Which is better, blown-in or batt insulation?

Whether blown-in or batt insulation is better depends on where you are installing it, but for attics blown-in is almost always better, while for walls batts can be better in new construction. Blown-in insulation, which includes cellulose and fiberglass, is loose material blown by machine to fill cavities. It is superior for attics because it conforms perfectly around joists, wiring, and irregular spaces with no gaps, provides more complete coverage, is faster to install, and is cheaper at $0.60 to $1.20 per square foot DIY to reach R38 to R60. It also easily adds on top of existing insulation. Batt insulation, which comes in pre-cut rolls of fiberglass, is better for new construction walls where stud cavities are open and uniform. Batts are easy to install friction-fit between studs, do not require a blower machine, and maintain consistent R-value when installed without compression or voids. However, batts perform poorly if installed poorly. Gaps, compression around wires, and voids where batts do not touch the drywall can reduce effective R-value by 20 to 40 percent. Blown-in dense-pack cellulose or spray-in fiberglass like Johns Manville Spider is better for existing closed walls because it fills all gaps. For soundproofing, batts and mineral wool are good, but blown-in cellulose is slightly better due to density. Overall, use blown-in for attics and retrofit walls, and use batts or mineral wool batts for new open walls where you can ensure perfect installation.