Hi, we bought a 1900 farmhouse in northeastern Massachusetts that has real 2 x 4 framed walls, no insulation, and board sheathing, a layer of wood shingles covered by a layer of cement shingles. We don’t think we can afford to redo the exterior, so thinking about removing interior plaster and lathe on outside walls and air sealing and insulating from interior.  A number of options or combination of options under consideration: 1) spray foam or dense pack cellulose the 4″ bays and sheet rock, 2) create internal rain screen with blocked rigid foam and then spray foam or dense pack cellulose, 3) create rain screen with caulked membrane held out with batten in middle of bays and fill with dense pack and sheet rock.  Also thinking about furring out studs with strip of rigid foam and 1×3 or cross strapping to get more depth.  Any opinions on best option at reasonable cost?  Do we need an internal rain screen? Is it worth thickening and thermally breaking walls? (we’re planning to replace single pane windows and sash weight cavities at same time so have opportunity to extend jams)  Do we need any vapor adjusting membranes under sheetrock?  And random question: can you paint a vapor permeable air barrier such as StoGuard on the back of the sheathing and then insulate to interior? Thanks, I’m total amateur at this stuff, but love learning from the discussions here on GBA.