ᐅ Waterproofing of Bathroom Floor Joint Sealant

The house is old, but the attic is new ... and a bathroom will be installed there.

The 6.5 cm (2.6 inches) thick screed lies on top of a polystyrene impact sound insulation with a membrane that was also used to secure the underfloor heating pipes.
Here you can see a spot where the screed is not yet complete (a level-access shower will be installed there).
The green plastic foam strip will be removed, and the floor – extending up the walls – will be covered with a waterproof membrane so that any moisture penetrating through the tiles cannot seep INTO the floor.
So far, everything is as it would normally be.

Because I learned here on the forum with surprise that there is a swelling paste which significantly expands when exposed to moisture, sealing joints in the process, I think it would be clever to fill the empty space left when the plastic foam strip is removed with such a paste. That way, if the second waterproofing barrier (1. tiles, 2. waterproof membrane) fails, there would still be a third seal that actually becomes tighter when water arrives (a great system).
Any water potentially passing through would then not run under the screed ... and I think that would be great 😎

Strangely, the descriptions of the "Betonit swelling paste" (found under this name online) do not explicitly mention this use, which puzzles me because it seems obvious, and a company would usually promote such an advantage if it could.

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

Best regards,
Wolfgang

ateliersiegel schrieb:

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

No, I can’t. But I wonder why you want to use it in the first place. By the way, the foam strip does not get completely removed. It is only trimmed above the screed. There must also be no connection between the wall and the screed—so you shouldn’t replace the strip with any kind of Betonit paste either.

Do it the way it is normally done. That is also sufficient.

Benutzer200 schrieb:

By the way, the "foam strip" doesn’t get completely removed. It is only cut off above the screed. There must be no connection between the wall and the screed.

The joint is there to allow the screed to expand. The paste is, as far as I understand, elastic.

Why bother at all?
Because tile grout joints eventually become permeable, and I don’t trust that waterproof membrane to last permanently.
Many of the nowadays “standard” methods still have to prove their long-term durability … and knowing myself: in 30 years (I’ll be a hundred :cool 🙂 I don’t expect to be thinking again about whether to renew the bathroom tiles 😉

Normally, sealing tape is applied in the corner (if you mean your membrane – that’s good!). On top of that, a liquid waterproofing membrane is usually applied completely (e.g., from Knauf, which is that blue material). This is generally sufficient.

Regarding the bentonite, I imagine it does not act as a decoupling layer (like the perimeter insulation strip) but rather connects the floating screed to the wall. This would result in impact sound, which is supposed to be prevented by the insulation under the screed, being transmitted through the walls into the lower floor.

I would advise against your idea, as creative as it may seem. There are proven methods (see above) that comply with technical regulations and are therefore sufficient.

ateliersiegel schrieb:

Why at all?
Because grout joints always become permeable over time, and I don’t trust this membrane to remain reliable indefinitely. Many of today’s "standard" methods still need to prove their long-term durability... and knowing myself: in 30 years (I’ll be one hundred then :cool 🙂 I don’t plan to be thinking again about renewing bathroom tiles 😉

Grout joints are permeable from the very beginning, meaning water seeps through.

But if you know better than the professionals anyway, why are you asking? And what are you doing in addition to the perimeter joints, especially in the shower area? Three layers of membrane plus three coats of liquid waterproofing plus two layers of tiles, because you don’t trust anything?

How can you build at all with such mistrust? This is a serious question! Because what is installed today (membranes or liquid waterproofing) has been used as a reliable standard since the last millennium. If a tradesperson follows the current state of the art (and does a proper job), it works, has been tested, and is safe.

Benutzer200 schrieb:

If a tradesperson works according to current technical standards today (and does a proper job), it works, is proven, and safe.

That’s exactly what I’m questioning.
“For the short term” it’s fine; for the long term, there’s no planning.
“Nowadays,” it’s assumed that installations and buildings will be replaced after a certain time anyway. (Which is okay, and it does work.)
Most of our bridges in Germany are made of concrete (like the Pantheon dome), need to be rebuilt, and aren’t even 100 years old.
With flat roofs, they say: they last 15 years, then they need to be replaced.
I’m not distrustful—most of our house was built according to standard criteria—but I try to question current construction methods and I like the idea of building for eternity.

The increased impact sound insulation wouldn’t be a big problem since you can’t really walk around in the bathroom anyway (there’s hardly any space to walk), and below it there are no living spaces, just a stairwell.

The “swelling paste” mentioned earlier was brought up here in the forum. It was emphasized that professional knowledge is needed to apply it.
The idea of a sealing material that swells with moisture made sense to me, and I’m trying to find out whether it would be useful in my bathroom.

The discussion so far is more about whether it’s necessary to be more cautious than “normal.”
That can certainly be questioned.
My question is aimed at whether the paste would technically work.
Probably only someone who has already worked with it can answer that for me.