I'm working on renovating a 90 year old house. It had a cast iron/ceramic tub that never had a shower, but I want to add one with a tile surround above the tub. The room is gutted right now down to the studs. I'll be applying closed cell foam insulation to the exterior wall. I want to put the tub along the exterior wall, but I'm not sure if I should put greenboard up first or just put the tub against the bare studs. I know the greenboard or cement board should come down and overlap the rim, but I'd need a double layer of wallboard (whichever) to do that. I'd also heard not to put greenboard behind cement board. Basically I'm not sure how far away from the wall I should put the tub. Should it be a half inch away or up against the studs? I'm not going to put the tub there till I notch out the joist below the tub to fit the drain tube. It won't be much of a notch. It just sits maybe an inch below the floor. Just didn't want to make the notch any wider than I have to in order to fit.