Air/Weather barrier and flashing when there is no WRB/sheathing – GreenBuildingAdvisor

Hi-

Our house was built in 1922.  It had a covered vestibule that had an exterior door recessed about 6 feet in.  The siding is clapboard and shingles.

The previous owners moved the exterior door to the same plane as the exterior walls. Since the vestibule was originally exterior, the clapboard and shingle siding wraps around the corner from the exterior into what is now our interior.  The jamb legs are rotting and the trim is soft, plus the door was once kicked in and rather dented; so we are replacing the door and jamb. Thankfully the floor planks under the threshold are good.

The previous owners framed the RO for the jamb, slapped on some sheet rock on the interior and an 8″ wide trim board on the exterior.  The trim board slightly overlaps the clapboard and shingles and is “sealed” with GreatStuff foam that is disintegrating due to UV exposure.  

Since I have the sheet rock removed, I can see the interior of the wall.  It’s been raining, but I can’t really tell where water intrusion was coming from to rot the jamb but there was obviously intrusion. Yesterday, I removed the exterior trim. You can see in the pictures where the exterior trim was attached to the exterior finishes with spray foam.

I am putting everything back together but am completely stumped how to make this weather protected/air sealed.  I also don’t know but am 90% certain that I don’t have a WRB.  I am not even sure if I have sheathing because the house is OG exterior materials.  What does one do in this situation?

There is also a tile floor with a 4 1/2″ threshold that the door jamb currently butts up against. 

I know I should probably use liquid applied flashing for the floor of the sill but how do I integrate this into the siding when there is no WRB or sheathing? Is it even possible or advisable to try to add flashing to the header? Or should I just repeat the same, caulk the crap out of everything and re-caulk every year?