Measure what you have and if fiberglass figure 3.5 per inch R value.
All insulation settles.

Codes are minimums. Previous sites provided will give you optimal R
values for your Zone.,

Also check out Owens Corning, Dow, and other R value energy sites.

Im Zone 5 , Code is R 35, optimal is stated at several sites R 60 -
70. I did R 100 and it has settled to R 80. My utilities are the lowest
in the area confirmed by the utility co, but I did the walls , windows
and basement also.

Up what you have past optimal ratings for settling and you will save
big time. Remember Heat Rises, the attic is the cheapest place to start
Be sure to instal side roof deck baffles.

Codes are basicly 30 + years old, minimums, and do not reflect todays
utilities prices .