Attic Insulation Calculator

Work out how much R-value, depth and how many bags of blown-in cellulose, blown fiberglass or batts you need to bring your attic up to ENERGY STAR recommendations for your climate zone.

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sq ft
R
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R
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Results will appear here once you enter your values.

How much insulation to add: a worked example

The math has three steps. Existing R = depth x R per inch. R to add = target R - existing R. Depth to add = R to add / R per inch of the new material.

Example: a 1,000 sq ft attic in climate zone 5 has 3.5 inches of fiberglass batts, about R11. ENERGY STAR recommends R49 when adding to 3-4 inches of existing insulation, so you need to add R38. Blown cellulose at R3.5 per inch means 38 / 3.5 = 10.9 inches of settled depth. If the bag chart says one bag covers 21 sq ft at R38, you need 1,000 / 21 = 47.6, so 48 bags. At $15 a bag that is about $720 in material.

Bag coverage shrinks as depth grows, so the calculator scales the chart row you enter: coverage at your R = chart coverage x chart R / R you are adding.

ENERGY STAR attic targets by climate zone

ENERGY STAR's Recommended Home Insulation R-Values for retrofitting wood-framed attics:

ZoneUninsulated atticAdding to 3-4 in
1R30R25
2-3R49R38
4-8R60R49

Zones follow the IECC climate zone map: zone 1 is South Florida and Hawaii, zones 2-3 cover most of the South and California, zone 4 runs from Kansas City to Baltimore and the Pacific Northwest coast, and zones 5-8 are the northern states and Alaska. These are cost-effective retrofit levels; local code for new construction may differ.

R-value per inch of common attic materials

  • Fiberglass batts: about R3.1-R3.4 per inch. A 3.5 inch batt is R11-R13; older or compressed batts perform lower.
  • Blown fiberglass (loose-fill): about R2.2-R2.9 per inch, so it needs more depth for the same R-value but weighs less per square foot.
  • Blown cellulose: about R3.2-R3.8 per inch. It settles roughly 15-20% after installation, which is why bag charts list both an initial (blown) and a settled depth.

Always use the R per inch and coverage chart printed on the product you buy; they vary by brand and density.

Common mistakes and safety

  • Skipping air sealing. Seal top plates, wiring holes, plumbing chases and the attic hatch first. Insulation does not stop air leaks.
  • Blocking soffit vents. Install baffles so blown insulation does not choke eave ventilation.
  • Burying non-IC recessed lights, chimneys or flues. Keep a 3 inch clearance or use rated covers.
  • Measuring the blown depth instead of settled depth. Blow to the initial depth on the bag so the settled depth reaches your target.
  • Counting on a tax credit. The federal 25C Energy Efficient Home Improvement Credit ended for property placed in service after December 31, 2025. Check utility and state rebate programs instead.

Frequently Asked Questions

Common questions about the Attic Insulation Calculator

Divide the attic area by the coverage per bag at the R-value you are adding, and round up. For 1,000 sq ft of R38 cellulose at about 21 sq ft per bag, that is 48 bags. Coverage varies by brand, so use the chart on the bag.

Sources & References

Recommended Home Insulation R-Values

Attic, floor and wall R-value recommendations by climate zone for uninsulated attics and attics with 3-4 inches of existing insulation.
→Source: ENERGY STAR

Types of Insulation

R-value per inch ranges for fiberglass, cellulose and mineral wool loose-fill and batt insulation.
→Source: U.S. Department of Energy, Energy Saver

Insulation

How R-value works, where to insulate and why air sealing comes first.
→Source: U.S. Department of Energy, Energy Saver

Energy Efficient Home Improvement Credit

IRC 25C credit details, including the end date for qualifying property placed in service.
→Source: Internal Revenue Service

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