In general, we look at a basket of technologies. Obviously, one of the weakest links in a building's envelope is with the glazing and windows. Airtightness is often overlooked. If you have a leaky envelope, then make it airtight.
It comes back to this: What are we starting with? At this juncture, all we can say is that there's a basket of technologies and there are many different pathways to get where you want to go. It's kind of like going from Ottawa to Montreal. You can take a detour via Cornwall or some other location. If you know the right path, it's comparatively short. If you don't know the right path, you'll go in a large number of different directions and log a lot more miles than you really need.
I think the key issue, as Michel indicated, is to understand what you have and develop a sensible plan to get to it. There are audits and programs that will help you develop that plan. It scales from a home all the way up to a national level.