How much insulation do I need for a 2,500 square foot attic?
Bringing a 2,500 square foot attic to R-38 takes about 87 bags of blown cellulose at 10.3 inches deep, or 82 bags of blown fiberglass, or 62 bags of R-38 batts. For R-49, recommended in cold climates, it is 113 bags of cellulose at 13.2 inches. Most home centers lend the blowing machine free with a purchase of this size.
Insulation for a 2,500 sq ft attic by product and R-value
| Product | Bags (5% extra) | Installed depth | Material cost, typical |
|---|---|---|---|
| Blown cellulose, R-30 | 69 | 8.1 in | $828 to $1,104 |
| Blown cellulose, R-38 | 87 | 10.3 in | $1,044 to $1,392 |
| Blown cellulose, R-49 | 113 | 13.2 in | $1,356 to $1,808 |
| Blown fiberglass, R-38 | 82 | 14.1 in | $2,460 to $3,280 |
| Blown fiberglass, R-49 | 108 | 18.1 in | $3,240 to $4,320 |
| R-30 batts | 46 | 10 in | $2,530 to $3,450 |
| R-38 batts | 62 | 12 in | $4,340 to $5,890 |
For 2,500 square feet of attic floor. Bag coverage follows typical manufacturer charts; check the bag you buy, since coverage varies by brand.
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Blown-in or batts for a 2,500 sq ft attic?
Blown-in wins in most attics. It flows around wiring, braces, and odd joist spacing with no gaps, and 87 bags of cellulose for this attic run about $1,044 to $1,392 at $12 to $16 a bag, with the machine free for the day. Two people can blow 2,500 square feet in 6 to 10 hours. Batts make sense if the attic is open and clean, you want to be able to walk on a deck later, or there is no existing insulation and the joists are standard 16 or 24 in: 62 bags of R-38 batts cost about $4,340 to $5,890. Already have some insulation? Measure its depth and subtract; 4 inches of old fiberglass is worth about R-10, so you would need the difference to reach your target.
Before you blow 2,500 square feet
- Air seal first: foam or caulk the top plates, wire and pipe penetrations, recessed lights (use IC-rated covers), and the attic hatch. This matters as much as the insulation.
- Baffles: about 39 rafter baffles so the insulation does not block the soffit vents, one per rafter bay along the eaves on both sides.
- Depth markers: 9 rulers stapled to the joists so you can see 10.3 inches as you go.
- Hatch: insulate the attic door with rigid foam and weatherstrip it; it is the biggest hole in the ceiling.
- Dam around the hatch and chimney: keep loose fill 3 in away from a chimney and out of the hatch opening.
A professional insulation contractor charges about $1.50 to $2.50 per square foot for blown cellulose to R-38, or $3,750 to $6,250 for this attic, often with utility rebates available.
Common questions
How many bags of blown insulation for 2,500 square feet?
About 87 bags of cellulose or 82 bags of fiberglass for R-38, 69 bags of cellulose for R-30, or 113 for R-49. Bag coverage varies by brand, so check the chart on the bag.
How deep should blown insulation be in a 2,500 sq ft attic?
About 10.3 inches of cellulose for R-38 or 13.2 inches for R-49. Blown fiberglass needs about 14.1 inches for R-38 because it has a lower R-value per inch. The area does not change the depth, only the bag count.
How much does it cost to insulate a 2,500 square foot attic?
About $1,044 to $1,392 in cellulose for a DIY job to R-38 with a borrowed blower, or $3,750 to $6,250 installed by a contractor. R-38 batts cost roughly $4,340 to $5,890 in material.