Is it worth adding insulation to a ceiling with 8″ already?

Q: Could you talk a little about financials of adding iso board to a pitched roof-cathedral ceiling with 8″ cellulose in rafter bays and shingles that otherwise wouldn’t be ready for replacement? 

A: If you have 8” of insulation already, then paying a contractor to add more insulation is unlikely to save enough money on the bills to pay back the investment in a reasonable period of time. However, it might make sense if you did it yourself, which is easy if you are installing fiberglass. I have found with my client work that even with 6” of insulation already, the savings on going to 12” is only about $150 a year. If you have no insulation today it is worth adding 12” but going from 6” to 12” will not save very much money. If you did this very cheaply, by just buying rolls of fiberglass and installing it yourself it would shorten the payback period. Please see also my other answer (below) on cathedral ceilings.