I am reviewing the requirements of the new Maine Uniform Building and Energy Code that goes into effect 12/1/2010. The required R value for a wood framed wall is R20. If I assume the Dense Packed Cellulose has an R value of 3.4 per inch and a 2x6 is really only 5.5" deep, then 5.5" of cellulose is only R18.7 which doesn't quite meet the standard.
I could put in R21 fiberglass and meet the standard, but I am sure the cellulose would out perform the glass in the real world.
According to National Fiber's website a 2 x 6 wall at 5.5 " thick has an R value of R20 when dense packed at 3.5 lbs per cubic foot. The reason the Code has a minimum wall insulation requirement of R20 is because thats as good as it gets with celluloce, if the cellulose manufacturers hadn't been allowed their input the minimum would be R21 (fiberglass batts).