Moisture around ventilation pipe of controlled residential ventilation

Hello everyone.

Yesterday we made a troubling discovery. Water has appeared around the ventilation pipe in the bathroom. So much that it was dripping from the ceiling.

Current situation: Since April 17th, the screed drying program has been running in our house. The humidity and temperature are accordingly high. On Saturday, we additionally applied primer. So far, we have only ventilated in the morning and evening with shock ventilation throughout the whole house. The ceiling in the bathroom is currently clad with drywall and plastered. Also directly around the pipe. Holes for the spotlights have been drilled in the drywall. There are no spotlights installed yet. The insulation wool (we have a thin layer in front of the vapor barrier) is dry at the spotlight holes. But moisture can of course currently move behind the drywall.

Right after painting on Saturday, the whole place was almost falling apart from moisture. For example, the bathroom window was completely fogged up and wet. Lesson learned: We are currently ventilating longer and three times a day. Previously, we ventilated less because the installer told us to do so (to maintain warmth).

The ventilation system is not yet in operation. The ventilation pipes are closed with caps. The pipe was completely dry on the inside.

What could be the cause of the problem? Our installer says to wait and observe. He also suggested that maybe the vapor barrier has detached from the pipe and created a thermal bridge. Possible, but I consider it unlikely. The craftsmen glued the pipes very poorly. We ourselves went over every pipe and glued every one again for two hours each. Of course, the work was done by amateurs but to the best of our knowledge and rather glued five times over than simply. I did not tell the installer that we glued them again. If there were a defect, shouldn’t he actually initiate an inspection?

I looked in the attic. Nothing is wet there. Since the weather has been good in recent weeks, it is not particularly cold there either. I think about 15 degrees. Certainly about 30 degrees in the house. The attic is insulated with 80mm Pavatex boards. We insulated the ventilation pipes ourselves with insulation material.

Do you think it is possible that the water formed on the outside of the pipe and still before the vapor barrier because the inside of the pipe might have been about 10 degrees cooler? The question would then arise why it is only in the bathroom. However, the bathroom has always been the most humid room upstairs. Two weeks ago, screed was also applied in the shower (2 sqm).

What would you recommend? What else could be the problem? How should we proceed?

Addendum: Only the bathroom upstairs and the hallway have outlets for the spotlights. So moisture can increase behind the drywall there.

Dripstone cave during screed drying is not unusual. I haven't quite understood your setup yet, where an open (?) ventilation pipe is hanging in the bathroom that condensation could form on? You should also be aware that in a dripstone cave there are no good conditions to sensibly carry out painting work. Everything is much too wet, not just the screed but also the plaster.

The pipe itself is not open. It is a normal vent pipe for the WRL. The ventilation system is just not in operation yet. Since the screed drying has been running since 17.04 and the house has already become noticeably drier, we have started painting. Until the painting, there was no moisture either. Last Friday, I was still working on the pipe (ventilation pipe). The conditions were so good that the last remaining spots on the plaster were completely dry. So it was already relatively dry up to that point. In the morning, however, the windows (only at the bottom) and the frames were still wet, of course.

I will send a picture shortly.