BattWiseR-value targets by climate zone, and what it takes to hit them.

Blown-in cellulose for a 1200 ft² attic: R-30 up to R-30

Already at target
0bags needed
R-30 already meets the R-30 target — spend the money on air sealing instead
R-value to addR-30 target − R-30 existing0
Settled depth to addat R-3.5 per inch0in
Blow to this depth20% settling allowed for0in
Bags0× 25 lb
Coverage per bagat 0″ settled—ft²
Material volume0ft³
Assumed R per inchsettled, not as-blown3.5
Assumed settled density25 lb bag ⇒ 15.6 ft³1.6lb/ft³
Attic area
1200 ft²
Existing R-value
30
Target R-value
30
Material
cellulose

The number, and the two assumptions under it

Going from R-30 to R-30 means adding R-0. In cellulose that is 0 inches settled over 1,200 ft² — 0 ft³ of material, or 0 bags.

This is worked from two constants printed on the page rather than copied from one manufacturer's coverage chart, because those charts differ by up to 15% between brands. Check both against the bag you are holding: R per settled inch, and settled density.

Blow deeper than the finished depth

Blow to 0 inches, not 0. Loose fill settles, and insulation blown to the finished depth ends up short of the R-value you paid for six months later. Staple depth rulers to the trusses before you start — it is the only way to know what you actually laid down.

Air sealing comes first, and it is not close

Insulation slows heat moving by conduction. It does nothing about air moving through gaps around can lights, top plates, ducts and the attic hatch. Blowing over unsealed penetrations buries the problem somewhere you can no longer reach, and the return per dollar on sealing those gaps is far higher than the return on the last R-10.

Keep it out of the eaves

Baffles in every rafter bay hold the material back from the soffit vents. Blocked soffits stop the roof breathing, and in a cold climate that is where ice dams begin.

Notes

  • Nothing to buy. R-30 is already at or above the R-30 target, and adding more insulation to an attic that already has enough returns very little. Air sealing the penetrations below it returns far more per dollar.
  • Blow to 0″, not 0″. Cellulose settles about 20% over the first months — insulation blown to the finished depth ends up short of the R you paid for.
  • Air sealing comes first, always. Insulation slows heat that moves by conduction; it does nothing about air moving through gaps around can lights, top plates and the attic hatch. Blowing over unsealed penetrations buries the problem where you can no longer reach it.
  • Keep it out of the eaves. Baffles at every rafter bay hold the insulation back from the soffit vents — blocked soffits stop the roof breathing, and in cold climates that is how ice dams start.

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