Insulation Calculator
Calculate how many rolls or bags of insulation you need by area and R-value.
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How to Calculate Insulation Needed
Proper insulation is one of the best investments you can make in energy efficiency. The insulation calculator helps you determine how many bags, rolls, or batts to buy based on your area and R-value target. Getting this right prevents both costly over-ordering and frustrating mid-project shortfalls.
R-Value by Location
R-value measures thermal resistance — higher values mean better insulation. DOE recommendations by location: attics in most US climates: R-38 to R-49. Exterior walls: R-13 to R-21 depending on framing size. Floors over unconditioned spaces: R-25 to R-30. Crawl spaces: R-19. Basement walls: R-11 to R-15. Check the DOE's Zip Code Insulation Program for exact recommendations for your location.
Batt vs. Blown-In
Fiberglass and mineral wool batts are pre-cut to fit standard stud cavities (16 or 24 inches OC). They're easy to install DIY. Blown-in insulation uses loose fibers or cellulose blown into place — ideal for attic floors, existing wall cavities, and irregular spaces. A blower machine is typically rented or provided free with purchase of a minimum number of bags at hardware stores.
Coverage per Package
Every package of insulation lists coverage area on the label for the target R-value. This is the number to use in your calculation. Coverage varies by product — blown fiberglass for R-38 might cover 40 sq ft per bag while R-49 covers only 30 sq ft (because you need more depth). Always use the number from the specific product you're buying.
Air Sealing First
Before adding insulation, air seal all penetrations — around pipes, wires, can lights, and at the top plates. Air sealing prevents convective heat loss that insulation alone cannot stop. Use caulk for small gaps and expanding foam for larger gaps. Air sealing + proper insulation can reduce heating/cooling costs by 15–30%.
Frequently Asked Questions
Most US climates: R-38 to R-49. Cold climates (zones 6–7): R-49 to R-60. Warm climates (zones 1–2): R-30 to R-38. Check the DOE's recommendations for your specific climate zone for the most accurate guidance.
For R-38 blown fiberglass at ~40 sq ft/bag: 1,000 ÷ 40 = 25 bags. Add 10% overage = 28 bags. Coverage per bag varies by product — always check the label for your specific target R-value.
Yes — you can layer additional batts or blown-in insulation over existing insulation in attics. Do not use faced batts (kraft or foil) against existing insulation — use unfaced. Never compress insulation, as this reduces its R-value.
Open-cell spray foam (R-3.5/inch) and closed-cell spray foam (R-6/inch) provide excellent air sealing along with insulation. Closed-cell also acts as a vapor barrier. Cost is $1.50–$5 per sq ft installed vs. $0.25–$1.50 for batts. The payback period can be 5–15 years depending on climate and energy rates.
An uninsulated 2×4 stud wall with drywall on both sides has about R-4 total. Adding R-13 fiberglass batts brings it to roughly R-14. Adding exterior rigid foam (R-5 per inch) can push it to R-20+. For a 2×6 wall with R-19 batts: approximately R-20 total assembly value.