Heat rises, insulating an attic is the cheapest way to save. 3% or
whatever loss you figure you save on utilities and increase resale
value.
Adding 6" or R 21, well next year it will have settled 15-20% and be R
18. I put in R 100 it settled to R 80, an R figure that is optimal not
what your local Minimum codes require. Through all I did I cut Ng by
60%, everything has a payback when it comes to insulation since
standards do not reflect 1.70 a therm Ng, they are outdated from the
50s.