Part of the Construction collection — 4 tools available
Insulation Calculator — R-Value & Coverage
Calculate insulation coverage area, R-value needed, and number of bags or rolls. Determine how much batt, blown, or rigid insulation your project requires.
About This Calculator
Proper insulation is the single most cost-effective way to reduce energy bills and improve year-round comfort, yet the Department of Energy estimates that 90% of American homes are under-insulated. Whether you're topping up attic insulation to meet modern R-38 standards, insulating basement walls to R-13, or adding spray foam to a crawl space, knowing the target R-value and material quantity is the critical first step. Fiberglass batts, blown-in cellulose, and rigid foam boards each have different R-values per inch, which means the same R-38 target requires different depths depending on your material choice — 12 inches of fiberglass versus roughly 10 inches of cellulose. Buying too few bags means another trip to the store mid-project, while overbuying ties up budget. Our insulation calculator determines the exact depth needed, number of bags or rolls, and estimated total material cost based on your area, target R-value, and chosen insulation type.
The Formula Behind This Calculator
Area = Length * Height R-value = Thickness * R per inch Batts = Area / Coverage per batt
Understanding the math helps you verify results and make better decisions for your project.
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How to Use
- 1Enter the square footage of the area to insulate.
- 2Set your target R-value (R-38 for attics, R-13 for walls in most climate zones).
- 3Enter the R-value per inch of your chosen insulation type.
- 4Enter the coverage per bag at your target R-value (from the bag label).
- 5Enter price per bag and click Calculate.
When to Use
- →Planning an attic insulation upgrade to reduce heating and cooling bills
- →Determining how many bags of blown-in insulation to order from the home improvement store
- →Comparing the cost and depth needed between fiberglass, cellulose, and spray foam insulation
Tips
- ✓Don't compress existing insulation when adding more — compressed insulation loses R-value and defeats the purpose
- ✓Seal all air leaks in the attic before adding insulation — air sealing often saves more energy than the insulation itself
- ✓Mark the target depth on roof rafters with a yardstick so you can verify even coverage across the entire attic
FAQ
What R-value do I need for my attic?
Climate zones 1-3: R-30 to R-38. Zones 4-5: R-38 to R-49. Zones 6-8: R-49 to R-60. Check DOE recommendations for your specific zip code.
What is the R-value per inch of common insulation?
Fiberglass batt: 3.1-3.4/inch. Cellulose blown: 3.2-3.8/inch. Spray foam (open cell): 3.5-3.7. Spray foam (closed cell): 6.0-7.0.
Can I add new insulation over old?
Yes for unfaced fiberglass or cellulose over existing. Remove any moisture-damaged insulation first. Do not stack faced insulation — the facing creates a moisture trap.
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