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Calculators / Energy and insulation

How much attic insulation do you need?

Find your climate zone by ZIP, measure what's up there now, and pick what you're adding. You get the R-value to add, how deep to blow it and how many bags to buy.

Updated · How we check our numbers

These are example numbers. Put in yours.

Where you live

Climate zone

Your attic

sq ft

Usually the house footprint, less any vaulted ceilings.

Insulation there now
in

Push a ruler down to the drywall in a few spots and average.

What you'll add

Adding

Add R50

38 bags of blown fiberglass

Owens Corning AttiCat at R60: 20.5 in deep over 1,200 sq ft.

Target
R60Zone 4 (4A or 4B)
You have
R10
To add
R50
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Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.

Zone 4 (4A or 4B): ENERGY STAR recommends R60 in an attic

existing = 4 in of loose fiberglass × R2.5 per in = R10

to add = R60 − R10 = R50

Owens Corning chart, R60 row: 1,200 sq ft ÷ 31.8 sq ft per bag = 38 bags

By brand

maker coverage charts
  • Owens Corning AttiCatR60: 20.5 in deep, 31.8 sq ft per bag
    38 bags

    27.5 lb bags. Chart row R60 for your R50.

  • Johns Manville Climate Pro B-7700R60: 20.25 in deep, 35.8 sq ft per bag
    34 bags

    bags. Chart row R60 for your R50.

  • CertainTeed InsulSafe SPR60: 21.75 in deep, 34.5 sq ft per bag
    35 bags

    bags. Chart row R60 for your R50.

ENERGY STAR's shortcut: with 3 to 4 in already down, add R49.

Seal before you insulate

EPA recommends air sealing the attic first: foam around plumbing vents and wiring holes, weatherstrip the hatch. Insulation doesn't stop air.

Depth markers

Staple attic rulers to the joists every 300 sq ft so you can see when you reach 20.5 in.

See attic rulers on Amazon
Recessed lights

Keep insulation 3 in back from recessed lights unless they're rated IC, and out of the air space around flues and chimneys.

Soffit vents

Staple baffles between the rafters at the eaves so insulation doesn't block the vents. Insulate and weatherstrip the hatch to the same R-value.

See rafter baffles on Amazon
Tax credit

The federal 25C credit ended for insulation installed after December 31, 2025. Check your utility for rebates.

How we work it out

The target. ENERGY STAR's retrofit table, based on the 2021 IECC, recommends R30 in zone 1, R49 in zones 2 and 3, and R60 in zones 4 to 8. With 3 to 4 inches already down, it says to add R25, R38 or R49.

What you have. Measure the depth in a few places. ENERGY STAR's rule of thumb for R per inch: loose fiberglass 2.5, rock wool 2.8, cellulose 3.7, fiberglass batts 3.2.

R to add = target − (inches × R per inch)

How much to buy. Federal rules make every bag of loose-fill print a coverage chart: for each R-value, the minimum settled depth and the most square feet a bag may cover. We take the first row at or above the R you need:

bags = attic sq ft ÷ sq ft per bag, rounded up

Fiberglass barely settles, so its chart depth is what you blow. Cellulose settles, so GreenFiber lists a deeper depth to blow it to. Don't work it out from R per inch: the FTC rule tells installers not to, because the bag count sets the R-value as much as the depth does.

Climate zone. Your ZIP takes the IECC 2021 zone of the county that holds most of it, from DOE's county list (PNNL-33270).

Where the numbers come from

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