Speaking as someone who spent a good part of my life renovating homes, I can tell you that there is not much of an "upper limit" to the energy efficiency of an older home. If you're already planning on renovating a home, there is an incremental $25-$50k energy efficiency improvements that you can make to turn the house into a nearly net-zero energy home.

The issue is that the young couple who do renovate their home have a far better ROI on updating the kitchen + bathroom than the efficiency of the house.