How much insulation do I need? Enter your dimensions and R-value target to get exact material quantities, thickness, and cost estimates for fiberglass, cellulose, or spray foam.
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Last updated March 31, 2026 by our expert review team
R-3.2/inch
$0.64/sq ft
R-3.5/inch
$1.2/sq ft
R-6.5/inch
$2.5/sq ft
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 March 2026
How we verify our calculatorsHot climates
Attic: R-30
Wall: R-13
Mixed climates
Attic: R-38
Wall: R-13
Cold climates
Attic: R-49
Wall: R-20
Very cold
Attic: R-60
Wall: R-21
Higher R-value per inch means less thickness needed to reach the same insulation level.
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.
The calculator starts with a fundamental thermal equation: your target R-value divided by the material's R-value per inch gives you the required insulation thickness. For example, hitting R-49 with fiberglass batts (R-3.2/inch) means you need roughly 15.3 inches of material. That is why most cold-climate attics use two stacked layers of R-25 batts rather than a single impossibly thick piece.
From there, we multiply your length by width to get total square footage, then add a 10% waste factor. This waste allowance covers irregular joist spacing, cuts around obstructions like ductwork and electrical boxes, and the material lost when trimming batts to fit around plumbing stacks. For blown cellulose, the waste factor also accounts for the 15-20% settling that occurs in the first few years after installation.
The cost estimate uses current average pricing for each insulation type: fiberglass batts at roughly $0.64/sq ft, blown cellulose at $1.20/sq ft (including rental of blowing equipment), and professional spray foam at $2.50/sq ft. These are material-only costs scaled proportionally to the thickness your R-value target requires. For a detailed budget breakdown, try our insulation cost calculator. Actual project costs may vary based on your region, accessibility of the space, and whether you hire a contractor.
Formula
Target R-value ÷ R per inch = Thickness needed
Area ÷ Coverage per bag = Bags needed
What R-value do I need for my attic?
Zones 1 to 3 require R-30 to R-38. Zones 4 to 8 call for R-49 to R-60. Use our attic insulation calculator for zone-specific recommendations.
Can I install new insulation over old?
Yes, if the existing material is dry and mold-free. Use unfaced batts or blow cellulose on top. Never add a second vapor retarder between layers.
Is DIY insulation realistic?
Fiberglass batts and blown cellulose are DIY-friendly. Rent a cellulose blower for about $50/day with a material purchase. Spray foam requires professional equipment.
How much can I save on energy bills?
Properly insulating an under-insulated attic saves 10 to 50% on heating and cooling. Going from R-11 to R-49 typically saves $200 to $600 per year.
Faced vs. unfaced insulation?
Faced insulation has a vapor retarder that goes against the warm-in-winter surface. Unfaced batts are for second layers or climates that do not need a retarder.
Spray foam vs. fiberglass?
Closed-cell spray foam delivers R-6.5/inch vs. R-3.2 for fiberglass, but costs $2.00 to $3.50/sq ft installed vs. $0.50 to $1.00 for DIY batts. See our spray foam calculator for detailed estimates.
Do I need a vapor barrier?
Cold climates (zones 5 to 8) require one on interior walls. Mixed climates often need only latex paint over drywall. Check your local code.
Is my existing insulation enough?
Measure depth in the attic. Fiberglass provides R-3.2/inch, cellulose R-3.5/inch. If you fall short of your zone's target, topping off is high-ROI.
Compressing batts to fit narrow cavities
An R-19 batt compressed into a 3.5-inch cavity loses about 35% of its R-value. Use R-13 for 2x4 walls and R-19 or R-21 for 2x6 walls.
Skipping air sealing before insulating
Insulation slows heat transfer but does not stop air leakage. Seal penetrations with caulk or spray foam first. Air leaks account for 25 to 40% of energy loss.
Installing the vapor barrier on the wrong side
In cold climates (zones 5 to 8), the retarder must face the heated interior. In hot-humid climates, it sometimes goes on the exterior. Always check local code.
Leaving gaps and voids around obstacles
Split fiberglass batts around wiring and pipes so material fills behind and in front. A 5% void can reduce R-value by up to 25%.
Blocking soffit vents with insulation
Install polystyrene rafter baffles in every bay before insulating. Maintain at least a 1-inch air channel to prevent ice dams and heat buildup.
Ignoring fire blocking at garage and chase walls
Code (IRC R302.11) requires fire blocking at concealed stud cavities. Use mineral wool or 0.5-inch drywall at floor and ceiling lines.
1,200 sq ft | R-19 to R-49 | Blown cellulose
Tip: Rent a free blower from Home Depot with a 10+ bag purchase to save $75-$150.
960 sq ft net wall area | R-4 to R-13 | Dense-pack cellulose
Tip: Best done during a siding replacement when cavities are already accessible.
600 sq ft | R-19 fiberglass batts | Solo DIY
Tip: Lay 6-mil poly ground cover to control moisture before installing batts.
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.