vapour barrier question
My basement currently has a four foot cement block foundation wall topped with a 3+ foot wood stud wall to the ceiling. I am building a 2x4 wall in front of the existing walls flush to the ceiling, which will give me room to insulate the block wall and add electrical receptacles as well. This means there will be a lot of insulation in the top half (existing stud wall), but that's ok with me (I don't want to create a ledge either).
To create a thermal break between the foundation and the stud wall, I was going to attach some xps to the foundation wall, and add batt insulation over top in between the new studs. It is my understanding that a foundation wall covered in this way does not require a vapour barrier on the warm side, since the xps is a vapour barrier in itself. However, the top half of the wall (existing stud wall) is above grade and therefore requires a vapour barrier on the warm side of the insulation.
My question is, am I going to create problems if I add a poly vapour barrier over the batt insulation on the entire wall, considering that I will have xps attached to the foundation on the bottom half of the wall?
I am trying to do this as economical as possible, which is why I'm not insulating entirely with xps or spray foam.
A compromise may be to add a layer of xps against the sheathing on the top part as well and skip the poly altogether.
Any insight/suggestions would be appreciated.