Hardibacker w/sloped tub flange and poly surround

Need advice on proper placement of hardibacker when working with a cast iron tub with sloped flange and the installation of a five piece surround. The surround is an American Bath Systems Sturdibuilt and appears to be fairly well made but will still not be close to the "solidness" of ceramic tile. Want that lower edge to be adequately backed but also don't want to risk the hardibacker getting wet. Am at bare studs and can fill out slightly from studs if required.

Would also appreciate info on using felt behind the hardibacker on interior walls and using fiberglass insulation as a sound barrier. Concerned that putting up a vapor barrier may not be a good idea in this case. Main tub wall shares a common wall with tiled shower on other side. Much damage on that wall but can't have both bathrooms gutted at once.

Any pitfalls concerning gluing a surround to hardibacker appreciated too.

Many thanks in advance for any and all help!

You do need to fur out the backer board so it can properly overlap the tub flange and still be plumb. You should use a vapor barrier. The subject of insulation and vapor barriers is complex. I would post this question over on the tile forum www.johnbridge.com Someone there will fill in the details on this issue. Let us know here what you end up with.

Jimbo, thanks for the quick response! Will continue on with the furring out process to get the hardibacker down over the flange by at least an 1/8. Will try your suggestion on the vapor barrier question and post again with any info.