How much insulation do I need for a 800 square foot attic?
Bringing a 800 square foot attic to R-38 takes about 28 bags of blown cellulose at 10.3 inches deep, or 27 bags of blown fiberglass, or 20 bags of R-38 batts. For R-49, recommended in cold climates, it is 37 bags of cellulose at 13.2 inches. Most home centers lend the blowing machine free with a purchase of this size.
Insulation for a 800 sq ft attic by product and R-value
| Product | Bags (5% extra) | Installed depth | Material cost, typical |
|---|---|---|---|
| Blown cellulose, R-30 | 22 | 8.1 in | $264 to $352 |
| Blown cellulose, R-38 | 28 | 10.3 in | $336 to $448 |
| Blown cellulose, R-49 | 37 | 13.2 in | $444 to $592 |
| Blown fiberglass, R-38 | 27 | 14.1 in | $810 to $1,080 |
| Blown fiberglass, R-49 | 35 | 18.1 in | $1,050 to $1,400 |
| R-30 batts | 15 | 10 in | $825 to $1,125 |
| R-38 batts | 20 | 12 in | $1,400 to $1,900 |
For 800 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 800 sq ft attic?
Blown-in wins in most attics. It flows around wiring, braces, and odd joist spacing with no gaps, and 28 bags of cellulose for this attic run about $336 to $448 at $12 to $16 a bag, with the machine free for the day. Two people can blow 800 square feet in 2 to 3 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: 20 bags of R-38 batts cost about $1,400 to $1,900. 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 800 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 12 rafter baffles so the insulation does not block the soffit vents, one per rafter bay along the eaves on both sides.
- Depth markers: 3 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 $1,200 to $2,000 for this attic, often with utility rebates available.
Common questions
How many bags of blown insulation for 800 square feet?
About 28 bags of cellulose or 27 bags of fiberglass for R-38, 22 bags of cellulose for R-30, or 37 for R-49. Bag coverage varies by brand, so check the chart on the bag.
How deep should blown insulation be in a 800 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 800 square foot attic?
About $336 to $448 in cellulose for a DIY job to R-38 with a borrowed blower, or $1,200 to $2,000 installed by a contractor. R-38 batts cost roughly $1,400 to $1,900 in material.