Screed on underfloor heating -> Is wall plaster necessary before screed?

Hello everyone,

I am renovating an old house (built around 1900) and want to install underfloor heating on the ground floor. The old floor has already been removed and a new floor slab has been poured, the electrical wiring and pipes are already in place, and the next step is the underfloor heating.

Now we cannot plaster directly because, for various reasons, there are delays. Therefore, my craftsmen are currently in a dispute about whether the screed can be laid directly on the underfloor heating or not, because apparently the walls should be plastered first. I now have to make this decision, whether the interior walls should be plastered first, but I am unsure.

The screed would border an old brick wall, which is also not completely even. The house stands on a plinth, so everything seems to be dry from below. The floor will be tiled afterwards.

I am aware that there will be a mess when we plaster later. I would try to cover the floor then. But in your opinion, is there anything significant speaking against laying the screed directly against the "open" wall and plastering later?

Kind regards Tom

The typical procedure is first to plaster the wall, then apply the edge insulation strip around, and finally the screed. The wall and the screed must not touch each other for sound reasons. If you plaster afterwards, you must not plaster down to the screed, but a gap must remain. You will have the least fun if you have to remove the plaster residues from the screed again.

I don't think you can cover it that well... especially if pipes have already been laid.

And the underfloor heating is relatively sensitive... I wouldn't let anyone walk over it!

Depending on the exterior wall construction, the interior plaster is the airtight layer of the wall. Therefore, plastering is first done up to the floor slab to ensure airtightness.

Depending on the exterior wall structure, the interior plaster is the airtight layer of the wall.
Therefore, plastering up to the floor slab is done first to ensure airtightness.

Here I kindly ask for the provision of the technical regulations in which an interior wall plaster is defined as an airtight layer!!
I am happy to learn!
Regarding the actual question:
Yes, it is indeed a problem if the wall plaster is applied after the screed.
Just do it like this:
Typically, for heated screeds, an edge joint with a thickness of 10mm to the wall plaster is required.
When installing the screed, preferably leave it continuous by placing 20mm thick (double) screed spacing strips of 10mm, which must be fixed against slipping, toward the brick wall. When using wooden strips, these are removed after the screed mortar has hardened and replaced with 10mm thick spacing strips before the wall plaster is applied.
The double-laid spacing strips remain later in the joint chamber; the wall plaster should only be applied up to the top edge of the screed.
To do this, during the application of the wall plaster, you can also lay 5mm thick screed spacing strips or MDF strips on the floor/wall transition (meaning laid on the screed surface for protection, the spacing strips are thereby covered up to the brick wall).
A bit more elaborate, but this will work!
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Regards: KlaRa

I cannot tell you if there is a technical regulation for this. However, it is repeatedly pointed out that, for example, with brick walls, the interior plaster takes over this function if no exterior plaster is applied. For example, articles in the DBZ, ikz, from the Architektenkammer BW and others.