Insulation Calculator
Calculate insulation area, thickness, bags, and cost for fiberglass, cellulose, or spray foam.
Area Dimensions
Fiberglass Batts
R-38
DIY-friendly batts and rolls. Moderate attic; $0.64/sq ft.
Ready to calculate
Enter your area dimensions
Last updated June 12, 2026 by our expert review team
Insulation Types
Fiberglass Batts
R-3.2/inch
$0.64/sq ft
Cellulose
R-3.5/inch
$1.2/sq ft
Spray Foam
R-6.5/inch
$2.5/sq ft
Expert Contributors
Hawkin
Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileEhsan Ghazanfari
Licensed Structural Engineer
FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
See full profileUpdated June 2026
How we verify our calculatorsR-Value Adds by Layer
R-value comes from material type and installed depth. Air seal first, then add enough loose fill, batts, or foam to reach the target without compressing the material.
Layers can add R-value, but gaps, air leaks, and compressed batts reduce real-world performance.
Recommended R-Values by Climate Zone
Zone 1-2
Hot climates
Attic: R-30
Wall: R-13
Zone 3-4
Mixed climates
Attic: R-38
Wall: R-13
Zone 5-6
Cold climates
Attic: R-49
Wall: R-20
Zone 7-8
Very cold
Attic: R-60
Wall: R-21
R-Value per Inch by Material
R/inchHigher R-value per inch means less thickness needed to reach the same insulation level.
Pro Tips
Seal every air leak before insulating. Caulk around wiring penetrations, plumbing stacks, and recessed light cans. Air sealing alone can cut heating costs 10 to 20%. Use our BTU calculator to see how upgraded insulation reduces your HVAC sizing needs.
Never compress fiberglass batts to fit a cavity. An R-19 batt crammed into a 3.5-inch stud bay drops to about R-13. Match batt thickness to your framing depth.
Check Energy Star's R-value recommendations for your climate zone. Zones 5 to 8 need a 6-mil poly vapor barrier on the warm side of exterior walls; zones 1 to 3 use unfaced batts or a smart vapor retarder instead.
For attic floors in cold climates, layer unfaced R-30 batts perpendicular over existing R-19 batts to hit R-49 without compressing either layer. If you're re-roofing, use our roofing calculator to estimate shingle materials before insulating the attic.
Blown cellulose settles 15 to 20% over time. Blow to 12.5 inches initially if you need R-38 (10.5 inches settled). See the DOE insulation types guide for material-specific settling rates.
Install fire blocking where insulation meets garage walls, dropped soffits, or balloon-framed walls. IRC R302.11 requires mineral wool or 0.5-inch drywall at these junctions.
After insulating, use our drywall calculator to estimate sheetrock for covering the wall cavities.
Better insulation means a smaller HVAC unit. Use our mini split calculator to size a ductless unit after upgrading insulation.
How the Calculator Works
Start with the surface you are insulating, not the whole room size. For an attic, enter the attic floor area. For a wall or crawl space, enter the net area after subtracting large doors, windows, hatches, or access panels.
The calculator uses the target R-value you select as the finished goal. If you already have insulation in place, measure its depth, estimate the existing R-value, and use the calculator for the added R-value you still need. Dry fiberglass is roughly R-3.2 per inch and settled cellulose is roughly R-3.5 per inch.
Bag counts include a 10% allowance for rim areas, uneven framing, small gaps around ducts and wiring, and the reality that opened rolls or partial bags rarely land perfectly. For a labor and material budget, use the insulation cost calculator.
Formula
Added R-value ÷ R per inch = installed depth, rounded up
Net area x 1.10 ÷ adjusted bag coverage = bags or rolls, rounded up
Estimated material cost = adjusted area x material rate x depth factor
Quick Reference
- Fiberglass R-value
- R-3.2/inch
- Cellulose R-value
- R-3.5/inch
- Spray foam R-value
- R-6.5/inch
- Energy savings
- Up to 40%
Frequently Asked Questions About Insulation
Should I enter total R-value or added R-value?
Use the R-value you still need to add. If your attic has 6 inches of fiberglass, estimate about R-19 already in place. To reach R-49, calculate for R-30 of added insulation.
How do I measure existing attic insulation?
Push a ruler through the insulation in several spots without compressing it. Average the depth, ignore matted or wet areas, and check whether the material is fiberglass, cellulose, or mineral wool.
Can I blow cellulose over fiberglass batts?
Yes, if the batts are dry, unfaced, and not moldy. Air seal first, keep soffit vents open with baffles, and avoid adding a second vapor retarder between layers.
When should I choose batts instead of loose fill?
Batts work best in open wall bays, crawl-space joists, and simple rectangular framing. Loose fill is usually better for attics with wiring, uneven joists, ducts, or many small obstructions.
Why does the bag count change so much at higher R-values?
Bag coverage drops as depth increases. A bag that covers a large area at R-19 covers much less at R-49 because you are installing several more inches of material.
What should I do before adding attic insulation?
Seal ceiling penetrations, mark electrical junction boxes, install baffles at soffits, repair roof leaks, and keep insulation away from non-IC-rated recessed lights and hot flues.
Do I need faced or unfaced insulation?
Use unfaced material for second layers and attic top-offs. Faced batts belong against the warm-in-winter side only when your local code and wall assembly call for a vapor retarder.
When is spray foam worth pricing separately?
Use spray foam for rim joists, metal buildings, air-leaky roof decks, and tight cavities where high R-value per inch matters. For full budgets, use the spray foam cost calculator.
Insulation Mistakes That Cost You Money
Calculating from room area instead of insulation area
A 12 by 12 room is not always 144 sq ft of insulation. Walls need height times wall length minus openings. Attics use attic floor area. Crawl spaces use the joist or wall area being insulated.
Not subtracting existing R-value
If you already have R-19 and want R-49, calculate the added R-30. Ordering for the full R-49 can overbuy material and bury wiring, baffles, or attic access details.
Compressing batts to fit narrow cavities
An R-19 batt squeezed into a 2x4 wall cavity performs closer to an R-13 batt. Match batt thickness to cavity depth or use a product made for that bay.
Skipping air sealing before insulating
Seal plumbing stacks, wire holes, top plates, bath fan gaps, and attic hatches first. Insulation slows heat flow, but it does not stop warm air from leaking through holes.
Covering vents, flues, or recessed lights
Keep soffit paths open with baffles, maintain clearance at hot flues, and verify recessed lights are insulation-contact rated before covering them.
Adding insulation over moisture problems
Wet, moldy, or rodent-damaged insulation should be removed. Fix roof leaks, bath fan venting, and crawl-space ground moisture before adding more material.
Insulation Estimates for Common Projects
Attic Top-Off in Zone 5
1,200 sq ft | R-19 to R-49 | Blown cellulose
Before ordering, air seal the attic floor, install baffles, and confirm bath fans vent outside. Those tasks change the quality of the upgrade more than one extra bag.
Exterior Wall Retrofit in Zone 4
960 sq ft net wall area | R-4 to R-13 | Dense-pack cellulose
This is easiest during siding work. Ask the installer how they verify cavity fill, especially around fire blocks and older balloon framing.
Crawl Space in Zone 3
600 sq ft | R-19 fiberglass batts | Solo DIY
Install ground vapor protection and fix standing water before batts. Sagging insulation loses contact with the subfloor and performs poorly.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.