Interior

Insulation Calculator: Bags, Batts and R-Value

Square footage and bag count for batt or blown-in insulation, with the R-value your climate zone actually calls for.

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  • Rounded to what you actually buy
  • Working shown, so you can check it

Your measurements

Blown-in insulation

5bags

168 sq ft at R-38 — 38 sq ft per bag at that depth

Area to insulate
168 sq ft
Target R-value
R-38
Bags of blown-in38 sq ft per bag
5
Material cost$0.60–$2.20 per sq ft depending on R-value and material
$101 – $370
  • Air-seal the attic floor before adding insulation. Insulation slows heat conduction; it does nothing about air leaking up through top plates, wire penetrations and can lights — and that leakage is usually the bigger loss.
  • Check your climate zone before trusting a default. The US Department of Energy publishes recommended R-values by zone, and they range from R-30 in the far south to R-60 in the far north for attics.

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How this is calculated

  1. Attic area is the floor footprint; wall area is perimeter × height; floor area is the footprint over the crawlspace.
  2. Blown-in coverage per bag comes from typical manufacturer depth charts and drops steeply with R-value — roughly 100 sq ft at R-13 down to 24 sq ft at R-60.
  3. Batt bundles cover about 40 sq ft at 16 in on centre and 49 sq ft at 24 in on centre.
  4. R-value targets follow US Department of Energy climate-zone guidance. Local code may require more; it rarely requires less.

Quantities are rounded up, because materials are sold in whole units and a second trip costs more than a spare box. Cost figures are typical US ranges and are not a quote — always price against your own supplier.

Frequently asked questions

How many bags of insulation do I need for a 1000 sq ft attic?

At R-38 a bag of blown-in fibreglass covers about 38 sq ft, so a 1,000 sq ft attic needs roughly 27 bags. At R-49 the same attic needs about 35 bags, because coverage per bag falls as depth increases.

What R-value do I need in my attic?

R-38 covers most of the US, R-49 for cold climates, and R-60 in zone 7 and above. The far south can meet code at R-30. Check the Department of Energy zone map for your county rather than a national average.

Is blown-in or batt insulation better for an attic?

Blown-in, in almost every case. It fills irregular spaces and flows around joists, wiring and framing without the gaps batts leave. Batts make more sense in open stud walls where the cavity is a clean rectangle.

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