ᐅ The vapor barrier is discolored brown, and the insulation is damp.

Hello everyone,

Our house construction started in 2017, and we are now just before the screed stage. The interior plasterwork, underfloor heating, and so on are currently completed.

Two weeks ago, I noticed a brownish spot on the vapor retarder.

We contacted the building supervisor, cut into the vapor retarder, and the result was damp wood and insulation.

At other spots where we cut in, it was not wet but also not 100% dry—maybe around 95% dry, if that can be quantified.

The building supervisor now suggests that once the screed is poured and the heating system has run through its full cycle, the vapor retarder should be opened, the insulation removed, allowed to dry, and possibly replaced.

This may help at that location temporarily, but perhaps not permanently, since it is unclear where the moisture is coming from (the roof has been inspected, so it cannot be from above).

Construction layers:

Brick
Battens
Breathable underlay membrane
22mm OSB board with tongue and groove for structural bracing
22cm (9 inches) insulation
Pro Clima Intello Plus vapor retarder

Later, there will be a suspended ceiling with gypsum board.

I currently suspect a design issue where the moisture, which can be absorbed by the variable vapor retarder, cannot escape upwards because of the OSB board. However, we need the OSB with tongue and groove for structural bracing.

Are there any experts here who can advise how this could be ventilated? Cutting slots with a router? Or drilling holes? If so, how many slots or holes would be recommended for an area of 10 by 15 meters (33 by 49 feet)?

Or is there no solution at all?

Thank you in advance for your support.

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

Thank you for your feedback. There might be a misunderstanding.

The vapor retarder mentioned is a Pro Clima Intello Plus, a climate membrane. I deliberately chose this one after consultation because it is supposed to be very effective and flexible, and it also cost significantly more than a standard retarder or foil.

Are you referring to something else when you say membrane, or are we talking about a climate membrane like the one we have?

No, that’s what I meant. I’ve reached the limit of my layman’s knowledge, especially since the OSB boards provide bracing.

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

That is exactly the question, but unfortunately not easy to define clearly. One thing seems certain: no water is coming from above, as the attic is bone dry and has been checked multiple times, and it doesn’t appear that any moisture is seeping in. Additionally, moisture would be easily visible on an OSB board if it had traveled from top to bottom through the panel.

The possible causes are:

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

If holes or slits are made in the OSB without compromising structural stability, the moisture should be able to escape upwards. However, whether it then settles on the roofing underlay membrane and drips down again (which would also be problematic) is the next question.