Blown Insulation for Attics: Fiberglass vs. Cellulose
Both perform better than batts and are less expensive than spray polyurethane. Neither is perfect.
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 ceiling cavities. Because gaps are virtually eliminated, either product outperforms fiberglass batts. To install blown-in insulation, contractors or homeowners use a blowing machine. Small do-it-yourself machines can be rented or borrowed from many home centers; insulation contractors usually buy more-powerful commercial blowing machines. The blowers include a hopper where the fibers are poured, a motor, and a 2-in.- or 3-in.-dia. hose that delivers the insulation.
The R-value per inch of either type of insulation increases with density, so packing insulation tightly improves performance. High densities and thicker layers also help to lower air-infiltration rates.
Insulation costs vary widely from region to region. Cellulose usually costs more than fiberglass batts, and blown-in fiberglass usually costs more than cellulose. Both cellulose and blown-in fiberglass are considerably less expensive than spray polyurethane foam.
Builders concerned about convection and air leakage are more likely to choose cellulose, while builders worried about the ability of cellulose to hold moisture are more likely to choose blown-in fiberglass. As long as the R-value is appropriate and the insulation is installed correctly, however, either product will perform well.
Convection is a worry only in the coldest climates
In 1991, researchers at Oak Ridge National Laboratory reported that when attic temperatures drop below about 0°F, convection currents passing through air-permeable loose-fill fiberglass cut the effectiveness of the insulation by 30%.
In response, fiberglass manufacturers pointed out that most U.S. attics are rarely cold enough for the problem to matter much. (While this may be true, the explanation provided little comfort to homeowners in northern Minnesota.) Energy experts advised concerned homeowners living in very cold climates to cap the loose-fill fiberglass in their attics with 2 in. of cellulose insulation; this effectively stops the problematic convection currents.
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