That is true, and I guess technically you don't want a house with no leaks, only with leaks that you have complete control over.

Spray foam insulation can seal essentially any structure, and it doesn't require much more than a small hole drilled into the wall between the beams which is easily spackled over.

It can also be put on as insulation in attics and crawlspaces.

Combine that with a full ducting audit if any of your ducting pierces the envelope, a full intrusions audit for power boxes and the like, and new windows/house sheathing/ proper roofing, you can get very close to as good as a new built.

Most of these I would not suggest doing as a DIY, therefore I still say it is expensive, and even moreso if you have lathe and plaster instead of more modern drywall or encounter any of a myriad of issues likely to be uncovered when doing these things to a very old house.

> Spray foam insulation can seal essentially any structure, and it doesn't require much more than a small hole drilled into the wall between the beams which is easily spackled over.

A hard maybe on here. Done right it can of course, but you are depending on it filling the cavity without so much pressure that is breaks the walls. The industry as figured a lot out, but this is still compromise and some guessing and so it won't always work (it usually will). There are also many ways they used to put voids in old houses that would not be reached by this.