Insulation Calculator
Calculate depth, bags, and cost for blown cellulose, blown fiberglass, and fiberglass batts.
Insulation Type
Blown cellulose: best value for attics. Blown fiberglass: moisture-resistant. Fiberglass batts: for walls and floors between joists.
Bags Needed
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bags · includes 10% waste
Coverage per bag/roll
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Depth to add
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R-Value to Add
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Total Depth at Target R
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Cost Estimate
Based on typical retail pricing
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How this was calculated
How to Use This Calculator
Select your insulation material using the buttons at the top: blown cellulose and blown fiberglass are for attics and open cavities, fiberglass batts are for walls and floors between joists. Enter the area to insulate in square feet — length times width for rectangular spaces, add sections for irregular shapes. Enter your target R-value. For most attics, R-38 to R-60 is standard depending on climate zone. If adding to existing insulation, enter your current R-value; the calculator finds only what you need to add. The result shows depth to add, bags or rolls needed, and a cost estimate. For fiberglass batts, roll coverage varies by product; verify the square footage on your specific package before ordering.
How to Calculate Insulation
The formula for blown insulation: Bags Needed = Area ÷ (Bag Volume ÷ Depth Needed) × 1.10 waste factor. Depth comes from your target R-value divided by the material's R-value per inch.
Example: 1,200 square foot attic, R-38, blown cellulose (3.7 R per inch): Depth = 38 ÷ 3.7 = 10.3 inches Bag volume = 30 lbs ÷ 1.5 lb/cu ft installed = 20 cubic feet per bag Coverage = 20 ÷ (10.3 ÷ 12) = 23.3 sq ft per bag Bags = 1,200 ÷ 23.3 × 1.10 = 56.7, rounded up to 57 bags.
Blown fiberglass uses 2.5 R per inch instead of 3.7, requiring more depth and more bags for the same R-value. Blown cellulose is denser and typically costs less per bag with better coverage per dollar. For fiberglass batts, roll coverage is approximately 40 square feet per roll but varies by product width and length; always check the package.
Insulation by Application
| Location | Best material | Target R-value | DIY-friendly |
|---|---|---|---|
| Attic floor (new) | Blown cellulose | R-38 to R-60 | Yes |
| Attic floor (top-up) | Blown cellulose over existing | Add to reach R-38 to R-60 | Yes |
| Wall (new construction) | Fiberglass batts | R-13 (2x4) / R-19 (2x6) | Yes |
| Wall (retrofit) | Dense-pack blown cellulose | R-13 to R-21 | Harder |
| Floor over crawl space | Fiberglass batts | R-19 to R-30 | Yes |
| Basement rim joist | Rigid foam + fiberglass batts | R-15 to R-25 | Yes |
Insulation Tips
Check your attic's current R-value before buying anything. Many homes built before 1990 have R-11 to R-19 in the attic, well below current recommendations of R-38 to R-60 depending on climate zone. Adding to existing insulation is the most cost-effective upgrade; you do not need to remove old batts before blowing cellulose over them.
Rent a blowing machine when buying bags. Most hardware stores that sell blown insulation bags offer free or low-cost machine rental with a minimum bag purchase. A two-person team — one loading the machine, one directing the hose — can complete a 1,500 square foot attic in four to six hours.
For batts in walls, seal all electrical boxes and penetrations with acoustic sealant before installing. Insulation pressed around unsealed gaps performs significantly below its rated R-value. Air sealing matters more than adding extra thickness once a wall is already partially insulated.
What to Buy
For blown cellulose, bags typically weigh 25 to 30 lbs. Coverage per bag is printed on the packaging for common R-values. Always use the bag's own coverage chart, not a generic formula, because installed density varies between products.
For blown fiberglass, bags run 19 to 25 lbs each. Coverage per bag is lower than cellulose at equivalent R-values because fiberglass is less dense and requires more depth. It is faster to blow and does not absorb moisture, which makes it a better choice in humid climates or areas with potential water exposure.
For fiberglass batts, match the batt width to your framing: 15 inches for 16-inch on-center studs or joists, 23 inches for 24-inch on-center. Unfaced batts are used when adding insulation over existing material. Faced batts with a kraft-paper vapor barrier are for new construction and specific climate zone requirements where vapor control is required on a particular side of the assembly.