I use two layers of 1/4" backer board, for reasons including secure tile overlap of a copper seal edge.
I will not rely on tile or caulk in sealing the tub edge. Here two-part epoxy seals a copper edge to the tub lip and exposed areas of the backer board.
After several trials, I chose a Kohler porcelain tub of ordinary height, with sharpest inside corners. I believe least maintenance and best suitability if handicapped, is with a tub this height, with good shower curtains, not glass doors.
See the pencil tracings on backer board above? Yes, my plumbing supplier took in exchange, an undamaged too-tall fiberglass Maax Skyline tub. I thought the Maax tub flanges would be conducive to edge sealing, but they are not. I find no tub built to not rely on caulk. The copper idea was volunteered by friend Copper By Design. I inquired about stainless steel.
Tiling was done atop 1/4" ACX plywood framed below as a strong and flat table. Bottom tiles will rest on the plywood, and that tile rest edge will remain exposed out of sight, sanitary with full gap filling of flexible grout. Flexible grout has added mildicide. I offer free samples and seek to inspire manufactur
Finished bathroom outside wall. I will towel dry the tiles daily, as is experienced where we find shiny walls in a hotel.
The tile is backed with plywood set in the walls, and two layers of 1/4" backer board. Many screws ensure tiles and grout should never be distressed by any action in the wall. Thermal stability with the insulation will help too.
The plywood is 3/4" thickness and should have extended to the ceiling, for secure mounting of the outer layer of 1/4" backer board even where there are no tiles. This plywood is moderately-well fastened. It may not adequately support grab bar attachment points. I might have toed many more screws into the joists. Wish for more space, such that plywood is atop the framing, for mounting of whatever, anywhere. More-ideally, plan grab bar attachment points of solidly-mounted 2x lumber.
Choose batts always wider than bays filled, commonly 15" in a 14 1/2" space. In a 2x4 wall, I will accept R19, but must struggle to bring edges out to the wall surface. These are a layer and a half of R15, R21. I didn't want the minimal fill of R19 in this wall, and enjoyed the better body of R15 strands stuffed into non-California corners. R19 is so wispy.
I hope others will find this example instructive, and will comment. Do you have a better way? If you have ample bathroom space, perhaps you would clad plywood over over walls to be tiled, plywood inboard of joists. Who has had bad experiences with tile placed over drywall, not over very solidly-set backer board? If putting a one-piece shower unit directly against framing, what is desired as insulation covering? Is a good plastic barrier like Tenoarm, sufficient then? Please know I believe that in my marine environment with low relative humidity, I should have a full interior vapor barrier at all exterior walls.
I do this for for very-rare customers where I have new-drywall opportunity. I have failed to inspire other contractors, to my knowledge.
This is a bedroom in my home, with all Tenoarm edges sealed with double-backed butyl tape.
Did I do right in my bathroom, to despair of Tenoarm with difficult blend of backer board/ tile, and drywall facings?
I must report that Swedish Tenoarm is no longer offered in USA.