The vapor barrier has a brownish position, insulation is wet

Hello everyone,

Our house construction started in 2017, currently we are just before the screed stage, interior plaster, underfloor heating etc. are currently completed.

I discovered a brownish spot on the vapor barrier two weeks ago.

Called the construction supervisor, cut open the vapor barrier, result: wood and insulation damp.

Cut open other spots, not damp but also not 100% dry, maybe 95% dry, if you can quantify it in numbers.

Now the construction supervisor suggests that as soon as the screed is in and the heating is running and the heating program is completed, the vapor barrier should be opened, insulation removed, dried and insulation possibly replaced.

This may help at the spot for the moment but possibly not permanently, because it is unclear where the moisture is coming from... (Roof has been checked, it cannot come from above)

Structure:

Brick
Battens
Underlay membrane
OSB 22mm with tongue and groove as stiffening panel
22cm insulation
Pro Clima Intello Plus

Later followed by suspended ceiling with gypsum.

I currently suspect an error in our construction, so that the moisture, which can be absorbed by the variable foil, cannot escape upwards because of the OSB. However, we needed the OSB with tongue and groove statically as the stiffening panel.

Are there by any chance experienced experts here who can tell me how the whole thing should be ventilated? Making slots with a milling cutter? Or holes? If yes, how many slots or holes for an area of 10x15 meters?

Or is there nothing to be done at all?

Thank you in advance for the support.

Yes, indeed the OSB boards are the problem there. It is still often done. The result can be seen here. I would recommend a membrane instead of a vapor barrier / vapor retarder, which can also release the moisture back into the room.

Thank you very much for the feedback. It may be a misunderstanding.

The mentioned vapor barrier is a Pro Clima Intello Plus, a climate membrane. I deliberately chose this after consultation because it is supposed to be so good and versatile and also cost much more than a normal barrier or foil.

Do you mean something else by membrane or are we talking about such a climate membrane as we have?

No, that's what I meant. Then I am at the limit of my layman knowledge, especially since the OSB panels are stiffening.

If the moisture did not come from above, where did it come from? Were there any leaks visible anywhere?

Exactly that is the question, unfortunately not clearly definable. Only one thing seems certain: no water is coming from above, as the attic is bone dry and has been checked multiple times, and it also seems that no moisture is coming from below. In addition, moisture on an OSB board could be easily detected if it had passed through the board from top to bottom.

Options are:

- Leak at the vapor barrier, moisture rising/condensing through the barrier and settling on the OSB from below, which is now running down again
- Ring beam was exposed to rain, possibly a bubble in the concrete that had filled with water and is now releasing it again to the aerated concrete
- Generally damp aerated concrete blocks at the ring beam because it rained one day after the concrete was poured, and the blocks are now releasing the moisture again

If holes or slots are made in the OSB, so that the statics are not affected, the moisture should be able to escape upwards, right? Whether it then settles on the underlay membrane and drips down again (which would not be so good either) is the next question.