On the payback period, that’s probably just outlay divided by energy savings. I’m sure you’d get more enjoyment from a more comfortable house and the next owner will appreciate the modernisation too so those need to be factored into the investment appraisal.
I doubt the architect puts such a miserly lens on the other projects they’re involved in.
This is actually becoming an important point. In parts of the world where energy ratings matter, they have an impact on house value as well. They unlock incentives, etc. A house that is up to modern standards is simply worth more because any new owner does not have to do expensive renovations to modernize. In the Netherlands house flipping is pretty common. Buy something old, modernize it, live in it a few years and make a profit. The lower energy cost is both a nice bonus and a key selling point.
I doubt the architect puts such a miserly lens on the other projects they’re involved in.