Blown-In vs Batt Insulation for an Attic
The short comparison
| Blown-in | Batts | |
|---|---|---|
| Coverage quality | Seamless — flows around wiring, boxes and odd bays | Only as good as the fit; gaps and compression cost real R |
| Best surface | Open, flat attic floor | Walls, rafter slopes, floors — anywhere vertical or sloped |
| Equipment | Blower machine (often free to borrow with the insulation purchase) | A utility knife and a straightedge |
| Crew | Two people — one feeds, one holds the hose | One person |
| Topping up old insulation | Ideal — blow straight over | Fine — lay unfaced batts across the joists |
R-value per inch, and the depth that follows
| Material | R per inch | Depth for R-38 | Depth for R-49 |
|---|---|---|---|
| Blown cellulose | 3.2–3.8 | ~11 in | ~14 in |
| Blown fiberglass | 2.2–2.7 | ~15 in | ~20 in |
| Fiberglass batts | 2.9–3.8 | R-38 batt: 12 in nominal | R-38 + R-11 layered |
R-38 to R-49 covers most mixed climates and R-49 to R-60 the cold ones — the same ranges as the climate table on the insulation calculator page. Two consequences of the depth column are easy to miss. First, blown fiberglass needs a lot of headroom: R-49 is 20 in deep, which can bury the top plates and reach the roof deck near the eaves — baffles at every soffit vent are not optional. Second, cellulose settles 10 to 15% over the first months; the bag chart gives a settled-depth figure, and that is the one to install to.
When blown-in is the right call
An open attic floor with joists every which way, wiring stapled across them and a dozen odd bays is exactly what a hose does well and a knife does slowly. Blown material fills the framing irregularities that make batts underperform their label — the insulation calculator applies a framing factor of 0.90 at 16 in centres for exactly that reason. Cost per delivered R is usually the lowest of any DIY option, and big-box stores commonly lend the blower free with a minimum bag purchase. Budget a weekend morning: a typical 1,000 sq ft attic to R-38 in cellulose is roughly 25 to 30 bags through the machine.
When batts are the right call
- Vertical and sloped surfaces. Loose fill will not stay on a wall or a rafter slope; batts and their friction fit will.
- Small areas. For one room over a garage, machine logistics outweigh the coverage advantage.
- Future access. Batts lift out and go back; blown material does not. If you expect to re-wire or add ductwork soon, batts keep the attic serviceable.
- No helpers. Blowing is genuinely a two-person job; batts are not.
Whichever you choose, seal first
Insulation slows heat; it does not stop moving air. Foam or caulk the top-plate gaps, wire penetrations and the chase around the flue before covering them, keep can lights IC-rated or boxed, and keep the soffit baffles clear. Air sealing an attic before insulating it is routinely worth more comfort per dollar than the last R-10 of depth.
Work out how many bags of batt insulation a wall or attic needs. The tool deducts the framing — batts go between the studs, not over them — and divides by the coverage printed on your bag.
Frequently asked questions
Can you blow insulation over old batts?
Yes, and it is the standard top-up. The old batts must be dry and any faced layer stays where it is, facing down against the ceiling — never add a second vapor retarder in the middle of the stack.
How deep is R-38 blown insulation?
About 11 inches of settled cellulose or about 15 inches of blown fiberglass. The exact figure is printed on the bag chart, and the settled depth is the one that counts.
Is blown-in insulation a DIY job?
Yes for an open attic floor. It takes two people, a borrowed blower, dust protection and a few hours. Dense-packing closed walls is a different job and is usually left to an installer with a proper machine.
You might also need
Related calculators
Spotted a wrong number? Report it — corrections ship in the next deploy.