The first part of this project was to have a bathroom to use while the one and only toilet in the house is out of commission. So here is the solution.
I want to get the underside of the house insulated with closed cell spray-foam for insulation and vapor barrier purposes. I knew that the bathroom floor would need new framing and the water pipes replaced. All which needed to get done prior to spray-foaming. I removed the old drywall from the walls, broke up the old cast iron tub and took out the layers of backer board, plywood and rotted and termite damaged wood.
Here is a picture with the new backer-board, drywall and tub installed. The toilet is installed temporarily.