Dry screed installation in the basement of an old building under renovation

Hello everyone,

I want to set up a room in the basement as living space.
1. The raw, uninsulated floor slab will be sealed with a Börner Erika vapor-tight sealing tape and slightly raised at the edges.
2. Then, a dry screed with a max height of about 2 cm will be applied, and on top of that, 22mm or 25mm OSB 3 boards will be laid. (The room is dry (the outside is also newly sealed and the exterior walls are plastered with cement plaster on the inside).
3. Should wood fiberboard panels still be placed on the screed before the OSB?
4. The screed must not be any thicker as there is not much room height available.

Which screed would you recommend for this? I was thinking of Knau Bituperl or possibly the dry leveling screed from Fermacell?

Should an additional vapor barrier be laid on the OSB board? After that comes laminate.

Many thanks in advance!

Hello "chrissi88".
No matter how you choose to set it up: it doesn't matter. The damage pattern is already programmed by the fact that you have a non-insulated, ground-contacting slab and the indoor air humidity will definitely condense within the construction during the cold season.
By laying a foil, you can only determine at which point the water condenses.
Note: for a ground-contacting slab, 10cm of thermal insulation WL035 should be used.
Besides condensation problems, you will get quite cold feet, don't underestimate that.
The more you heat later to compensate, the more moisture the warmed indoor air can absorb and release again in the floor construction.
No, that doesn't work that way!
------------------------------------
Regards: KlaRa

But that is not what I wanted to hear...
but unfortunately I have no other option, since the room also borders other already finished rooms, which were constructed at the end of the 1960s with cast asphalt screed.
So I cannot change heights either because of the ceiling height or because of the adjoining floor.
It worked in the past, so why not today anymore?
The cast asphalt also has an SD value > 1500, just like the boerner erika.
Therefore, similar conditions prevail from the floor slab upwards. And in the old bathroom, the tiles were even glued directly onto the floor slab with speiss....

Hello "chrissi88". I understand well that such feedback does not bring joy. That other floor constructions have worked in the building may be true. I myself only rely on (building) physics and listen to what it whispers to me. I am happy to write it to you: It says that at an assumed room temperature of a cozy 22°C and a relative humidity of 65%, there are pretty exactly 17.1 g of water per m³ of air. We can simply disregard that, or listen further! Because physics says that with earth-contacting components, i.e. floor slabs, whether sealed with "Börner Erika", "Katja" or something else, we can expect a floor temperature of about 8°C in the winter months. I already pointed out the "cold feet" at the beginning. Building physics further says that at these temperatures, the saturation limit for air is 6.65 g of water per m³. If we now calculate the difference in the amount of water in the air (namely 17.1 g), then about 10.5 g of water per m³ of your room volume will condense on your beautiful laminate floor. That is exactly what I wanted to hint at in my first post on this topic. Usually, this situation is addressed by shifting the condensation point below the sealing layer through construction type and material thicknesses (especially of the thermal insulation). If that is not possible, well, there are only two options: accept that it will get wet inside the room or use it for other (secondary) purposes. In many areas, we can "trick" building physics with modern building materials. But there are limits we must accept! In any case, you are free to build as you see fit. However, it will be little help to listen to those voices who have become supposed experts through self-teaching according to the motto "I once had ..." Any damage that may occur you will have to bear alone, without expressions of sympathy from public forums, and the self-proclaimed experts will also keep a discreet distance then. -------------------------------------------- Good luck: KlaRa

Thank you very much for your detailed answer!
Does it make a difference compared to the other rooms, or should I expect something similar? The poured asphalt has poor heat/cold transfer compared to cementitious floors.
Therefore, I also wanted to use OSB, as it is warmer for the feet.
Am I shifting the condensation into the OSB board or the loose fill, or is it on the surface of the finished floor?
By the way, the room is supposed to become a bedroom.
Is there a way to somehow minimize this with other building materials without having to accept high insulation thicknesses?

Best regards, Chrissi1988

Alternatively, I also came across this board
LIN LINITHERM PGF
20(+18)mm, 1235x635mm, 024
possibly also in 30mm insulation thickness, but preferably 20mm