How to stop walls from rotting when insulating an old house

My experiance with an older house suggests that the air sealing phase is where the real pain is. I was working on a 1920s build where the original plaster was peeling off the studs. It proved much more complicated than just stuffing insulation in. I found that most of the air leakage came from the small gaps where the cast iron plumbing came thru the rough framing. You’d think the framing was done, but those tiny penetrations create massive convection. After I sealed every joint and gap with a thick mastic sealant, the drafts stopped immediatly. I used an open-cell foam board for the cavities, but the real win came from finding all those tiny gaps first. We even had to replace a few sections of compromised wood around a defunct radiator, which was surprisingly soft. Getting that vapor barrier placed correctly afterwards was the last hurdle, but managing where the ambient moisture settles outside the envelope made a big difference in keeping the interior dry.

I don't really agree that the real win is finding all those tiny gaps first. While leaks around plumbing are a pain, focusing too much on the minuscule holes might just lead you to seal over foundational issues. When I did a similar project, I found that the constant work on the small penetrations just kept blowing back up with more moisture once the insulation was up. You should really be more worried about the total R-value achieved after you are done sealing, becasue air doesn't matter if you've got thin walls.

I've found that once the air sealing's totally locked down, opting for spray foam instead of batts realy changes the game because it inherently stops airflow. The main thing for old frames though is making sure you're not trapping condensation against the studs if the moisture problem wasn't fully fixed first. It's all about the relative humidity shift when you make the walls tighter. My biggest worry is always about getting that vapor barrier on the exterior side oriented correctly for my climate, not just getting it installed.

Update: I was looking into that spray foam suggestion since it inherently stops airflow. But given the warning about condensation if the moisture problem wasn't fully resolved, I'm wondering what the best approach is. Should I completly dry out the studs first, or can I proceed with sealing and then foam, managing the moisture later?