Water on outside of PVC vent pipe in basement

[bgrant1]

I have water dripping (very slowly), on the outside of a PVC vent pipe that runs between my tub and toilet (I think it's condensation). The good news is the plumbing can be accessed from my basement and easy to see that it's the outside of the PVC pipe where the water drips from, not from the trap for the tub. The drip fills maybe a shot glass or two daily. I tried insulating the vent pipe VERY well but that did not stop it (just caused the insulation to get wet). I considered the leak may be from water running down the vent pipe possibly leaking water from supply line to tub, shut the water supply off for multiple days but no change. I then removed the baseboard molding and cut a hole in the drywall, the vent pipe above the floor is 100% dry and so is the wood around the floor that the vent pipe passes through. Below the floor, water still collects and drips. The basement is not that cold (~60 degrees). Is it possible that water in the vent pipe might causing the condensation? The tub drains fine, toilet flushes fine, no gurgling. Above the floor all is good, below the floor is the problem....I'm running out of ideas? The vent pipe runs through the floor and inside the wall between the toilet and tub. In the attic it joins the main vent. Is my next step cutting the vent pipe in the attic and snaking that?

 

[Reach4]

I would wrap something absorbent, like a towel held in place with tape or bungee, around the pipe high on the more accessible area. Does it get wet below that? If so, I would consider condensation. You can buy a hygrometer (measures humidity) cheaply today. See what your basement air shows.

 

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[Cacher_Chick]

Post a photo of the piping installation so we can can better see why this problem might exist.