The basic formula is straightforward: attic square footage multiplied by target depth in inches, divided by the coverage rate listed on your insulation bag. Coverage rates are printed for specific R-values, so a bag rated for R-38 coverage at a certain thickness won't cover the same area if you're installing for R-49.

Start by measuring your attic floor area, subtracting anything you won't be insulating like storage platforms or mechanical equipment footprints. Then check the Department of Energy's recommended R-value for your climate zone: most northern regions call for R-49 to R-60 in the attic, while milder southern climates often target R-30 to R-38. Colder climates need more depth, which means more bags per square foot.

If you already have some insulation in place, measure its current depth and subtract that from your target depth rather than starting the calculation from zero. Adding 4 inches of new cellulose over 6 inches of existing fiberglass gets you to a different total R-value than starting fresh, so it helps to know both your existing depth and material type before ordering bags.

Depth requirements shift depending on whether you're using cellulose or fiberglass, since each material has a different R-value per inch. Cellulose generally runs R-3.2 to R-3.8 per inch, while loose-fill fiberglass sits lower at R-2.2 to R-2.7 per inch, which is why fiberglass installations need more depth to hit the same R-value.

For R-30, cellulose needs roughly 8 to 9 inches while fiberglass needs 11 to 13 inches. For R-38, cellulose runs 10 to 12 inches against fiberglass at 13 to 15 inches. Stepping up to R-49, cellulose needs about 13 to 15 inches compared to fiberglass at 17 to 19 inches. If your climate zone calls for R-60, cellulose depth climbs to roughly 16 to 18 inches.

Keep in mind that cellulose settles over time, typically losing 15 to 20 percent of its initial loose-fill depth within the first year. Manufacturers account for this in their coverage charts, so always install to the initial fill depth listed on the bag rather than the final settled depth, or you'll end up under-insulated once things compact.

Most independent rental yards stock insulation blowers as walk-behind hopper units that feed material through a long hose into your attic access point. A working machine can typically process 3 to 5 bags per minute once it's running, though setup and hose management usually take longer than the blowing itself.

If your attic access doesn't have a nearby outlet, you'll need an external power source for the blower motor. Many DIYers pair the blower rental with a portable generator rental for the day, especially on job sites without finished electrical near the attic hatch.

Daily rental rates for insulation blowers typically run $50 to $100. Some home improvement stores waive the fee entirely if you buy your insulation bags there, though independent rental shops usually offer more flexibility on pickup times and weekend availability. Expect a refundable deposit in the $50 to $150 range, held against the machine and hose kit.

Pickup is standard since blowers and hose kits are bulky, but some shops offer delivery for an added fee, worth asking about if you're doing a whole-house job. If you're tearing out old, moisture-damaged insulation before adding new material, you'll also want a way to haul the debris away. A dump trailer rental handles that load without turning your truck bed into a mess, and if you're transporting a large quantity of new insulation bags home from the supply store, an enclosed trailer rental keeps everything dry and contained on the drive over.

Measure irregular areas separately rather than eyeballing an average. Attics with cathedral sections, knee walls, or dormers rarely have a uniform square footage, so breaking the space into rectangles and adding them up gives a more accurate total than a single rough estimate.

Build in a 10 percent buffer on top of your calculated bag count. Joist spacing, hopper efficiency, and minor spillage all eat into your material faster than the math suggests, and running out mid-job means a second trip to the rental counter.

Check local building code minimums before you buy, since some municipalities require R-values higher than the DOE's general recommendation. It's worth a quick call to your local permit office if you're unsure, particularly for homes being resold or inspected soon after the insulation work.