How to Use This Calculator
Select your insulation type using the material buttons at the top of the calculator. Blown cellulose is the best value for most attic projects. Blown fiberglass is faster to install and better in humid environments. Enter the square footage of the area to insulate: for a rectangular attic, multiply length by width. For irregular shapes, divide into rectangles and add the areas. Set your target R-value (R-38 is the standard for most US attics). If you already have insulation, enter the current R-value to calculate only what you need to add. The result shows depth to add, bag count with 10% waste, and a cost estimate. See our Attic Insulation Calculator for attic-specific guidance including depth by climate zone.
How to Calculate Blown-In Insulation
The blown insulation formula: Required Depth = Additional R-Value Needed ÷ R-Value per Inch. Volume = Area (sq ft) × Depth (ft). Weight = Volume × Installed Density (lb/cu ft). Bags = Weight ÷ Bag Weight × 1.10 waste factor.
| Property | Blown cellulose | Blown fiberglass |
|---|---|---|
| R-value per inch | 3.7 | 2.5 |
| Installed density | 1.5 lb/cu ft | 1.25 lb/cu ft |
| Typical bag weight | 30 lbs | 19 lbs |
| Depth for R-38 | 10.3 in | 15.2 in |
| Settles over time? | Yes (~20% first year) | Minimal |
Example: 800 sq ft area, R-38 target, existing R-0, blown fiberglass. Depth = 38 ÷ 2.5 = 15.2 in = 1.27 ft. Volume = 800 × 1.27 = 1,013 cu ft. Weight = 1,013 × 1.25 = 1,266 lbs. Bags = (1,266 ÷ 19) × 1.10 = 73.3, so order 74 bags.
Bags per 1,000 sq ft (starting from R-0)
| R-value target | Blown cellulose bags (30 lb) | Blown fiberglass bags (19 lb) |
|---|---|---|
| R-19 | 24 | 46 |
| R-30 | 38 | 73 |
| R-38 | 48 | 92 |
| R-49 | 61 | 119 |
| R-60 | 75 | 145 |
Includes 10% waste factor. Bag counts assume 30 lb cellulose bags and 19 lb fiberglass bags at standard installed density. Verify against the coverage chart on your specific bag before purchasing.
Blown-In Insulation Tips
Mark your target depth on your attic joists before you start blowing. Cut small pieces of wire or use a yardstick as depth gauges and place them at intervals across the attic floor. Without physical markers, it is very easy to under- or over-shoot your target depth. Attic insulation that is 2 inches short of the target loses significant R-value.
Seal attic bypasses before blowing. The top plates of interior partition walls, gaps around plumbing stacks, and gaps around recessed lights are the biggest sources of heat loss in most attics. Close them with rigid foam and acoustic sealant before blowing insulation. Air sealing adds more energy savings per dollar spent than additional insulation thickness in most homes.
Move methodically from the attic perimeter inward toward the access hatch. Blowing from the center toward the hatch means you would be walking on fresh insulation, disrupting depth uniformity. Start at the far end, work back toward the hatch, and finish at the access point. Close off the attic immediately to prevent disturbance from air movement.
What to Buy
The bag's own coverage chart is your most reliable guide to how many bags to buy. Manufacturers print R-value coverage tables directly on the bag: bags per 1,000 sq ft at R-30, R-38, R-49, and R-60. These tables account for the specific product's installed density. Use your calculated bag count as a check against the chart, not as a replacement for it.
Most large hardware stores rent blowing machines free with a minimum purchase of 10 to 20 bags. Ask about the machine when you buy the insulation; some stores will hold the machine for you if you call ahead. Bring the machine back the same day if possible, as daily rental fees apply after the free period. Pick up a respirator rated N95 or better and disposable coveralls; both are available in the same aisle as the insulation.