I am an architect working on the remodel of a 1915 stone and wood frame home.  Specifically, I am looking for some advice on the best way to insulate the existing wood frame roof.  A quick background on the project….This is a 3-story residence with 18-inch-thick exterior stone walls and wood framed floors and roof.  We are in Maryland CZ 4 and the footprint is about 1800 SF.  We plan to strip out the plaster to expose the interior side of the stone walls and expose the interior walls, floors, and roof framing.  We are going to insulate the exterior walls with a furred 2 x 4 wall with an R=15 Rockwool insulation.  The 12” basement floor cavities will be filled with blown-in cellulose.  The third floor is a wood frame “cape style” roof line with 2 x 6 rafters 24” OC.  The house has multiple dormers and a couple of valley areas and a slate roof that has been repaired and will remain.  The slate is installed over building paper and 1 x 6 sheathing.  There are no ridge vents installed, and I am not sure how effective they would be because of the multiple dormers and a couple of valleys interrupting air flow.  I am thinking about specifying closed cell spray foam or rigid board insulation on the underside or the roof sheathing to R=15 depth then furring down the rafter cavities and filling in with dense pack cellulose to gain at least R=30 total.  Do others agree with this approach or have any other advice or suggestions to deal with this roof condition?

TIA