I have a 3" PVC vent stack that goes up three stories from my basement through first and second floor interior walls, into the attic, goes horizontal for about six feet, then vertical through the roof. It's causing moisture problems in the wall through which it goes. The problem seems to disappear in the summer and come back in the winter. It doesn't seem to be related to rain events. The house is about 10 years old.

Plastic usually does not cause condensation. Is that pipe on exterior wall ? Do you have insulation in your walls ?

I would inspect the pipe for possible leak first.

Thanks for the quick response. In the basement the vent stack is exposed and there is no problem; on the first floor it goes between an interior wall and the garage wall, which is insulated, and there is no problem I've detected; on the second floor the stack goes inside an interior wall (uninsulated), through the wall top plate and into the attic space. Moisture is inside the wall on the second floor and the ceiling of the second floor near where the pipe comes through the wall top plate. Where the pipe comes through the top plate, the pipe and top plate are sealed with polyurethane insulating foam. It was dry through the summer and just started to get damp again when the nights began to get into the 40's and 50's. I can't tell where the moisture is coming from.

Hello Buckomatic: The vent stock vents moist, warm, sewer gases. When it gets cold outside, the gases will liquidify once they hit cold section of pipe. Also, it is true about the pipe: once warm pipe enters cold area, it will / may produce condensation.

Go to the attic with flash light and put your hand on the pipe. Feel if you feel moisture. If positive, I would recommend wrapping the pipe in the attic with a thick layer of insulation.

Sometimes happens that carpenter or drywall people will accidentally drive a nail through the plate and right into the pipe. The nail will eventually rust away leaving a hole in the pipe. Then, there is a hole that will allow moisture to exit the pipe. Far fetched, but want you to be aware of it...

Also, if it is plastic pipe, it may have snapped under pressure as the house settled after it was built...

My theory is that there is an unsealed joint in the pipe in the second story wall. This joint would not show up as a leak because it is a vent pipe only.

Thanks! I'll check it out this weekend and let you know!