How the insulation bag calculator works
The label-coverage method
Every bag of loose-fill prints a coverage chart: square feet per bag at each nominal R. The math underneath is simple — bags = (area × target depth in inches) ÷ (coverage per bag). Our calculator encodes typical chart values (cellulose ~2.7 bags per 100 sq ft per R19-layer, fiberglass ~4.2) so you can sanity-check any quote or rental-machine plan.
The buy margin
Add ~10%: joist-bay edges, hard-to-reach corners, and machine priming all consume material, and unopened bags usually return for refund. Undershooting by two bags is how attics end up R44 instead of R49 — invisible to the eye, measurable on the heating bill.
Rental machine reality
Home-improvement stores loan blowing machines with bag purchase. The learning curve costs time, not money: expect the first 20% of the attic to go slow, keep the hose low and angle it into the joist bays, and have a second person feeding the machine. A two-person crew does a 1,200 sq ft attic in 4–6 hours.
Fluff versus weight
Insulation vendors are paid by the bag; performance comes from installed density. Blown-too-fluffy fiberglass hits its labeled R at labeled depth on paper and loses it to air movement in practice. The honest check is the bag count per square foot — if an installer's bid uses conspicuously fewer bags than this calculator, ask why.