Blown-In Insulation Calculator
Calculates compressed bags of loose-fill blown fiberglass (Attic Cat / ProCat) or treated cellulose insulation needed to achieve target DOE Energy Star attic R-values (R-38 for South, R-49/R-60 for North).
Formula
Bags = Ceil((Attic Sq Ft ÷ Sq Ft per Bag at Target R) × (1 + Settling %))
1. Coverage per Bag at Target R: Cellulose: R-30=39.8 sq ft; R-38=31.4 sq ft; R-49=24.3 sq ft; R-60=19.8 sq ft Fiberglass: R-30=47.0 sq ft; R-38=37.0 sq ft; R-49=28.5 sq ft; R-60=23.0 sq ft 2. Base Bags = Attic Sq Ft ÷ Coverage per Bag 3. Total Bags with Settling = Ceil(Base Bags × (1 + Settling %)) 4. Baffles (Cardboard / Foam Attic Rafter Baffles) = Ceil(Attic Perimeter ÷ 4 ft)
Inputs
- Attic Floor Area
- Desired Added R-Value
- Blown Insulation Material
Worked example
Blowing 12.5 inches of cellulose insulation over a 1,200 sq ft attic floor.
Outcome: At R-38, each 25-lb bag of cellulose covers 31.4 sq ft. 1,200 ÷ 31.4 = 38.2 bags. Adding 10% settling gives 42.0 bags, rounding to 42 bags. Most big-box stores provide free 24-hour machine blower rental when buying 20+ bags.
Is blown cellulose or blown fiberglass better for attic insulation?
Blown cellulose has a higher R-value per inch (R-3.7 vs R-2.5 for fiberglass) and packs denser, dramatically reducing air convective loops in extreme sub-zero weather. Fiberglass is lighter, non-absorbent, and does not generate paper dust during blowing.