I vapor retarder over closed-cell creates a double vapor barrier that traps moisture with nowhere to go. Open-cell spray foam is highly vapor permeable (~16 perms at 3") and is NOT a vapor retarder; you may need a separate vapor retarder in Zones 5–8 depending on assembly design. Both open and closed-cell foam require a 15-minute thermal barrier (minimum ½" gypsum board) before occupancy per IRC.