I’m finishing the basement of a row house in Philadelphia. The bottom 2/3 or so of my brick foundation is covered in parged concrete about 4″ thick. In other words, the vertical surface of the wall runs up from the floor to about 5′ in height, then there is a 4″ horizontal shelf leading to the brick foundation, which makes up the top 2′ of the wall. (The pictures should make this clear.)

I am planning to use XPS board to insulate the walls. Should I do anything to make the XPS against the brick flush with the XPS against the concrete? I was thinking I could stack two more layers of 2″ thick XPS against the brick, then run a single panel that should adhere flush to the concrete and the two extra layers of XPS on top. Or should I just adhere a single layer of XPS to all the surfaces (including a 4″ horizontal strip/shelf) on top of the parged concrete? This would leave an unfilled cavity behind the wood framing at the top of the wall.