Position the cut drywall sheet on the wall.
Rotten wall behind bathroom enclosure.
Make sure the bottom side edge of the drywall sheet extends down over the thin lip of the tub s mounting flange.
Leave some of the old drywall exposed where it will meet the new backerboard.
Any insulation that is wet or moldy will also need to be replaced.
If any of the wood studs appear wet and moldy you can clean them with a water bleach solution.
Leave a 1 8 inch gap between the bottom.
Drop those pieces into a bucket of hot water to separate the gypsum backing from the tiles.
If you find that the wood is rotted you will need to replace it.
Allow the wood and wall cavity to completely dry.
Then pull the tile and small squares of drywall out together.
Install an air barrier behind showers and tubs installed on exterior walls.
Install insulation without misalignments compressions gaps or voids in all exterior wall cavities behind tubs and showers.