I am remodeling my bathroom and I have torn down the old greenboard around my bathtub but left the ceiling thats the old drywall and still in good condition. I plan on using cement board and tile from the tub to the ceiling. My question is, how do I seal the area between the cement board and the old drywall on the ceiling? I'm trying to avoid messing with the integrity of the ceiling.
When I've done this in the past I just butted the cement board within a 1/8" or so of the ceiling, and then tiled up to the ceiling, and applied a bead of silicone caulk between the tile and the ceiling. Not sure if that's Kosher, but it seemed to work for me.