# How much insulation do I need?

Finds the R-value to add to an attic or floor for your climate zone, the depth of insulation that gives it, and the volume to cover the area.

- Page: https://www.acalculator.org/construction/insulation-calculator
- JSON spec: https://www.acalculator.org/construction/insulation-calculator.json
- Version: b3c2cfd67efd

## Default answer

Example with the default inputs (Climate zone Zone 4A or 4B, Where Attic, none yet, Area to insulate 1,000 ft², R-value per inch 3): Add R-60: about 20 inches of insulation, 1,667 cubic feet over the area.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| zone | Climate zone | Your IECC climate zone, from 1 (hottest) to 8 (coldest). |
| where | Where | An attic with no insulation, an attic that already has 3 to 4 inches, or a floor over an unheated space. |
| a | Area to insulate | The floor area of the attic, or the area of the floor. |
| rpi | R-value per inch | The R-value one inch of your insulation gives, from its label (about 3 for blown fiberglass or cellulose). |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| r | R-value to add | The recommended R-value to add for this zone and place (US units, ft²·°F·h/Btu). |
| inches | Depth (in) | R-value to add ÷ R-value per inch. |
| cm | Depth (cm) | The same depth in centimetres. |
| cubicFeet | Volume (ft³) | Area × depth: the insulation to fill the space, in cubic feet. |
| cubicMeters | Volume (m³) | The same volume in cubic metres. |
| rsi | RSI to add | The R-value in metric units (m²·K/W): R × 0.1761102. |

## Method

R to add = the zone’s value for the place; depth (in) = R ÷ R per inch; volume = area × depth.

## Assumptions

- The R-values are ENERGY STAR’s recommendations for adding insulation to existing wood-framed homes; your local code may ask for more in new work.
- R-value per inch differs by product and how it is installed; use the number on the bag or batt label when you have it.
- Blown insulation settles; the bag label gives the installed depth and the number of bags for each R-value, which this page does not count.

## Worked examples

1. zone = 4, where = attic, a = 92.90304, rpi = 3 gives r = 60, inches = 20, cm = 50.8, cubicFeet = 1,666.666667, rsi = 10.566612. Source: ENERGY STAR, Recommended Home Insulation R-Values (retrofit of existing wood-framed buildings, based on 2021 IECC Table R402.1.3), https://www.energystar.gov/saveathome/seal_insulate/identify-problems-you-want-fix/diy-checks-inspections/insulation-r-values (zone 4A and 4B, uninsulated attic: R60); ENERGY STAR, How to Check Your Home’s Attic Insulation Level (fiberglass, cellulose and mineral wool about R-3 to R-3.5 per inch; depth × 3), https://www.energystar.gov/products/ask-the-experts/how-check-your-homes-attic-insulation-level.
2. zone = 2, where = topup, a = 139.35456, rpi = 2.5 gives r = 38, inches = 15.2, cubicFeet = 1,900. Source: ENERGY STAR, Recommended Home Insulation R-Values (retrofit of existing wood-framed buildings, based on 2021 IECC Table R402.1.3), https://www.energystar.gov/saveathome/seal_insulate/identify-problems-you-want-fix/diy-checks-inspections/insulation-r-values (zone 2, 3–4 in existing: add R38).
3. zone = 7, where = floor, a = 100, rpi = 3.5 gives r = 38, inches = 10.857143, cubicMeters = 27.577143. Source: ENERGY STAR, Recommended Home Insulation R-Values (retrofit of existing wood-framed buildings, based on 2021 IECC Table R402.1.3), https://www.energystar.gov/saveathome/seal_insulate/identify-problems-you-want-fix/diy-checks-inspections/insulation-r-values (zones 7 and 8, floor: R38).
4. zone = 1, where = attic, a = 92.90304, rpi = 3 gives r = 30, inches = 10. Source: ENERGY STAR, Recommended Home Insulation R-Values (retrofit of existing wood-framed buildings, based on 2021 IECC Table R402.1.3), https://www.energystar.gov/saveathome/seal_insulate/identify-problems-you-want-fix/diy-checks-inspections/insulation-r-values (zone 1, uninsulated attic: R30); DOE, Energy Efficient Home Improvement Credit: Insulation and Air Sealing (zone 1 uninsulated attic R30), https://www.energy.gov/cmei/buildings/articles/energy-efficient-home-improvement-credit-insulation-and-air-sealing.

## FAQ

### How much attic insulation do I need?

It depends on your climate zone. For an attic with no insulation, ENERGY STAR recommends adding R30 in zone 1, R49 in zones 2 and 3, and R60 in zones 4 to 8. With 3 to 4 inches already there, add R25 in zone 1, R38 in zones 2 and 3, and R49 in zones 4 to 8.

### How deep should the insulation be?

Divide the R-value by the R-value per inch of your insulation. Fiberglass, cellulose and mineral wool give about R-3 to R-3.5 per inch, so R60 at R-3 per inch is 20 inches deep, and R38 at R-2.5 per inch is 15.2 inches.

### How much insulation do I need to buy?

The volume is the area times the depth: 1,000 ft² at 20 inches is 1,000 × 20 ÷ 12 = 1,667 ft³. Each bag of blown insulation lists how many square feet it covers at each R-value; use that chart to count bags.

### What climate zone am I in?

The International Energy Conservation Code splits the US into zones 1 (hottest) to 8 (coldest), some with A, B or C parts. The DOE page in the sources has a map and a link to look up your zone.

### What R-value does a floor need?

For a floor over an unheated space such as a crawl space or garage, ENERGY STAR recommends R13 in zones 1 and 2, R19 in zones 3 and 4A or 4B, R30 in zones 4C, 5 and 6, and R38 in zones 7 and 8.

### What is RSI?

The metric R-value, in m²·K/W. One US R unit is 0.1761102 RSI, so R60 is RSI 10.57.

## Sources

- ENERGY STAR (US EPA), Recommended Home Insulation R-Values: recommended insulation levels for retrofitting existing wood-framed buildings by zone (attic uninsulated, attic with 3–4 in, floor), based on the 2021 IECC Table R402.1.3. https://www.energystar.gov/saveathome/seal_insulate/identify-problems-you-want-fix/diy-checks-inspections/insulation-r-values (retrieved 2026-10-02)
- US Department of Energy, Energy Efficient Home Improvement Credit: Insulation and Air Sealing: minimum ceiling R-value in an uninsulated attic by climate zone under the 2021 IECC (R30, R49, R49, R60 …). https://www.energy.gov/cmei/buildings/articles/energy-efficient-home-improvement-credit-insulation-and-air-sealing (retrieved 2026-10-02)
- ENERGY STAR, How to Check Your Home’s Attic Insulation Level: fiberglass, cellulose and mineral wool have an R-value of about 3 to 3.5 per inch; depth in inches × 3 estimates the R-value. https://www.energystar.gov/products/ask-the-experts/how-check-your-homes-attic-insulation-level (retrieved 2026-10-02)
- NIST Special Publication 811, Appendix B (1 ft²·h·°F/Btu = 0.1761102 m²·K/W; 1 in = 2.54 cm, 1 ft = 0.3048 m exactly). https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-02)