Need to tap into the "fountain of knowledge" here....
Bathroom. Wood Cabinets above double sinks shows evidence of leak/rot coming from above ceiling (like attic). Crawl in attic find ONLY about a 3 inch PVC vertical vent pipe. It goes straight up, does a 90 degree zig-zag above cabinet (2- 90degree elbows) then goes straight up and out composite shingle roof.
Pipe is moist on outside in small deadspace between cabinet and hole in ceiling to attic. Siliconed all joints (although I saw no evidence of leakage) , also at roof line where it goes in. STILL getting moisture. Slow slow drip but enough where it is again dripping in from top of cabinet. Checked it early (6AM) yesterday morning. It was wet. Afternoon it was dry. Put drip pan under it, probably about a shot glass amount of water.Temp inside about 78, outside about 40-50, later in day up to 70+.
It's just a vent pipe. No running water. No a/C or water lines anywhere near it. WHADDAHELLIZGOINONHEREANYWAY??
I suspect it may be pipe sweating. Any other ideas? Suggestions to remedy?? Need a pumbing guru!!! HELP!
I would bet it is pipe sweating. For example: if you have a indoor temperature of 78 (dry bulb) , wet bulb temperature of 65, and relative humidity of 50%, then the dew point would be about 58 degrees. This is the point where moisture will condense. Your overnight temperatures of 50 to 40 degrees is well below that and would provide plenty of condensation.
You need to insulate the vent pipe where it is exposed to cold air.
A Psychrometric chart will show the various changes in dew point, depending on what the data is.