ᐅ Proper Ventilation and Drying of Interior Plaster / Drying Time

Hello everyone,
are there any physicists or experts familiar with physics here?
I have questions regarding the drying of interior plaster.

In our new build, the interior plaster was completed the day before yesterday, and we are now considering how to remove the moisture from the house or walls as effectively as possible without causing damage, especially in the basement. The basement was also underwater by about 25 cm (10 inches) roughly two months ago. (Our house is built on a slope, so the basement is half underground, and the other half extends above the ground level by about half its height. On the basement side, there are four standard windows above ground level.)
According to various guides, some construction workers, and a drying company, in summer you should only ventilate at night when it’s at least 5°C (9°F) cooler outside than inside, and NEVER during the hot daytime hours. This makes sense from a scientific standpoint based on the water vapor saturation curve. Now, our question is: does this advice apply only to finished houses, or also to fresh interior plaster to help it dry quickly and effectively? We have read in other forums that, after plastering, continuous ventilation during the day is recommended, and that it should not be ventilated at night.

We want to avoid any mistakes in the basement in particular, to prevent mold growth!

Currently, inside the house during the day, we have about 82% relative humidity at 22–23°C (72–73°F). In the basement, it’s about 90% relative humidity at 18–19°C (64–66°F).
When we ventilate at night, the temperature drops to approximately 17–18°C (63–64°F) with about 70% relative humidity.

Another question is: how long should we wait after plastering before installing the floor insulation? Could any remaining moisture from the plastered wall migrate into the insulation and cause damage?

Thank you very much in advance for your answers!

Best regards,
hausi2016

For the next 4 to 6 to 8 to 10 weeks, you won’t be able to do much. Your house will currently resemble a dripstone cave, and the current weather is not helping the situation. Your target will be to reduce the indoor humidity to below 60% at 20°C (68°F). But keep in mind, eventually the screed will be laid, which will add several hundred liters of water into the building.

First: What does the interior plasterer say about ventilation? What does the architect or site manager say?

And then, from what I understand as a layperson: It doesn’t make sense to ventilate continuously if the outside climate is not significantly better. Use shock ventilation (short bursts) instead of continuous ventilation. And if it dries too fast, that’s not good either, because cracks may form. The latter probably won’t happen at the moment unless technical measures are applied.

Thank you for your quick response!

What about the underfloor insulation? Can it be installed now, or is it better to wait until the plaster is less damp?
When is it recommended to do this?

Thank you!

He doesn’t really mind because it doesn’t absorb any water.

But given the current situation, I would definitely use technical drying and try to ventilate mostly at night, which is usually a bit more difficult to organize.

So, run dehumidifiers during the day, turn them off at night, and ventilate then.

Yes, we will just go ahead and do it like this. It will probably be fine.

We are just a bit worried that due to the remaining construction moisture, mold could develop in the basement.

Thanks for the tips!

Mold can only grow on organic materials.

I would not intervene technically after 3–4 days, but rather spend about a week ventilating properly first. Drying too quickly is also not ideal.