Subfloor Preparation Click Vinyl

Good day. I want to install click vinyl with HDF carrier board of the brand Coretec on an old wooden floorboard subfloor, underneath the floorboards there is clay. What should I consider regarding the vapor barrier and should I use an impact sound insulation despite the HDF carrier board? To additionally even out unevenness? If yes, which one?

Depends on how uneven the old floorboards are.
With leveling compound you get an absolutely even surface.
Everything else "floats" and gives way more or less.
Ever thought about gluing the vinyl?!
Fill the floor with reinforcing mesh, sand roughly beforehand and don't forget to prime. It's extra effort, but really worth it.

Hello Sven.
What is really worth it is: to think!!
According to your information, there is a clay subfloor beneath an old wooden flooring (according to photos).
This means that the relative humidity between the clay and the wooden floorboards is around 100% (no calculations or tables are needed for that).
Currently, the old floorboards absorb the moisture and transport it through their cross-section to the room air. Without anyone noticing it.
Now comes the idea of a click vinyl.
Completely independent of whether you install a vapor diffusion-tight floor covering on a carrier plate here or a sheet material: the water vapor transport from the clay subfloor to the room air is disturbed, hindered by this vapor barrier.
This means that – quite quickly moreover – a moistening of the wooden floorboards will occur throughout their entire cross-section. With the result that the floorboards will rot away within the next few years.
No, not an unnecessary horror scenario, but lived construction practice!
There are now three possible ways to proceed:
a) dismantle the wooden floorboards, level the clay layer and install a sealing layer on top,
b) refurbish or dismantle the old wooden floorboards and replace them with new floorboards, or
c) sand the old floorboards, level with a suitable (!) leveling compound and install a vapor diffusion-open covering.
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Anything else will rather sooner than later lead to a major building damage.
Best regards and good luck:
KlaRa (the expert)

what about a "normal" screed under the click vinyl? Do you also have to pay attention to moisture in the floor there?

@ "bauenMK": If we assume a floating screed installed on an insulation layer and talk about a so-called "click system", at least a PE foil should be installed as a separating layer even in the case of a heterogeneous structure. PVC itself is moisture-resistant and -proof. And if it is not glued to the substrate, we would not need an intermediate layer due to moisture, but as a sliding or separating layer I can only recommend it. Regards: KlaRa

We are building a terraced mid-terrace house from a developer. Throughout the entire house (basement, ground floor, upper floor, and attic), a floating screed consisting of thermal insulation, separation layer, and cement screed will be installed. Construction will start shortly, and after handover, the flooring work will be done by ourselves. We also want to lay click vinyl on the screed.

In the purchase contract / construction description, it states that “leveling the screed with appropriate leveling material and filler is not part of the flooring work. The standard specified installation height of approx. 10mm must be observed.”

What does this specifically mean in the end for the following future scenario? If I ultimately have, for example, a 2 cm difference between or within the individual rooms, do I have to accept that? Is this not taken into account during the screed work?
Regardless, we want a building surveyor to accompany the new construction. Could he also exert more pressure with regard to defect remediation? We would like to avoid the leveling work on the screed...