Blown-In Insulation Calculator

It depends on your attic area, the target R-value, and the loose-fill type. For a 1,000 square foot attic at R-38, plan on about 32 bags of cellulose or 28 bags of blown fiberglass, then round up and buy a spare.

Divide the target R-value by the material R-per-inch. Cellulose at about R-3.5 per inch needs roughly 10.9 inches for R-38, while blown fiberglass at about R-2.7 per inch needs about 14.1 inches for the same R-38.

Cellulose packs a higher R-per-inch so it needs less depth, resists air movement well, and uses recycled content. Blown fiberglass is lighter, will not absorb as much moisture, and does not settle as much over time. Both work well; the calculator lets you compare bag counts for each.

Fiberglass has a lower R-per-inch, so you must blow it deeper to reach the same R-value. But each blown fiberglass bag holds more settled volume than a cellulose bag, so over a given area you often end up buying fewer total bags even though the layer is thicker.

Loose fill settles, blows unevenly, and gets overfilled near the edges and baffles. A 5 to 10 percent allowance keeps you from running a bag or two short partway through the attic.