Blown-in vs batt attic insulation compare
Same attic, two ways: see the blown-in bag count next to the batt bundle count, and drop in your $/bag and $/bundle to compare the material spend head to head.
Calculator
For 1,200 ft², blown-in is about 33 bags vs 30 batt bundles. Blown fills irregular joist bays; batts suit open, accessible attics — a labeled note, not a verdict. Enter your $/bag and $/bundle for the cost columns.
Formula
blown_in_bags = ceil( attic_area_ft² ÷ blown_coverage_per_bag )
batt_bundles = ceil( attic_area_ft² ÷ batt_coverage_per_bundle )
blown_cost = bags × your_$/bag · batt_cost = bundles × your_$/bundle
Both are the same area divided by whatever one unit covers at your target R, rounded up to whole units. Feed the two coverage numbers off the actual bag and bundle you are pricing, then add your unit prices for the cost columns.
Worked example
A 1,200 ft² attic to R-38, blown-in covering 37 ft²/bag and R-38 batts covering 40 ft²/bundle:
Blown-in: ceil(1,200 ÷ 37) = 33 bags. Batt: ceil(1,200 ÷ 40) = 30 bundles.
Add your prices to see the money side. The unit counts alone tell the story: blown-in flows into every joist bay and around wiring with no cutting, while batts mean cutting and fitting each bay — slower, and worse if the framing is irregular.
When to blow & when to batt
Blown-in wins on irregular attics; batts suit open, accessible ones. Loose fill self-levels over cross-bracing, truss webs and wiring, and there are no gaps at the ends of bays. Batts make sense in a clean, walkable attic with standard joist spacing where you can lay full-length pieces and want to keep the material dry-stored and simple to handle.
Coverage numbers must be at the same R. The comparison is only fair if both the bag and the bundle coverage are read at your target R-value. A bundle of R-38 batts covers about 40 ft²; a bundle of R-30 covers about 58. Mixing R-values makes the counts meaningless.
Batts leave gaps if you rush. The single biggest field mistake is compressing batts or leaving voids at the ends and around obstructions — a batt stuffed to 3 inches in a 3.5-inch bay loses R fast. Blown-in avoids that but needs baffles at the eaves so it does not bury the soffit vents.
Cost is only half the decision. This tool prices material; it does not price your time or a rental blower (often free with a bag purchase). For a large or awkward attic, the labor saved with blown-in usually outweighs a small per-unit price difference. It is a labeled comparison, not a verdict — get itemized quotes if you are hiring it out.
Reference table
Labeled coverage at each R — blown fiberglass (per bag) vs batt (per bundle). Read your own product.
| Target R | Blown ft²/bag | Batt ft²/bundle |
|---|---|---|
| R-13 | 108 | 88 |
| R-19 | 74 | 75 |
| R-30 | 47 | 58 |
| R-38 | 37 | 40 |
Labeled planning typicals; coverage falls as R rises.