Cellulose and fiberglass are the two most popular types of blown insulation, and they each have some pros and cons. Here are some details on the differences between these kinds of insulation.
Cellulose and fiberglass have similar insulating values. Cellulose has a slightly higher R factor (a unit of thermal resistance) with an R-value of approximately 3.0 per inch while fiberglass ranges from R-2.1 to R-2.7. However, R-value chnges depending on the thickness and amount of material you use, so check with local experts to figure out the likely R-value for fiberglass or cellulose based on your job's specification.
One key insulation difference is that cellulose settles over time, potentially leaving some areas (in walls particularly) with little or no insulation. Fiberglass manufacturers have developed blown fiberglass insulation they claim won't settle over time. This concern can actually be avoided in both cases if the insulation is more densely packed in the spaces, but fiberglass will require special training and tools to do so and even then, it proves more difficult than cellulose.
Cellulose retains its insulating value no matter the temperature while fiberglass has been shown to lose some of its insulating value as the temperature drops. In extreme temperatures, the loss of insulating value can be as much as 50 percent.
Loose-fill, or blown, cellulose insulation is manufactured primarily from recycled newspapers, a very benign product, so it poses virtually no ongoing health risk.
Blown fiberglass, on the other hand, is made up of very fine strands of glass, and these tiny fibers are a carcinogen that can easily be inhaled into your lungs. To offset this potential health concern, fiberglass insulation is usually covered with something after it's installed, or it's installed in an area where it won't be disturbed (such as an attic), so the fibers won't get into the air where they could be inhaled. With these precautions, it's no threat to your health.
Since cellulose is made from newspaper, it obviously will burn if ignited. Cellulose insulation manufacturers have responded to that concern by treating it with fire-retarding chemicals such as boric acid, ammonium sulfate, or sodium borate in the manufacturing process. These chemicals have the additional benefit of repelling mice and other rodents.
Fiberglass, again because it's made from glass, simply won't burn (although it will melt at extremely high temperatures).
Cellulose contains the higher percentage of recycled materials. While the fiberglass industry does a good job of recycling and uses approximately 35 percent recycled material, cellulose manufacturers average over 75 percent recycled content.
Fiberglass insulation has proven over the years to provide effective temperature and sound insulation while more recently, cellulose has established itself as a viable alternative. Since either will do an effective job insulating your home, the insulation you choose should be based on these and other factors such as cost, availability of the product, quality and reputation of the installers (if you're not doing it yourself), and your personal environmental concerns.