Mass Save has just completed insulation of my 1900 balloon framed house.  They did a great job, but one of the gable ends of my house is still very leaky–I can feel air pouring in–due to gaps in the exterior side of an inaccessible foundation wall that allows air intrusion under the ridge beam. The guys left me some extra polyiso, and I have a good amount of mineral wool as well. Could folks look at my plan and advise on improvements?  Essentially, I want to block off the air intrusion by laying the polyiso horizontally and spray foaming it in lightly, possibly using foil tape as well, then also damn up the stud bays better by adding poly iso there as well (it was too deep for them to reach in some places, but I’m pretty sure I can do it, either with polyiso or mineral wool… they placed fiberglass there as best they could to dam up when they blew in cellulose. 

Note that just spray foaming the hell out of this situation is not an option–as the diagram shows, some of the joists are not touching the ridge beam (ie some joists are resting on the foundation wall and the finished framed wall inside), and I’ll need to be able to access the meeting of the beam<>joist to install some tension ties etc.  Instead of 100% air seal of the polyiso against the joists, I’ve thought about just tucking mineral wool tightly between the foam and joists for easier removal of the foam board with the structural work is done. 

Thoughts and recommendations?