Blown in insulation or batts?

Short answer: In an attic or a closed wall, blown in usually wins: the DOE says loose fill usually costs less than batts and provides better coverage, because it conforms to the space into which it is blown [1]. Batts are the right call for open framing: new walls, vaulted ceilings, floors.

Blown in (loose fill)

The DOE: The blown-in material conforms readily to odd-sized building cavities and attics with wires, ducts, and pipes [2]. It fills behind and around electrical boxes, wiring and plumbing [1]. It needs a blowing machine.

Batts

Batts are fiberglass or rock wool cut to fit standard stud and joist spacing [2]. The DOE lists their weak spots: not air-tight, hard to fit into irregular spaces, they lose R-value when compressed, and air can move around and through them [1]. Installed carefully in open framing, they work well.

You can mix them

ENERGY STAR: You can add loose fill on top of fiberglass batts or blankets, and vice versa. If batts go over loose fill, they need to be unfaced [3]. That is how most attic top ups work: blown in over whatever is there, as long as it is dry and clean.

What we do

Attics and closed walls: blown fiberglass or cellulose. Open walls, vaulted ceilings, floors over garages and crawl spaces: fiberglass or rock wool batts, or foam. New construction.

More questions

Can you blow insulation over old batts?

Yes. ENERGY STAR says you can add loose fill on top of fiberglass batts or blankets, and vice versa, as long as batts laid over loose fill are unfaced.