I have a 25 year old house with a timber frame and SIP roof system and (5) stick-built dormers.  We are on the coast of Maine, Bar Harbor area.  While refinishing the siding last year I noticed that the shed dormer sheathing may be failing (in a couple of places the trim was rotting and shingles could be pulled away without much trouble) probably because I did a poor job with flashing and let the shingle and MDO siding almost touch the roof surface as an inexperienced homeowner on a tight budget.   I also kept the window sills too close to the roof surface, but that area seems to be fine.  Also, when we have a decent snow cover those dormers definitely leak enough to cause ice damming, and I’ve noticed that the ceiling drywall feels cool and damp to the touch at the roof-wall joints during those periods, although there is only the slightest visible water damage at one of those roof-wall joints.

We are planning to re-shingle the roof this summer and I’ve decided to take the opportunity to have those dormers stripped down to the studs and rafters, hopefully only at the exterior, and improve them in several ways.  The wall and roof systems have fiberglass batt insulation with a poly vapor barrier.   Once the framing is exposed, the plan is to remove the cavity insulation and cut away the vapor barrier between the studs and rafters, then improve the insulation, replace the sheathing, new WRB/drainage plane and siding, A very capable builder will be doing the work this time, not the homeowner!  

The big question is, what is the best way to insulate those wall and roof cavities from the exterior?  I’ve been told that installing spray foam from the exterior, onto the outside face of the interior drywall is problematic, and I’m not really a big fan of foam.  So what about densepack cellulose, mineral wool, a wood fiber product like the new Timber HP batt, can those be effectively installed from the exterior?  Or, should I just go ahead and remove the drywall from the interior?  

You can see by the first photo that I tried to cross vent the dormers originally, but I doubt that has worked well.  The window walls are also mostly windows and framing, so is it even worth taking those apart?  Lastly, you can see from the last photo that I (cleverly?) made a couple of the dormers wider than the timber frame/SIP opening and installed built-in cubbies for the kids.  Those 2 areas are going to be difficult and costly to access from the interior.  

Thank you so much.  I appreciate any information you can provide!