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Attic Insulation Calculator: R-Value & Inches to Add

Find the ENERGY STAR attic R-value for your climate zone, estimate what your insulation is worth now and how many inches to add.

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Good to know: Scope: adding insulation to the attic floor of an existing wood-framed home, using the ENERGY STAR table of recommended levels for retrofits (based on the 2021 International Energy Conservation Code, Table R402.1.3) by climate zone, and ENERGY STAR’s rough R-value per inch for measuring insulation you already have: loose fiberglass about 2.5, loose rock wool 2.8, cellulose 3.7, vermiculite or perlite 2.7 and fiberglass batts 3.2. ENERGY STAR gives the R-value to add for an uninsulated attic and for an attic that already has 3–4 inches; for any other depth this tool estimates the gap to the uninsulated-attic level. Not included: walls, cathedral ceilings and roof decks, spray foam and rigid boards, new construction code compliance, number of bags (use the coverage chart on the bag), prices, rebates, tax credits and energy savings, air sealing and ventilation, and finding your zone by ZIP code (use the map on the ENERGY STAR page). Everything stays in this browser tab; nothing is sent or saved. Independent ToolsKo estimate, not an ENERGY STAR product. Sources reviewed November 13, 2026.

How to use it (step by step)

  1. 1Find your climate zone on the map on the ENERGY STAR “Recommended Home Insulation R-Values” page and pick it from the list.
  2. 2Look in your attic (walk only on the joists) and pick what kind of insulation you see. Push a ruler down to the attic floor and type the depth in inches, or type the R-value printed on the batts.
  3. 3Pick the insulation you plan to add and read the R-value and the inches to add. Before you buy, check the R-value and thickness on the bag or batt label.

Words to know

R-value
How well insulation slows heat from passing through it. The higher the number, the better it insulates. R-49 insulates better than R-30.
Climate zone
The U.S. is split into zones 1 (hottest, like Florida and Hawaii) to 8 (coldest, interior Alaska). Colder zones need more insulation.
Loose fill (blown-in)
Small pieces of fiberglass, rock wool or cellulose blown into the attic with a machine. It fills gaps well and you can add it on top of old insulation.
Attic floor joists
The wood beams across the attic floor. If your insulation is level with them or lower, you probably need more.

Explore tools for Australia, the US & Europe →

Sources & methods

References checked November 11, 2026. Read each tool’s scope and assumptions before relying on its result.

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