Attic Insulation Calculator: How Many Inches Do You Need?

How the calculation works

R-value measures how well insulation resists heat flow. The higher the number, the better it insulates. The calculator estimates your current R-value from the depth and type of insulation you have, compares it with the recommended level for your climate zone, and works out the inches of new material needed to close the gap.

Current R-value   = current depth (inches) × R per inch of current material
R-value to add    = recommended R-value − current R-value   (not less than 0)
Inches to add     = R-value to add ÷ R per inch of new material
Total depth       = current depth + inches to add
Volume to add     = inches to add ÷ 12 × attic floor area   (cubic feet)

R per inch. The tool uses the multipliers from ENERGY STAR’s do-it-yourself insulation check: loose-fill fiberglass 2.5, loose rock wool 2.8, fiberglass batts 3.2, and loose-fill cellulose 3.7.

Recommended R-value. The target is ENERGY STAR’s recommendation for an uninsulated attic in each climate zone: R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8.

ENERGY STAR also publishes a lower figure for attics that already have 3 to 4 inches: R-25, R-38 or R-49 depending on the zone. By default the calculator aims for the higher, uninsulated-attic level. If budget is tight, choose the add-to level in the Target level setting; it is a reasonable first goal.

Source: ENERGY STAR recommended home insulation R-values.

Worked example

The default case is a 1,200-square-foot attic in climate zone 5, the Midwest or southern New England.

1. The attic has 4 inches of loose-fill fiberglass. At R-2.5 per inch, that is about R-10.
2. The recommended level for zone 5 is R-60.
3. The homeowner plans to blow in loose-fill cellulose at R-3.7 per inch. To make up the difference takes about 13.5 inches.
4. The finished depth would be about 17.5 inches.
5. Over 1,200 square feet, that is about 1,351 cubic feet of material to add.

A second example

A 1,500-square-foot attic in zone 3, in the Southeast, has 6 inches of fiberglass batts. The homeowner plans to add loose-fill fiberglass.

• Current R-value: about R-19
• Recommended: R-49
• Inches to add: about 11.9 inches
• Total depth when done: about 17.9 inches
• Volume to add: about 1,490 cubic feet

If this homeowner aimed instead for ENERGY STAR’s R-38 level for attics that already have some insulation, the job would be smaller. Change the zone or depth in the tool to compare.

What your result means

“Inches to add” is the settled depth of new material you need. Installers and product labels work in R-values and bag counts, so use the R-value target when you talk with a contractor or buy material. Product bags list the bags per 1,000 square feet and the minimum settled thickness for each R-value.

The volume figure helps you sanity-check a quote or plan a do-it-yourself job, but it is not a bag count. Loose-fill is sold by weight, and coverage differs by product.

Insulation pays off most in attics with little or none. To see roughly what the upgrade could save on heating, take your before and after R-values to the insulation savings calculator. Our attic insulation guide covers materials, cost ranges and when to hire a pro.

Air sealing, climate and incentives that change the answer

Air seal first. EPA recommends air sealing the attic before adding insulation. Gaps around wiring, plumbing, chimneys and light fixtures let warm air escape, and new insulation does not stop air. EPA estimates that air sealing plus insulating saves an average of 15% on heating and cooling costs. Our air sealing guide shows where to look.

Ventilation and moisture. Fix bath, kitchen or dryer fans that vent into the attic before insulating. Keep baffles at the eaves so air can flow from soffit vents.

Local codes. Building codes in your area may set minimum levels for new construction or major renovations. Your local building department can confirm.

Incentives. The federal 25C credit does not cover insulation placed in service after December 31, 2025. Some state-run Home Energy Rebate programs, the Weatherization Assistance Program for income-eligible households, and many utilities help pay for insulation and air sealing. Check your state energy office, your utility, or a professional home energy audit, which many utilities subsidize.

Common mistakes

• Measuring only in the middle of the attic, where insulation is deepest.
• Treating batts as R-3.2 per inch when they are compressed, gapped or poorly fitted.
• Adding insulation before sealing air leaks.
• Covering soffit vents, which can lead to moisture and ice dam problems.
• Buying material by cubic feet instead of following the coverage chart on the bag.
• Using faced batts on top of existing insulation, which can trap moisture.

Limits of this calculator

The R-per-inch values are rough averages, and real products vary. Settled depth, compression, gaps and wet or damaged insulation all lower actual performance. The tool does not account for framing, air leakage, or attics with sloped ceilings or knee walls. It does not estimate cost or savings. This is an educational estimate, not a quote or advice. Get at least three written quotes that state the R-value, settled depth and bag count.