I have recently discovered a significant mold/decay problem at the underside of the roof sheathing above the ceilings of the bathrooms in my home.  They are back-to-back so the problem is contained to one area.  Half the house is low-slope insulated above the sheathing with open beams and decking.  The other half is low-slope with an insulated lid and minimal attic (14″ at the peak).  Problem, obviously is on the half with the lid.  Plan is to tear off the roof on that end of the house and insulate above the sheathing and eliminate the condensing surface.  We will also have to remove a strip of plywood sheathing around the perimeter so that we can spray-foam the top of wall condition to achieve continuity from wall to deck.  However, its unclear to me what the best practice is for dealing with the cavity above the lids and below the deck.  Builder wants to dense pack it with insulation but this would require removing all of the roof sheathing and I don’t see that it’s necessary.  Its an old home without a poly vapor barrier so the cavity can dry to the inside, technically.  Is it realistic to think that this cavity is now interior space and that it can just be left as a cavity?  Marine (4C).  Thanks.