We’d like to air seal and insulate our garage walls.  We are in zone 5.  The current configuration is as shown, though I don’t know if there is any sheathing paper on the sheathing.  We will heat and cool the garage.  We’d like to get the highest r-value that we can.  We can get Johns Manville R-13, 2-in x 4-ft x 8-ft AP Foil Faced Polyisocyanurate, and GreenGuard Unfaced Foam Board Insulation in various thicknesses locally.  I have been planning to air seal where the studs meet the sheathing, fill the spaces between the studs and air seal, cover the entire interior wall with board insulation and tape the seams.  I’m confused about where I should use foil faced versus unfaced.  I’m also unsure whether I need an air gap between the sheathing and the first layer of board insulation.  Thanks in advance for any help!