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Insulation Calculator
Estimate insulation thickness, batt bundles or loose-fill bags, and cost for any wall, ceiling, or floor. Supports fiberglass batt, blown cellulose, and open or closed-cell spray foam at any target R-value.
Insulation requirements
Insulation needed
9 bundles
9.38 in thick to reach R-30 across 500 sq ft.
- Layer thickness
- 9.38 in
- Effective R-value
- R-30
- Coverage area
- 500 sq ft
- Batt bundles
- 9 bundles
- Rough annual savings
- $750.00
Planning estimate only. Check the IECC for the recommended R-value in your climate zone and follow the manufacturer's coverage chart on the bag. Annual savings is a very rough order-of-magnitude figure, not a forecast.
Frequently Asked Questions about the Insulation Calculator
What R-value do I need by climate zone?
The US Department of Energy recommends R-30 to R-49 in attics for zones 1 to 3 (Florida, Gulf Coast, much of California), R-38 to R-60 for zones 4 and 5 (mid-Atlantic, Pacific Northwest), and R-49 to R-60 for zones 6 to 8 (Northeast, Upper Midwest, Alaska). Walls usually call for R-13 to R-21 in framed cavities and R-5 to R-10 of continuous exterior insulation in the colder zones. Floors over unheated crawl spaces want R-19 to R-30. Check the IECC table for your county before you buy.
How much R-value do I get per inch of each material?
Fiberglass batt gives about R-3.2 per inch, so an R-30 attic layer is roughly 9.5 in thick. Blown cellulose runs about R-3.7 per inch (R-30 in about 8 in). Open-cell spray foam is about R-3.6 per inch and expands to fill the cavity. Closed-cell spray foam is the densest at about R-6.5 per inch, so an R-19 layer fits in about 3 in, which is the only practical option in 2x4 walls when you need high R-value plus an air seal.
Batt, blown, or spray foam: which is cheapest?
Fiberglass batt is the cheapest at about $0.40 to $1.50 per sq ft installed (or $0.20 to $0.40 DIY for the batts alone). Blown cellulose runs about $1.00 to $2.50 per sq ft installed, or $0.50 to $1.00 DIY if you rent the blower. Open-cell spray foam is $1.00 to $1.50 per board foot installed, closed-cell is $1.75 to $3.00. Spray foam costs the most but seals air leaks at the same time, which usually saves an extra 10 to 20 percent on heating and cooling beyond the R-value alone.
When does closed-cell spray foam beat open-cell?
Pick closed-cell when you need a vapor barrier (rim joists, basement walls below grade, exterior of foundations), when the cavity is shallow and you need maximum R-value per inch (2x4 walls, van conversions, low-clearance roofs), or in flood zones where the foam needs to resist water absorption. Open-cell is fine and far cheaper for above-grade walls, attics, and interior soundproofing, where you have room to install the full thickness and do not need a vapor barrier on that side of the assembly.
How do I calculate U-value from R-value?
U-value is the inverse of R-value: U = 1 / R. An R-30 attic has a U-value of about 0.033 Btu per hour per sq ft per degree F, meaning each sq ft loses 0.033 Btu per hour for every 1 degree F difference between inside and outside. Doubling the R-value cuts the U-value in half and roughly halves the heat loss. The IECC sets minimum U-values for windows and assemblies, and your insulation choice is the biggest lever for hitting them.