Healthy Building - What is important?

With diffusion-tight buildings yes, with diffusion-open buildings you won't notice any stench, even if you grill kilometers of shrimp with garlic indoors.

You just have to believe it. My house (plaster pumice brick) is rather diffusion-open, but it would fail your stench tests.
Ventilation is the essential thing; if that’s right, you can live in a "plastic bag" and not notice anything.
No one gets sick from living in a modern house. Unless, perhaps, they had to work more than was good for them because of it.

Clay plaster helps in that it buffers an increase in moisture caused by temperature changes, weather shifts, internal vapor production, etc., somewhat and thus keeps it lower for a longer time compared to some other plasters. As a result, one has the feeling that the air is not as stuffy. However, this has nothing to do with CO2 or O2. Neither does it have to do with diffusion and breathable walls. If you solve the differential equations, you notice that orders of magnitude are missing to allow the air volume of a breathing person to diffuse through. I better don’t mention a simple self-test that would make you collapse behind the diffusion-open wall due to lack of air... PS: We also build diffusion-open.

Both clay and gypsum plasters have a moisture-regulating effect! The only difference is that clay plaster is used deliberately and therefore very rarely covered with liquid plastic, i.e., dispersion paint (>= 30% acrylic, meaning plastic!). Gypsum plaster, as a standard plaster, is covered that way! And then the wall is basically sealed like with plastic foil as a vapor barrier, no more regulation…

And then it also depends on how thick the moisture-absorbing layer of the wall is. If I have a drywall panel as wall cladding and on top of that several cm of gypsum or clay plaster, the above-mentioned moisture buffer is larger and more noticeable than with just a few mm of plaster on the wall.
I once read that the concept makes sense from a thickness of 20mm.

As a practical example:
Pay attention in a bathroom to whether the wall is tiled up to the ceiling or if drywall is installed above the splash protection layer in a diffusion-open manner.
In this case, after a hot shower or bath, the mirror will not or barely fog up.

As a practical example: Pay attention in a bathroom whether the walls are tiled up to the ceiling or if drywall is installed above the splash protection layer in a diffusion-open manner. In this case, after a hot shower or bath, the mirror will not fog up or hardly fog up.

Certainly not. And especially in the bathroom, absorbent materials are strongly discouraged. Better to air out than to create a basis for mold.

As a practical example: Sometimes in a bathroom you can check whether the walls are tiled up to the ceiling, or if gypsum board is installed diffusion-open above the splash protection level. In this case, after a hot shower or bath, the mirror does not or hardly fog up.

Shit. My bathroom only has tiles in the shower and directly behind the sinks. The rest of the room unfortunately did not get your diffusion-open theories and the mirror also doesn’t know that it’s not supposed to fog up.

In diffusion-open buildings, you won’t notice any stink, even if you grill kilometers of shrimp with garlic indoors.

Can you explain to me how the garlic-fish-stink can actually pass through walls? Or which walls can absorb so much stink that you don’t have to ventilate?

I once read that the concept makes sense starting at 20mm thickness.

You seem to really know your stuff – I once read... I also once read that the Earth is flat. Still not the case. Sorry. But once again. Your diffusion-open and non-stinky constructions don’t exist today. That is physically not possible.