I'm completely redoing a smallish bathroom. It will have tile from tub to ceiling and outside the tube area will be up to about chair rail height and be painted the rest of the way up. There will be Hardibacker under all the tile. Do I even need to bother with drywall or can I just use all 1/2" Hardibacker and prime/paint it? As small as the bathroom is, it'd only be another couple sheets. If I need to use drywall instead, how exactly do I do the transition?

U can use green or blue sheetrock in all areas outside of tub. If u want to use hardiback under tile, stop it below top of last tile. Butt s/r to it and tape to seal.

Chuck

I have also wondered about using Hardibacker as a finished surface in a bath. The smooth side paints up nicely and would certainly be moisture resistant.

I can think of no reason not to use the smooth side as a finished surface. You would have to finish your joints with joint compound and fiberglass tape. Make a joint about 3" below the level of where the tile will extend and use the smooth side for the upper wall.