Insulation Calculator
Calculate blow-in depth, batt coverage, and R-value for walls, attics, and crawl spaces.
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Result
Formulas
Depth = R-Value ÷ R-per-inch
Bags = Area × Depth ÷ Bag Coverage
Understanding Insulation
Insulation resists heat flow and is measured by its R-value — the higher the R-value, the greater the insulating effectiveness. Whether you're blowing cellulose into an attic, installing fiberglass batts in walls, or calculating the total R-value of a multi-layer wall assembly, accurate estimates help you meet building code requirements and reduce energy costs. This calculator supports blow-in depth and bag estimates, batt/roll coverage, and multi-layer R-value stacking with common material presets.
Formulas
How to Calculate Insulation
Frequently Asked Questions
What R-value do I need for my attic?
The recommended attic R-value depends on your climate zone. The U.S. DOE recommends R-38 to R-60 for most attics: R-38 in warm climates (zones 1-3), R-49 in mixed climates (zones 4-5), and R-60 in cold climates (zones 6-7). Existing insulation should be topped up to meet these targets. Check local building codes for minimum requirements in your area.
How many bags of blow-in insulation do I need?
First, calculate the required depth: Depth = Target R-Value ÷ R-per-inch. Then multiply the area by the depth to get total cubic feet needed. Divide by the bag coverage (sq ft per bag at 1 inch depth times the depth) to get the number of bags. For example, 1,000 sq ft at R-38 with cellulose (3.5 R/in) requires about 10.86 inches of depth. If each bag covers 40 sq ft at 1 inch, you need (1,000 × 10.86) ÷ 40 = 272 bags.
What is the difference between R-value and U-value?
R-value measures thermal resistance — how well a material resists heat flow. Higher R-value means better insulation. U-value (or U-factor) measures thermal transmittance — how easily heat flows through an assembly. U-value is the reciprocal of R-value: U = 1 ÷ R. Lower U-value means better insulation. Windows are typically rated by U-value, while insulation is rated by R-value.
Can I stack insulation layers to increase R-value?
Yes, R-values are additive. If you have existing R-11 insulation in a wall and add R-13 batts over it, the total R-value is R-24. This is why adding insulation over existing material is effective. However, compressed insulation loses R-value — make sure the cavity can accommodate the full thickness without compressing the new layer. Also, avoid mixing vapor barriers, which can trap moisture.
How much does insulation cost?
Insulation costs vary by type and region. Blow-in fiberglass runs about $0.80-$1.50 per sq ft at R-38. Blow-in cellulose is similar at $0.70-$1.40 per sq ft at R-38. Fiberglass batts cost roughly $0.30-$1.00 per sq ft depending on R-value. Spray foam is the most expensive at $1.00-$3.00 per sq ft for open-cell and $1.50-$4.50 for closed-cell. Professional installation typically adds 50-100% to material costs.