How much insulation do I need?
The U.S. Department of Energy says the ability of insulation material to reduce heat flow is called its R-value, and that the greater the R-value, the more this heat flow is reduced. [3] It says blanket insulation, as continuous rolls or pre-cut batts, is the most common type used in homes in the United States.
The formula
Rolls or bags = the area x (1 + the extra) / the area one roll or bag covers, rounded up. The depth in inches = the R-value you want / the R-value per inch. The coverage and the R-value per inch come from your product's label, so this calculator gives no standard values: the numbers it opens on are examples.
Worked example
An attic floor 30 ft long and 20 ft wide is 30 x 20 = 600 square feet. For R49 at R3 per inch, the depth is 49 / 3 = 16.33 in. If one bag covers 40 square feet at R49, 600 / 40 = 15 bags, with no extra. Blown-in coverage depends on the R-value or depth, so read your bag's chart at your own R-value: 40 is an example, not a standard.
What R-value for attic insulation?
ENERGY STAR lists recommended R-values for the attic and the floor by climate zone, for retrofitting wood-framed homes, based on the 2021 International Energy Conservation Code. [2]
| Climate zone | Attic, uninsulated | Attic, 3 to 4 in existing | Floor |
|---|---|---|---|
| 1 | R30 | R25 | R13 |
| 2 | R49 | R38 | R13 |
| 3 | R49 | R38 | R19 |
| 4A and 4B | R60 | R49 | R19 |
| 4C, 5 and 6 | R60 | R49 | R30 |
| 7 and 8 | R60 | R49 | R38 |
The DOE consumer guide gives a range for the same job: R30 to R49 for an uninsulated attic in zone 1, against R30 in the table above. [3] The tables differ, so use the R-value your local building authority or installer gives you.
How much insulation do I need for an attic?
Measure the length and the width of the attic floor in feet. Enter the R-value you want from your zone's row above, the R-value per inch and the coverage from your product's label, and the calculator gives the rolls or bags and the depth.
How much insulation do I need for walls?
ENERGY STAR says that, for an uninsulated wood-frame wall, you can drill holes in the sheathing and blow insulation into the empty wall cavity before installing the new siding. [2]
We found no source for the R-value of a wall by its framing depth. Measure each wall's area, take away the windows and doors, and use the R-value your building authority or installer gives you.
How many bags of blown-in insulation do I need?
The U.S. Department of Energy describes loose fill as an adaptable type of insulation, which resembles packing peanuts. [3]
Blown-in insulation is sold in bags with a coverage chart on the bag: it gives the area one bag covers at each R-value. Enter the chart's figure for your R-value under Area one roll or bag covers, and the calculator divides the area by it and rounds up.
How many inches of blown-in insulation?
The depth in inches = the R-value you want / the R-value per inch on the bag. The volume of loose fill = the area in square feet x the depth in inches / 12 cubic feet: for the example, 600 x 16.33 / 12 = 816.67 cubic feet. [1]
We found no source we can read for how much loose fill settles, so the depth here is the depth to install. Ask the maker for the settled thickness that gives your R-value.
How much does insulation cost?
We state no price. Ask local suppliers for the price of one roll or bag, then enter it under Price per roll or bag, in the section Price (optional) above, to see the total. The site does not store the prices you type.
Common mistakes and limits
- Using the example values. The R-value per inch and the coverage are examples: read them from your product's label.
- Mixing units. Enter the area in feet, the depth is in inches, and the coverage in square feet.
- What this tool does not cover: vapor barriers, air sealing, ventilation, spray foam by the board foot and the framing.
Frequently asked questions
How many rolls of insulation do I need?
Enter the area one roll covers, printed on the roll, under Area one roll or bag covers. The calculator divides the area, with your extra, by it and rounds up.
How thick should blown-in insulation be?
The depth is the R-value you want divided by the R-value per inch on the label. For the example, R49 at R3 per inch is 49 / 3 = 16.33 in.
Does blown-in insulation settle?
We found no source we can read for how much blown-in insulation settles. Ask the maker of your product for the settled thickness that gives your R-value, and use the coverage chart on its bag.
How do I calculate insulation for a house?
Work out each area on its own, for the attic, the floor and the walls, because each has its own R-value. Run the calculator once for each and add the rolls or bags.
Can I use this for spray foam?
We found no source for the coverage of spray foam. Spray foam is sold by the board foot or by the kit, so use your maker's coverage chart and treat this tool as a check on the area only.
Sources and references
- National Institute of Standards and Technology (NIST). NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically (SP 811, accessed November 2026). Used for: the inch and foot relationship.
- ENERGY STAR (U.S. EPA and U.S. DOE). Recommended Home Insulation R-Values (Based on the 2021 IECC, accessed November 2026). Used for: the recommended R-values for the attic and the floor by climate zone, and blowing insulation into a wall.
- U.S. Department of Energy. Consumer Guide to Home Insulation, DOE/EE-2476 (DOE/EE-2476, accessed November 2026). Used for: what R-value is, the kinds of insulation including blown-in, and a second table of R-values.
How we use sources: methodology.
Estimates only. Confirm quantities with your supplier. See the disclaimer. Found a wrong result? Report an error.