Fiberglass and acrylic tub and shower enclosures come with a flanged edge that attaches directly to the wall studs and the proper way to hide this flange is to install drywall over it.
Rotten wall behind bathroom enclosure.
Then pull the tile and small squares of drywall out together.
Position the cut drywall sheet on the wall.
If you find that the wood is rotted you will need to replace it.
Install insulation without misalignments compressions gaps or voids in all exterior wall cavities behind tubs and showers.
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.
Drop those pieces into a bucket of hot water to separate the gypsum backing from the tiles.
If any of the wood studs appear wet and moldy you can clean them with a water bleach solution.
Cover the wall cavities with a rigid air barrier or other supporting material to prevent cavity insulation from sagging and to create a continuous thermal barrier.
To remove tiles that won t budge reach into the wall and use a utility knife to score the drywall from behind.
Leave a 1 8 inch gap between the bottom.