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Attic insulation calculator — bags + R-value target

Estimate attic insulation bags from attic area, IECC climate-zone R target, existing R-value, and the coverage chart printed on your insulation bag.

Adjust these to match your project.

sq ft
zone
R
sq ft/bag

Use the coverage-at-R chart printed on your specific bag.

R/in

Blown cellulose is about R-3.5 to R-3.7 per inch; blown fiberglass is about R-2.5 per inch.

%

No waste is applied to blown insulation. For batts, use about 10% cut waste, but bundle coverage was not cited in the spec so this calculator does not output batt bundle counts.

You'll need

21 bags

Insulation bags

Target R-value
60 R
Additional R needed
60 R
Approx. added depth
17.1 in

What this assumes

  • Climate zone 4 maps to attic target R-60; existing R-0 leaves R-60 to add.
  • 48.8 ft² per bag is a user-entered value from the manufacturer's bag coverage chart.
  • 3.5 R/in is used only to estimate added depth.
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Coverage rates & sources

Every number this calculator uses is a published engineering constant — not an estimate we made up. Here is exactly what it assumes and where each value comes from.

  • IECC attic R-target, Zone 1: R-30Source: 2021 IECC / ENERGY STAR recommended (zone 1)
  • IECC attic R-target, Zones 2–3: R-49Source: 2021 IECC / ENERGY STAR
  • IECC attic R-target, Zones 4–8: R-60Source: 2021 IECC (raised from R-49 in 2018 code); R-49 satisfies if full depth over the eaves
  • Blown cellulose R/inch: 3.5–3.7 R/inSource: DOE / industry material data
  • Blown fiberglass R/inch: ~2.5 R/inSource: DOE / industry
  • Fiberglass batt R/inch: 3.1–3.7 R/inSource: DOE insulation material data
  • Closed-cell spray foam R/inch: 6.0–7.0 R/inSource: DOE (display only — spray foam is pro-installed)
  • Bag coverage at target R: varies by manufacturer chartSource: Manufacturer coverage chart printed on every bag (e.g. Greenfiber ~48.8 ft²/25 lb bag at a given depth) — user enters from bag

Before you buy

  • R-target comes from the 2021 IECC by climate zone; your local code is the authority.
  • Bags needed must come from the coverage chart printed on your bag because brands differ a lot.
  • Air sealing, attic ventilation, and baffles matter as much as R-value and are not counted here; spray foam is a professional install.

This is a planning estimate, not a substitute for a pro's on-site measurement. For load-bearing, structural, or code-regulated work, confirm quantities with a licensed contractor.

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