Hello GBA

Title says it all. We have scoured the Internet and GBA for a proper answer. I’m an architect and I still don’t have a good answer to this question. 

The house has 12” cedar groove shingles over 6” horizontal sheathing boards (no lap and imperfect seams) over studs over plaster board. It has cheap vinyl windows with popped seals so we are putting in replacements. No rot that we have noticed. 

Every installer recommends adding vycor and caulking the window in. Architect colleagues recommend the same, plus metal head flashing and a second layer of vycor that extends +10” outwards on all sides. I am concerned that if we put any vycor wrap around the existing window opening, it won’t have a proper seal to the sheathing boards at the board seams, nor at the head because there is no wrb to lap over it. So water that is undoubtedly behind the shingles will collect behind vycor and begin a very serious problem. 

Of course the proper answer is to tear off the siding and put on a rain screen. But I keep coming back to this fact: architects and builders do this all the time. Full stop. Every architecture firm in Seattle has added new window openings to a 1900s house that has no sheathing, insulation, or WRB. How are they dealing with this water issue?

My guess is they’re doing the vycor method, relying on caulk and hoping the homeowner reapplies the caulk every 5 year or so, long before it fails.

(attached is photo of typical sill and “sheathing” boards on a very small area we had re-sided last year. So much is wrong about this!)