1) Seems that this would bump out the shower beyond the rest of the wall. Ditch the sheetrock, staple the barrier to the studs and then add the Hardi & tile.
2) No...without a vapor barrier, moisture can migrate to the studs causing mold.
3) 15lb felt is also fine. Build it from the bottom up, overlapping the seams.
4) Plastic sheeting, sold in rolls. Use 6mil, if they have it. Thicker is OK as well.

Thank you!

This is an existing bath/shower enclosure. The sheetrock is already there. The old walls had tile with like an inch of mud underneath. Our GC is putting the hardibacker over the sheet rock to bring the depth back up for thinset and tile which will have a similar profile as the old tile in that there are 1/2" bead trim pieces for the edges...it will not bump the shower beyond the rest of the wall at all.

Given this, is it okay to have the following...

sheetrock
15lb felt
hardibacker 500
thinset and tile

Thanks again!

When I say "bead trim pieces"...these are like 1/2" x 6" quarter round pieces to go with our 6"x6" tiles.

While I've always managed to not go over drywall with a vapor barrier sandwiched between it and Hardi, I don't see any reason why that shouldn't work. If it were my job, I'd simply Kerdi or Hydroban the drywall and tile on that.

You didn't mention if this is a shower or a tub "wrap."

This is a tub installed in an alcove with a shower.

OK...if the Hardi is up with no vapor barrier behind it, coat the face with two coats of Redgard from Home Depot.

Use pure silicone to seal the edges to the tub.

Actually, the Hardi is not up yet. And actually, the sheetrock is down too on part of the wall. The contractor is not keen about using something like Redgard (he feels it makes it difficult to tile over).

That is why I am asking him to put the felt between the sheetrock and hardibacker.

Shouldn't make any difference if he's using a good quality latex-modified morter...and I like not having the Hardi ever even get wet, as it can be with a vapor barrier behind it, however, be that as it may, tarpaper or a plastic poly liner behind the Hardi is perfectly acceptable also. Just "dealer's choice" here........

Thank you!