Condensate on the underlay membrane

Hello everyone,

by chance, it was noticed yesterday that condensation is accumulating between the insulation felt and the underlay membrane in our new building.
The insulation setup is as follows: underlay membrane, 22cm insulation felt WLG 035, vapor barrier foil.
All overlaps of the foil are glued, the wall connections were sealed with foil adhesive, and roof penetrations were also sealed with tape.
In November, plastering was done and shortly before Christmas, flow screed was installed.
The underfloor heating has been running for almost 2 weeks now.
The moisture on the underlay membrane has already left initial marks on the roof rafters. How would you proceed further? We have three dehumidifiers running alongside the heating, which are collecting quite a bit of moisture. Unfortunately, the condensation will not disappear on its own quickly due to the current weather conditions...

How did you take the photo? Did you cut open the vapor barrier again?

That already looks like mold.

Had similar crap with our new build 14 months ago - end of the story:

Everything completely removed, dried completely in winter with >30kW heating output, beams treated preventively with H2O2 (already had blue stain like yours; if it were only blue stain, it's still not critical for health; but who wants to risk that in a new build), everything rebuilt with new insulation material. Afterwards blower door test.
Costs including construction power for heating and dryer were settled between the general contractor and the drying company.

Everyone involved was not in a good mood...

Doesn't have to be that bad for you. But don't let yourself be fobbed off, and have all areas of the roof checked including wood moisture measurement; do you have the possibility for targeted ventilation of the rafter/insulation area?

Edit: Your drywall ceiling is already completed, which of course makes inspection enormously difficult. And possibly the willingness on the part of the trades to inspect it thoroughly...

Extreme construction/residual moisture and cold outside temperatures tolerate no mistakes - in our case it was a non-optimal adhesion at the joint area of the vapor barrier, a missing layer of insulation up to the ridge beam combined with poorly sealed roof penetrations/empty conduits. Probably little would have happened in summer, but in winter the entire insulation ended up soaked - and I mean that literally! In the end, there were proper puddles between the vapor barrier and the insulation.

At the temperatures and construction stage, it is likely that some condensation can always occur between the underlay membrane and insulation (e.g., due to residual moisture in the masonry/ring beam in the area between the vapor barrier and the underlay membrane) and would probably dry out again to some extent in summer.

The blue discoloration is of course not a good sign; how does the insulation feel? Only surface damp or already heavy/saturated?

How did you take the photo? Did you cut the vapor barrier again?

At these temperatures and construction status, some condensation can probably always occur between the underlayment membrane and the insulation (for example due to residual moisture in the masonry/ring beam in the area between vapor barrier and underlayment membrane) and would probably dry out again to some extent in summer. The blue discoloration is of course not a good sign; how does the insulation feel? Only surface damp or already heavy/saturated?

I only came across the issue by chance. Since condensation was transported at the point of the roof rafter bracket via the antenna cables, I opened the vapor barrier foil at that spot. There I noticed that the top layer (about 5-10% of the insulation) is wet. I could only take the photo in this area because I pushed part of the insulation aside. As I became skeptical, I opened the foil at several places and found the same. I also noticed moisture directly behind the vapor barrier foil in some places, which is probably due to the fact that at those spots the respective clamp wool pieces do not lie tightly together, but rather a gap lets water through.