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Insulation Calculator

Find how much insulation to add to an attic or floor for your climate zone, the thickness needed for your insulation type, the volume and the number of bags.

Results

ItemValue
Recommended levels come from ENERGY STAR’s table for retrofitting wood-framed homes, based on the 2021 IECC. R per inch uses the low end of typical ranges, so the depth is not underestimated. Bag coverage varies by product, so use the chart on the bag. Check local code, and keep vents and recessed lights clear.

Quick Answer

Thickness = R-value needed ÷ R-value per inch. In zone 4A, ENERGY STAR recommends adding R-60 to an uninsulated attic. With loose-fill cellulose at about R-3.0 per inch, that is 60 ÷ 3.0 = 20 inches, or 2,000 cubic feet for a 1,200 sq ft attic.

Last updated: 3 November 2026 ·How we calculate this: ENERGY STAR R-value levels and R per inch

Table of Contents

How This Insulation Calculator Works

Enter the area of your attic or floor, choose where you are adding insulation and your climate zone, and pick an insulation type. The calculator looks up the recommended R-value to add from the ENERGY STAR table, divides it by the R-value per inch of your material to find the thickness, and works out the volume. If you enter the bag coverage from the label, it also counts the bags.

Quick explanation: R-value measures how well insulation slows heat flow.

  • Higher R: better insulation.
  • R per inch: varies by material, so the depth needed varies too.
  • Climate zone: colder zones need more insulation.

How to Use the Insulation Calculator

  1. Measure the attic or floor area in square feet.
  2. Choose where you are insulating, and your climate zone.
  3. Pick the insulation type.
  4. Optionally enter the bag coverage from the product label.
  5. Press Calculate and read the R-value, thickness and amount.

Core Formulas

  • Thickness (in): R-value to add ÷ R-value per inch
  • Volume (cu ft): area × thickness ÷ 12
  • Bags: area ÷ coverage per bag at your target R, rounded up
  • Layers add up: R-19 + R-30 = R-49

Key Terms

  • R-value: resistance to heat flow. Higher means better insulation.
  • Climate zone: a region grouped by its heating and cooling needs.
  • Loose-fill: insulation blown into place as small pieces.
  • Batt: a pre-cut blanket of insulation.
  • IECC: the International Energy Conservation Code.

Worked Example

A 1,000 sq ft attic in zone 5 already has 3 to 4 inches of insulation, and you plan to add loose-fill fiberglass.

  • ENERGY STAR suggests adding R-49 in zones 4C, 5 and 6 when 3–4 inches are already there
  • Thickness = 49 ÷ 2.5 = 19.6 inches
  • Volume = 1,000 × 19.6 ÷ 12 ≈ 1,633 cu ft

Real-Life Examples

  • Attic top-up: see how many inches of blown-in insulation to add.
  • Crawlspace floors: find the batt R-value for floors over unheated spaces.
  • Comparing materials: see how much thinner closed-cell foam is than loose fill for the same R.
  • Buying supplies: count bags using the coverage chart on the bag.

Loose-Fill vs Batts

  • Loose-fill: blown in, fills gaps around wires and pipes, good for topping up attics.
  • Batts: pre-cut pieces that fit between joists, easier for DIY in open areas.
  • Both: work well when installed at the full thickness without gaps or squashing.

ENERGY STAR Recommended R-Values

Source: ENERGY STAR, based on the 2021 IECC, for retrofitting existing wood-framed homes.

ZoneAttic, uninsulatedAttic, 3–4 in alreadyFloor
1R-30R-25R-13
2R-49R-38R-13
3R-49R-38R-19
4A/4BR-60R-49R-19
4C/5/6R-60R-49R-30
7/8R-60R-49R-38

What Changes How Much You Need

  • Climate: zones 5 to 8 need the highest levels.
  • Existing insulation: less is needed if some is already there.
  • Material: spray foam reaches a given R with less depth than loose fill.
  • Settling: some loose-fill settles over time, so labels list an installed and a settled depth.

To check what you have now, measure the depth of the existing insulation in a few places and multiply by its R per inch. If the old insulation is level with or below the tops of the floor joists, adding more is usually worthwhile. If you can see the joists clearly, you probably have less than the recommended amount.

Common Mistakes to Avoid

  • Squashing batts. Compressed insulation loses R-value.
  • Blocking soffit vents. Keep air flowing with baffles.
  • Covering recessed lights that are not rated for it. Check the fixture rating.
  • Skipping air sealing. Seal gaps before adding insulation.
  • Using the settled depth for buying. Follow the bag chart for installed depth and coverage.

Frequently Asked Questions

ENERGY STAR suggests adding R-30 to R-60 to an uninsulated attic, depending on climate zone.

It depends on the material: about 15 inches of loose-fill fiberglass or 12.7 inches of cellulose at the per-inch values used here.

Check a climate zone map from the U.S. Department of Energy or ENERGY STAR.

Usually yes, if the old insulation is dry and in good shape.

How much R-value one inch of a material gives. It differs by type.

Divide the area by the coverage per bag shown on the bag chart for your target R.

More helps up to a point. The recommended levels balance cost and savings.

ENERGY STAR suggests R-13 to R-38 for floors over unheated spaces, by zone.

Some loose-fill can settle a little. Bag charts list the settled thickness.

Yes. Sealing gaps first makes insulation work better.

Foam boards are more common for walls and basements. Follow product instructions and fire-safety rules.

Many people blow in attic insulation themselves, but spray foam is usually installed by pros.

References & Sources

SourceUsed ForLink
ENERGY STAR, Recommended Home Insulation R-Values Recommended R-values by climate zone, based on the 2021 IECC. https://www.energystar.gov/saveathome/seal_insulate/identify-problems-you-want-fix/diy-checks-inspections/insulation-r-values
Wikipedia, R-value (insulation) Typical R-values per inch of common insulation materials. https://en.wikipedia.org/wiki/R-value_(insulation)