Condensation on internal soil stack?

Maintenance

I purchased an old (100 years perhaps), end of terrace house in 2004 and converted it into 3 flats in 2005. The shower rooms on top and bottom flats are on the same side (inside wall) but middle on the other side so the soil stack snakes a the building a bit.

The problem I have is that in the bottom flat around the boxing from the middle flat soil stack, water has come through. The plumber opened up some of the boxing in bottom flat but couldn't see any sign of a leak (and said that if it was a leak he would expect the boxing to be wetter), although could see condensation around the pipes. He's not sure but thinks that condensation is the problem, as that pipe (and shower room) is on a north facing wall, and with cold weather and hot showers. He thinks this may have been going on for a while.

He is suggesting to cut into the boxing in both flats and put insulation, covered in foil, around the pipes. He would also know if a leak was present while doing this. 

Has anyone had a similar experience? If so, what was the solution? Or if anyone has any other suggestions on what it might be or other solutions to clear? I did suggest putting air brick in the wall but he thought this would just let more cold air in.

Any thoughts, much appreciated.