I am putting new composite siding up on an exterior wall that presently has sheathing, Tyvek, 1/2 inch foam insulation board, and the cedar in that order.  Since the new siding will go up vertical, I will put 2x4s horizontally nailed in to the wall studs at 20 inch intervals with the new siding nailed into those.  This will make the wall thicker and I will pull the old windows and upgrade to new windows to fit the new wall thickness. I plan to take insulatiog advantage of the 2×4 thinkness and put  foam board between the nailing studs (I live in Montana).

Now the question, is the vapor barrier in the correct position? Should I pull the old Tyvek and  thin foam and then nail the 2x4s directly in contact with the sheathing?  If I leave the existing Tyvek and foam on the wall after I pull the old cedar siding should I put a new layer of Tyvek on over the stud nailers and foam before I put up the new siding? If there is a vapor barrier, should it be the last layer on the wall before the siding or will it “work” if it is under the nailers and foam? 

Thanks for your experience.

CS