How to Use This Calculator
Select your insulation material: blown cellulose for most attic projects (best value), blown fiberglass for humid climates or attics with moisture concerns. Enter the area to insulate in square feet (attic length times width for a rectangular attic; add sections for irregular shapes). Set your target R-value. For most US attics, R-38 is the minimum and R-49 to R-60 is ideal. If you have existing insulation, enter its current R-value; the calculator finds only what you need to add. The result shows: depth to add in inches, number of bags with a 10% waste factor, total final depth, and a cost estimate. A full R-value guide is on our main insulation calculator page.
How to Calculate Attic Insulation
The formula for blown insulation bags: Required Depth (inches) = (Target R-Value - Existing R-Value) ÷ R-Value per inch of material. Then Bags = (Area × Depth ÷ 12 × Installed Density) ÷ Bag Weight × 1.10 waste factor.
R-value per inch by material:
| Material | R-value per inch | Depth for R-38 | Depth for R-49 |
|---|---|---|---|
| Blown cellulose | 3.7 | 10.3 in | 13.2 in |
| Blown fiberglass | 2.5 | 15.2 in | 19.6 in |
Worked example: 1,500 sq ft attic, R-38 target, existing R-11, blown cellulose. Additional R needed = 38 - 11 = 27. Depth = 27 ÷ 3.7 = 7.3 inches. Volume = 1,500 × (7.3 ÷ 12) = 912.5 cu ft. At 1.5 lb/cu ft, that is 1,369 lbs of cellulose. At 30 lbs per bag: 45.6 bags × 1.10 = 50.2, so order 51 bags.
Recommended R-Value by Climate Zone
| Zone | Typical states / regions | Code minimum | Recommended |
|---|---|---|---|
| 1–2 | South FL, Gulf Coast, HI | R-30 | R-38 to R-49 |
| 3 | Southeast, West Coast | R-30 | R-38 to R-60 |
| 4 | Mid-Atlantic, Pacific NW | R-38 | R-38 to R-60 |
| 5 | Midwest, Great Plains | R-49 | R-49 to R-60 |
| 6–7 | Northern states, Mountain West | R-49 | R-49 to R-60 |
| 8 | Alaska | R-49 | R-60 |
Attic Insulation Tips
Air seal before you insulate. Attic air leaks lose more heat than the insulation layer itself in many older homes. Before blowing insulation, seal around every penetration: recessed light fixtures, plumbing stacks, wire chases, and the tops of interior partition walls. Use acoustic sealant or spray foam. An hour of air sealing before insulating will double the effectiveness of your insulation upgrade.
Install proper attic ventilation before insulating. Insulation that covers soffit vents will cause moisture to accumulate in the attic, leading to mold and rot. Install baffles (cardboard or foam insulation dams) at every rafter bay along the eaves to keep a ventilation channel open between the insulation and the roof deck. Most building codes require 1 inch of clearance minimum.
Rent the blowing machine when you buy the bags. Most hardware stores that sell blown insulation offer free or low-cost machine rental with a minimum bag purchase of 10 to 20 bags. The machine is pneumatic and runs on a standard 110V outlet. The hose needs to reach all areas of your attic; pick up an extension hose if your attic is large.
What to Buy
For blown cellulose, look for bags labeled "R-value per bag" with a coverage chart printed on the packaging. Coverage varies by brand and bag weight (typically 25 to 30 lbs). Always use the bag's own coverage chart for your target R-value rather than a generic formula, because installed density varies between products.
For blown fiberglass, bags typically weigh 19 to 25 lbs each. Coverage per bag is lower than cellulose at equivalent R-values. Major brands (Owens Corning, CertainTeed) include coverage charts on every bag that tell you exactly how many bags per 1,000 sq ft at each R-value. Verify the chart matches your target R-value and buy 10% more than the chart suggests to account for irregular attic shapes and waste.