I pulled some 1/2" copper pipes upward in the summer with wire for clearance in preparation for installing T-bar ceiling tiles. I don't know that it did any damage to the pipe or joint, or even if I have more than a common situation now. I wrapped some pipe insulation, but not completely, this joint in question was tough to get to, and I left it until I was ready to finish it off. Today I was peeling back that same insulation; these pipes go to the sink and toilet in the basement washroom, and the last inch or two of the insulation was WET, meaning waters on the outside. Its been months since its been summer/humidity? in Toronto. I have taken off the insulation, chiseled off the little black rotting part of 2x4", and cleaned off the pipes, but other than the moist insulation I haven't seen any water at all, even when running tissue paper around. The water may even been wicking from down inside the 2x4" in the wall, a crack possibly on the other side of the elbow in the wall. . I don't know if either pipe is dripping from a leak/crack, or if the cold is condensing (its cold to the touch), or the insulation has just stayed wet since last it was humid, or if its bound to happen when I flush.. This pipe is at the end of the plumbing line, nothing else runs past them. The basement is where the hot and cold water are, both at the other far end of the house. The place in question is in the ceiling, running just under the first floor joists. More specifically, where both the hot and cold pipes have 90 degree elbows soldered, and where these pipes turn and go straight down into a stud wall. The drilled hole in the 2x4" top wall plate is just large enough to fit the hot and cold, each in individual holes, so nothing is getting/looking in there. The elbows are just barely solderable above this top 2x4", in fact the cold is half-buried. The two pipes are spaced diagonally, about 1" apart on centers; the pipes run very close together, both along the ceiling, and in the wall. The hot pipe is the upper one. Where the hot elbow is soldered some of the solder of that joint is actually soldered to the upper length of straight pipe of the cold.
How do I diagnose if I have a leak? I have not run any hot or cold water in the sink, nor flushed the toilet in a couple days. In the last few months I have only flushed the toilet twice, both times in the last couple weeks. I've just got tissue paper sitting there, waiting for me to look later. I could drill a hole-saw in the wall, but what about running the water, etc. to help find out. A problem is its a tight location, can't see all possibilities, and don't know how to check with a diagnosis. There are more pipes around, but they are all currently insulated.