In article ,
says...
So how did I get an 1800sq. ft. 110 yr old house to have an annual gas
bill of 463$ last year, down from 1850 $, Which includes tankless gas
water heater gas cooking and gas dryer. I did R 100 attic R 35 walls,
dual and tri pane, and complete basement insulation, even Under the new
concrete. But #1 I did not listen to the naysayers that I would never
get a payback. My utility co was so sure I was stealing they were out
twice. So all you who say don`t overdo it , it will never payback, I
laugh to the bank this year and maybe pay 6-700 for a years worth of
comfort.

I suspect that the 110 yr old house had air infiltration issues.

The energy savings could be more from plugging the air leaks with
insulation than from the insulation's R value.