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

Blown-in fibreglass for a 2500 ft² attic: R-11 up to R-49

Loose-fill fibreglass bags
61bags
15.4″ blown over 2,500 ft² to go from R-11 to R-49
R-value to addR-49 target − R-11 existingR-38
Settled depth to addat R-2.6 per inch14.6in
Blow to this depth5% settling allowed for15.4in
Bags61× 30 lb
Coverage per bagat 14.6″ settled41.1ft²
Material volume3,045ft³
Assumed R per inchsettled, not as-blown2.6
Assumed settled density30 lb bag ⇒ 50 ft³0.6lb/ft³
Attic area
2500 ft²
Existing R-value
11
Target R-value
49
Material
fibreglass

The number, and the two assumptions under it

Going from R-11 to R-49 means adding R-38. In fibreglass that is 14.6 inches settled over 2,500 ft² — 3,045 ft³ of material, or 61 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 15.4 inches, not 14.6. 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

  • Check these two assumptions against the bag before you buy: R-2.6 per settled inch and 0.6 lb/ft³ settled density. Manufacturer coverage charts vary by up to 15% between brands, and every one of them prints its own numbers on the bag.
  • Blow to 15.4″, not 14.6″. Loose-fill fibreglass settles about 5% 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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