Bright lines on fresh interior plaster - future cracks?

Plaster cracks are completely normal. When a reinforcing mesh is incorporated, it reduces the tension and you have fewer cracks. This also has nothing to do with drying too quickly. The cracks are part of it.

Cracking above the window is the classic case. Therefore, at least a plaster mesh should be applied around the windows. If only because of the different materials. Lintel, masonry, etc.

Question. Was a reinforcing mesh incorporated?

Unfortunately, with today's construction methods, natural drying no longer takes place. As a result, mold develops accordingly. Too much residual moisture in the house. And gypsum plaster is particularly prone to molding here. It practically attracts it. Where else should the moisture go in hermetically insulated houses? A natural indoor climate is mostly nowhere to be found in modern houses.

As the pictures look, everything is still quite damp. And the screed was applied immediately as well. There are 2–3000 liters of water in the house! It just takes time.

If we get a few days of cold east wind now, then open all windows and ventilate throughout day and night. The cold wind removes the moisture. In summer, you can’t dry something like this. The humidity is simply too high.

Plaster cracks are completely normal. When a mesh fabric is integrated, it relieves the tension and results in fewer cracks. This also has nothing to do with drying too quickly. The cracks belong to it.

Cracking above the window is the classic case. Therefore, at least around the windows, a plaster mesh should be incorporated. Alone because of the different materials. Lintel, masonry, etc.

Question. Was a mesh fabric integrated?

Unfortunately, with today's construction methods, natural drying is no longer done. As a result, there is corresponding mold. Too much residual moisture in the house. And especially gypsum plaster is predestined to mold here. It practically attracts it. Where else should the moisture go in hermetically insulated houses? A natural indoor climate is usually hard to find in modern houses.

As the pictures look, everything is still quite damp. And the screed was laid immediately as well. There are 2-3000 liters of water in the house! That just takes time.

If we now get a few days of cold east wind, then open all windows and ventilate. Day and night. The cold wind pulls the moisture out. In summer, you can't dry something like this. Humidity is simply too high.

Yes, I also see this very critically. And all this before we can start heating. The mesh fabrics you mentioned unfortunately were not installed...

If you cannot heat, unfortunately airing out does not help much either. Get dehumidifiers (or have them brought). It costs electricity but protects health. By the way, gypsum is mineral and cannot mold. But unlike lime or silicate, it is not strongly alkaline and therefore does not prevent organic materials adhering to it from molding. Also, if it becomes completely and too long soaked, it can deform. The drywall (Rigips) was obviously installed too early. I would have the drywall removed first and then properly let the building dry. Is the screed already in? If not, put it in first. Then, when the screed program runs and afterwards the covering readiness heating runs, measure the humidity everywhere with hygrometers. If the humidity at 20° before the morning airing is at most 60%, then put the drywall in. What does your expert say?

Reinforcement mesh is usually always embedded in exterior walls. It doesn't matter whether you work with lime cement plaster or with reinforcement plaster in ETICS. Indoors, it's used less often.

Since I plastered everything myself, plaster mesh was also incorporated. 100 sqm costs about 90 euros. That's nothing. Then, of course, nothing will crack.

Nevertheless, it's not necessary. If there are these plaster cracks in the wall, you can stick crack-bridging fleece mesh on and then either paint or finish with a textured paint. Otherwise, just put on fleece wallpaper and that's it. You won't see any cracks anymore.

Therefore, let the plaster dry properly until all the cracks have formed. That can take a few weeks or months.
Then you could fill them with slurry if you want.

All windows open today. We now have a few days of dry and cold east wind. It dries out more than any construction dryer. Free of charge. The nights will be cold with correspondingly low humidity, ideally with frost. It really helps a lot and these will be the last drying nights. Until next winter then.

I find that interesting. We have plastered some rooms ourselves already, and we never had such problems. Neither with gypsum nor with lime-cement plaster, and we also did not install any mesh except over wooden beams. And you think that is due to the lack of heating? We did not have new screed, or if we did, it was dry screed. And we plastered once in summer and once in winter, good ventilation was enough for us and we never had such cracks anywhere.