One of the 2 stacks in my house has started leaking, and was discovered during a recent extreme cold spell (regularly -0 F) in Indiana. (short version of house: 1920 built house. countless remodels to subdivide... current one to restore to a single family home in progress by me).

I'm not sure when the leak started, but it was discovered when the paint started bubbling on the wall in the 1st floor bathroom. Above the 1st floor bathroom is the 2nd floor bathroom. I opened up the ceiling access panel between the 1st and 2nd floor, and saw the rubber joint between the cast iron and PVC has started leaking.

Closeup:


Going to the basement, I discovered this:


The leak had flowed down the wall, froze as it went, and then accumulated frozen at the base of the stack. The stack runs along an uninsulated exterior wall. I chipped away and and put a space heater in to ensure the base-stack wouldn't freeze.

As things thaw out (as they are doing now) I'll have a water logged drywall and insulation to tear out and replace, as well as the stack to repair. See main floor bathroom pic:


2 questions:

1. Is having a drain stack on an uninsulated exterior wall ever ok, or should I move it? I could re-route to another drain stack that goes to another bathroom on the 2nd floor (that's on an interior wall).

2. If I keep the drain stack, what's the recommendation to repair the joint? Replace the cast iron T with a PVC one, like this? I'm not sure the cast iron T is advisable to keep using as there's evidence of past leaks...

 

One problem is the flexible coupling used. You need a jacketed no-hub coupling.
Assuming the pipe is 3", something like this:
https://www.homedepot.com/p/3-in-EPD...0-33/100372287

Fernco identifies cast iron as 'cast iron' or 'extra heavy cast iron'. If the P3000-33 doesn't fit snug, a P3005-33 might.