How the bag count is worked out
Every bag of loose-fill insulation carries a coverage chart. For each R-value, the chart gives the square feet one bag covers, the depth to install and the depth after settling. The calculator uses two of these charts: GreenFiber SANCTUARY for cellulose (25 lb bags) and Knauf EcoFill Wx for blown fiberglass (28.6 lb bags).
Here are two rows from those charts:
| Chart row | Sq ft per bag | Installed depth | Settled depth |
|---|---|---|---|
| Cellulose, R-38 | 19 | 11.2 in | 10.4 in |
| Blown fiberglass, R-38 | 50.2 | 13 in | 13 in |
The calculation:
- R to add = target − existing. If you need R-48.8, the calculator interpolates between the chart's R-44 and R-49 rows.
- Area to cover = attic floor × 1.10 (10% waste, adjustable).
- Bags = area to cover ÷ sq ft per bag, rounded up.
When the R you need falls outside the chart, for example a small top-up below the lowest row, the calculator switches to physics: depth from the material's R per inch, then bags from the settled density and the bag weight. Show the math tells you which method it used.
Worked example: 1,500 sq ft attic in zone 6A
A 1,500 sq ft attic in zone 6A with 4 in of old loose fiberglass:
- Existing: 4 in × 2.5 = R-10
- Target: R-60, so you add R-50
- Area with waste: 1,650 sq ft
| Material | Blow to | To buy | Material cost |
|---|---|---|---|
| Blown cellulose | 14.6 in, settles to 13.6 in | 119 bags | $1,903–$2,138 |
| Blown fiberglass | 16.7 in | 45 bags | $1,948–$2,189 |
Cellulose goes in about 2 in shallower but needs more than twice as many bags. That matters for the job itself: someone has to feed every bag into the hopper.
Topping up a well-insulated attic
Now take a 1,500 sq ft attic in zone 3A that already has 12 in of fiberglass batts (R-38). The target is R-49, so you only add R-10.6. That's below the lowest row on both charts, so the calculator uses density and bag weight:
- Cellulose: 3.14 in, settling to 2.9 in, 25 bags, $400–$449
- Blown fiberglass: 3.63 in, 10 bags, $433–$486
Bag count vs. depth: check both
Depth alone can fool you. A blowing machine set too fluffy can reach the chart depth with fewer bags, but the material is then too light to deliver its rated R-value. The chart lists both a depth and the most area each bag should cover, so:
- Count your bags. If you've reached the depth with bags to spare, the layer is probably too fluffy.
- Use attic rulers. Staple depth markers to the joists or trusses across the attic so you can see the installed depth from the hatch.
- Fill the far corners first and work back toward the hatch.
Settling
Cellulose settles after it's blown. On the GreenFiber chart, R-60 is installed at 17.3 in and settles to 16.1 in, and the calculator shows both. Blow to the installed depth. The Knauf fiberglass chart lists the same installed and settled depth, so with that product what you blow is what you keep.
Renting the blower
Home centers often lend a blowing machine free for a day with a minimum bag purchase. Check the current terms and have the attic prepped before you pick it up. It's a two-person job: one person feeds the hopper, the other works the hose in the attic. Agree on signals for start and stop, since the machine is loud.
Before you blow
- Air seal first. Seal gaps around pipes, wires, ducts and the top plates of interior walls. Loose-fill hides them.
- Install baffles at the eaves so insulation can't block the soffit vents.
- Dam around the hatch, chimney and any non-IC recessed lights.
- Wear a mask (N95), goggles, gloves and long sleeves.
For a full attic plan, including batts, see the attic insulation calculator. To compare depths across materials, see the R-value chart, and check your target on insulation R-values by climate zone.