Ive read about using the roofing paper and the plastic.
What about applying the foundation sealer/tar to the hardiebacker itself?
I have used pressure treated frame boards and applied foundation sealer to the side facing the concrete/brick wall, but did not apply it to the side facing the hardiebacker so as to allow some air flow through the wood (at least vertically).
Now Im thinking about applying the foundation sealer to the hardiebacker side facing the stud and wall.
This seems logical. Seems more water tight than roofing felt or even plastic.