Bathroom floor joint waterproofing protection

The house is old, the attic is new ... and a bathroom will go there.

The 6.5 cm thick screed lies on a styrofoam impact sound insulation with a foil that also served to fix the underfloor heating pipes.
Here you can see a spot where the screed is not yet complete (a level-floor shower will go in there).
The green plastic foam strip will be removed, the floor - up to the walls - will be covered with a sealing foil on the walls so that any moisture penetrating through the tiles cannot run INTO the floor.
So far, everything is as "normal."

Because I have learned here in the forum with amazement that there is a swelling paste that significantly increases its volume when exposed to moisture and thus seals joints, I think it would be clever to fill the empty space that is created when the plastic foam strip is removed with such a paste so that, if the second sealing protection (1st tiles, 2nd sealing foil) should fail, there is still a third seal that even - if water comes - becomes tighter than when applied (great system).
Any water that might penetrate would thus not run under the screed ... and I would find that cool :cool:

Strangely, the descriptions of the "Betonit swelling paste" (found under this name on the internet) do not directly mention this purpose and I wonder about that because it seems so obvious to me and if a company can advertise anything, they normally do.

Is there anyone here who knows "Betonit swelling paste" and can confirm or refute my theory?

Would be happy about that:
Wolfgang

Is there anyone here who knows "Betonit Quellpaste" and can confirm or refute my theory?

No, I cannot. But I do wonder why you want to use it at all. By the way, the "foam strip" does not completely disappear. It is only cut off above the screed. There must also be no connection between the wall and the screed – so the strip cannot be replaced with any Betonit paste either. Do it the way it is normally done. And that is also sufficient.

By the way, the "foam strip" does not completely disappear. It is only cut off above the screed. There must also be no connection between the wall and the screed

The joint exists so that the screed can expand. The paste is - as far as I understand - elastic.

Why at all?
Because tile joints always become permeable at some point and I don't trust this foil permanently.
Many of today's "normal" methods still have to prove their long-term durability ... and I know myself: in 30 years (I'll be a hundred then :cool:) I probably won’t be thinking about whether I should replace the bathroom tiles again ;)

Normally, sealing tape goes into the corner (if you mean that with your foil - good!) Usually, liquid plastic is then completely applied over it (e.g. from Knauf, it's that blue stuff). That is normally sufficient. With the bentonite, I could imagine that it does not act as a decoupler (like the edge insulation strip) but couples the floating screed to the wall. This would mean that the impact sound, which is supposed to be prevented by the insulation under the screed, would still pass through the walls to the lower floor. I would advise against your idea, as creative as it may seem. There are proven methods (see above) that also comply with technical regulations and are therefore sufficient.

Why at all? Because tile joints always become permeable eventually and I don't trust that foil permanently. Many of today's "normal" methods still have to prove their long-term durability ... and I know myself: in 30 years (then I'll be a hundred :cool:) I won't be thinking again about whether I should renew the bathroom tiles ;)

Tile joints are leaky from the start, or water seeps through there. But if you know better than the professionals anyway, why do you ask? And what are you doing, in addition to the perimeter joints, especially in the area of the shower? Three layers of foil and then three coats plus two layers of tiles because you don't trust anything? How can you build at all with your mistrust? This is a serious question! Because what is installed (foil or sealing coat) has been used as a functioning standard since the last millennium. If a craftsman today does something according to the current state of the art (and does it properly), it works and is proven and safe.

If a craftsman works according to the current state of the art today (and does it properly), it works and is proven and safe.

That is exactly what I am questioning. "For the short term" it works, for the long term it is not planned. "In our times" it is assumed that installations/buildings will be renewed anyway after a certain period of time. (which is okay. It works as well.) Our bridges in Germany are mostly made of concrete (like the dome in the Pantheon), need to be renewed, and are not even 100 years old. For flat roofs it is said: they last 15 years, then they have to be re-covered. I do not distrust – most of our house was built according to usual criteria – but I try to question current building methods and I like the idea of building for eternity. The increased impact sound would not be dramatic, because you cannot really walk around much in that bathroom anyway (there is no space to walk) and there are no living rooms underneath, but a staircase. The above-mentioned "swelling paste" was mentioned here in the forum. It was emphasized that expert knowledge is needed to apply it. The idea of a moisture-swelling seal made sense to me and I am trying to find out whether that would make sense in my bathroom. The discussion so far is more about whether it is necessary to be safer than "normal". That can certainly be questioned. My question aims at whether the paste would technically work. Probably only someone who has already worked with it can answer that for me.