I live in Michigan. My home is 100 years old. I've just discovered a wet patch in the plaster corner of the ceiling in a bedroom on the second floor of my home. It is actively dripping water, now that I've poked it a bit. It is 16 degrees outside. The patch is near where a toilet stack (through the common wall between this bedroom and adjacent bathroom) exits the roof above. From the attic, I can see frost on the pipe/stack where it exits the roof. I do not see water leaking down the pipe. Poking around in the insulation (cellulose) reveals some moisture, but it's not directly against the toilet stack. From outside, looking at this stack, I can see ice around it.
I am wondering: 1. If I should try and wrap the pipe from inside the attic; 2. If I should keep the insulation packed down as it was, or remove it where it is wet so it dries; 3. Any other advice as to what I should do in the months before we warm up?