After screed, mold or mildew stains?

Pitch black and really formed within one night.

Nothing happens there within one night at all!

Hm... I still don’t understand the site manager’s statement, but he must know what he’s talking about.

I would still recommend keeping an eye on the situation. You can get a small hygrometer for little money and place it in a (from an air exchange perspective "unfavorable") position just below the ceiling. The goal is to stay permanently below 70% relative humidity. Once doors and furniture are in, at most 60%. Above 90% relative humidity, you always have to expect mold. Wooden components are often affected first. So let your gaze wander there more often as well.

Usually, you then develop a feel for the drying effect and the extent of re-moistening that occurs when the windows are closed. If you feel it’s too humid, then ventilate one time too many, even if the site manager has a different opinion. It definitely won’t hurt.

Maybe the OP is confusing the recommendation for the first few days with a general recommendation? At the beginning, you should not ventilate too much (and definitely not cross-ventilate), so that the screed does not dry too quickly on the surface. This can then lead to curling and/or capillary break (the top layer no longer lets moisture through and the lower layers can no longer dry).

A mold expert is the solution

Hello,

how did it turn out in the end? We had a similar issue. With us, after the screed and yes, even overnight, mold spots appeared under the roof (OSB boards). This was dried over weeks and then hydrogen peroxide was vaporized by the "mold expert." This is supposed to kill the mold. The ceiling was then insulated and sealed with an airtight foil. Then plasterboards were installed. According to all experts, everything is fine. I am still worried though. I find mold a tricky subject.