Um how is it cheaper? cellulose that is? While R-value "per inch" is promoted by some manufacturers, the overall R-value installed is what counts. Fiber glass insulation products come in R-values ranging from R-11 to R-38 for fiber glass batts and rolls. Fiber glass insulation can be blown in an attic to nearly any R-value. More R-value alternatives provide greater flexibility in meeting code energy requirements. Also, In contrast, cellulose insulation not only settles to a much greater degree (approximately 20%), but also at a significantly higher rate. If cellulose insulation is being considered, make sure the installer understands that most cellulose insulations settle in attic loose fill applications - that's a significant loss of insulating effectiveness. In fact, the Insulation Contractors Association of America recommends that an additional 25% of thickness be added for cellulose insulation to compensate for this extreme loss of R-value.
also it is not as green as people say it is.
The environmental benefits of fiber glass insulation, however, go far beyond its recycled content when analyzed from a life-cycle perspective. A life-cycle analysis is an appraisal of the environmental impacts connected with a product through an examination of the product's environmental traits during many stages including pre-manufacturing; manufacturing; distribution/packaging; use, reuse, maintenance; and waste management. In reviewing each of these stages, a life-cycle evaluation of fiber glass clearly shows its environmentally beneficial attributes. As an example, consider fiber glass versus cellulose insulation.
Cellulose manufacturers claim environmental benefits, even though they may be removing newsprint from an existing recycling loop. More trees must be cut and more energy used to make new newsprint and cardboard to make up for what has been used for cellulose insulation. And when it comes to insulating the same size home, pound for pound, it takes up to three times more cellulose than fiber glass to achieve the same insulating efficiency. Both insulations use the same amount of virgin material but the virgin material in fiber glass is sand - which is classified by the U.S. Environmental Protection Agency as a "rapidly renewable resource."
Just my two cents worth on what I think. Just to point out both will do the job just that one needs to look at the whole thing to know both sides .