I have an existing 2 x 4 wall with kraft paper faced fiberglass insulation (no drywall has been installed yet) and would like to add a layer of foamular rigid insulation to the interior side of wall (On top of studs - not between studs) to add r value and reduce thermal bridging. I am concerned about the double vapor retarder ( but in this case, the vapor retarders would be immediately on top of (Touching) each other (ie, kraft paper touching foamular)) but wanted to know whether others think this would be okay. Please let me know your thoughts. Thanks.

This is for climate zone 4. Philadelphia pa area.

You're better off adding foam to the exterior if possible.
Adding it inside basically brings the dewpoint of the wall into the wall assembly.