Synopsis: When you’re preparing to insulate an attic, the traditional choice of fiberglass batts might not be the best option. Blown insulation performs better than batts, and it is less expensive than spray polyurethane. GBA editor Martin Holladay compares and contrasts two types of blown insulation, fiberglass and cellulose. Loose-fill fiberglass has a low R-value (2 to 2.7) per inch, so it is best applied in attics that have enough room to accommodate insulation 16 in. to 26 in. deep. Blown-in cellulose is made of ground-up newspaper mixed with a borate-based fire retardant. Because it is denser than fiberglass, it is more effective at reducing air leakage. Cellulose has an R-value of about 3.2 per in. Cellulose can be problematic if it becomes wet. Because it can absorb a lot of water, leaks can cause severe water damage before homeowners become aware of them via damaged drywall.

Fiberglass-batt insulation is inexpensive but difficult to install well. Framing members aren’t always spaced perfectly, and it’s awkward to fit batts into irregularly shaped cavities and around electrical wires and boxes. Because few insulation installers have the patience required to install batts carefully, insulation performance suffers.

How much of an effect does sloppy installation have? According to energy expert Bruce Harley, a ceiling assembly with perfectly installed R-38 attic batts will have an R-value of about R-33, but if an installer leaves gaps amounting to only 5% of the insulated area, the R-value of the assembly drops to R-20.

When insulation fibers are blown into an attic, though, you get better performance. The material fills the nooks and crannies of framing bays, and fills gaps behind blocking, wiring, and other obstacles.

The two most common types of blown-in insulation are cellulose and fiberglass. Either product can be blown onto attic floors or dense-packed into wall or…