Insulation Calculator
Calculate how many batts or rolls of insulation you need for walls, attics, or crawl spaces based on area and R-value.
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Total Area —
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Extended More scenarios, charts & detailed breakdown ▾
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Rolls/Batts Needed
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Total Attic Area —
Order Area (10% buffer) —
Professional Full parameters & maximum detail ▾
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Area Calculations
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Net Insulation Area —
Area to Order (with waste) —
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Cost Estimate
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Labor Cost (est.) —
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Total Cost per Sq Ft —
Specifications
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How to Use This Calculator
- Enter the area length and width to insulate in feet.
- Select the target R-value.
- Enter the coverage per roll/bag from your product label.
- Results show rolls/batts needed with a 10% waste buffer.
Formula
Total area = Length × Width
Rolls needed = ceil(Total area × 1.10 ÷ Coverage per roll)
Example
Example: 40×30 ft attic, R-38, 40 sq ft per roll → 1,200 sq ft → 33 rolls (with 10% waste).
Frequently Asked Questions
- R-value requirements depend on climate zone and location in the building. The US Department of Energy divides the country into 8 climate zones. Attic insulation typically needs R-38 to R-60 (Zones 1–3 need R-38; Zones 4–8 need R-49 to R-60). Exterior walls need R-13 to R-21 (R-15 is the current standard for 2×4 walls; R-20 for 2×6 walls). Crawl space floors need R-25 to R-30. Basement walls need R-5 to R-15. Check ENERGY STAR or your local building code for the specific requirements in your climate zone. Insulation is one of the best long-term investments in home energy efficiency.
- A batt is a pre-cut rectangular section of insulation, typically 15 or 23 inches wide to fit between studs spaced 16 or 24 inches on center. Standard batt lengths are 4 feet and 8 feet. Rolls are the same fiberglass or mineral wool material in a continuous strip that you cut to length as you install. Batts are easier to install in wall cavities, while rolls are more economical for large open attic floors. Both come in various R-values: R-11, R-13, R-15, R-19, R-21, and R-30 for batts, and R-38 and R-49 rolls for attics.
- Coverage depends on the target R-value and the specific product. Every bag of blown cellulose or fiberglass insulation has a coverage chart printed on the label that shows square feet per bag at each R-value depth. For example, a 25 lb bag of blown fiberglass may cover 40–80 sq ft depending on whether you are installing R-19 or R-38. Always follow the chart on your specific product. The number of bags needed = attic area ÷ sq ft per bag at your target R-value. This calculator uses typical average coverage values — always verify with your product label.
- Measure the attic floor area (length × width of the attic space at floor level). Do not measure the roof slope — insulate the attic floor, not the rafters, in a vented attic. Then select your target R-value based on your climate zone. The calculator shows the number of batt rolls or blown-in bags required. For blown insulation, the coverage chart on the bag gives sq ft per bag at your target R-value. For example, a 1,200 sq ft attic targeting R-49 might need approximately 25 bags of blown fiberglass, depending on the brand and installation depth.
- Yes — add 10% waste to your insulation order to account for cutting around ceiling joists, electrical boxes, pipes, and HVAC equipment. In attics with irregular framing, obstructions, and knee walls, the actual usable area can be 10–15% less than the measured floor area. For blown insulation, add a few extra bags since you can return unopened bags but cannot buy partial bags. For batts, order at least one extra roll or pack. Proper air sealing before installing insulation is equally important — gaps and penetrations are the biggest source of heat loss in attics.