Filling compound on XPS? Leveling 20-30mm

Hello,

I have a problem with the floor in my "new" house from 1953. The floor is about 20 mm lower than the hallway. Previously, there was 10 mm of styrofoam glued down and a high pile carpet. Below that is an unheated cellar and it was quite warm on the feet! The other rooms have some kind of screed with a wood fiber board(?). I had never seen that before. Therefore, I assume that in my bedroom there is no insulation and screed at all, only a concrete ceiling. The hallway will be tiled over, so it will be a bit higher. 30 mm?

QUESTION: Can I glue 20 mm XPS 300 and pour 10 mm leveling compound? Possibly with reinforcement mesh. Does that work?

Best regards, Olaf

Hello Olaf.
Basically, my advice at this point: thermal insulation on the underside of the basement ceiling! That is where it belongs, namely always on the cold side.
The thermal insulation or thermal insulation panels you mentioned are unsuitable as an underlay for floor coverings.
Therefore, drop the idea of sticking something onto this.
If the floor surface lies 20mm lower after the dismantling of the old structure, this only causes problems at component transitions.
Meaning: at stair landings or room transitions where the floor height may be higher.
Do it like this:
first sand the substrate exposed from old thermal insulation and textile covering (concrete? old screed?) dust-free, and thoroughly vacuum the dust from the surface.
Then prime the entire surface (emulsion is completely sufficient) and fill about 3mm of the visible surface of the substrate. This creates a floor-ready and flatness-compliant surface for later adhesion of a top covering.
At the transitions to other rooms which still have the original condition (i.e., higher), ramp-shaped filling is applied using a stable filler.
An approximate "ramp length" is calculated using the formula
>cm height difference x 0.6 = ramp length in meters<
With a 3cm height difference, this would be approximately 1.8m, which should be filled into the room.
This is a scale that does not present any tripping hazards.
Within the room area itself, it does not matter at all whether the substrate is 3cm higher or lower than in the neighboring room.
But better avoid regarding a thermal insulation panel as a surface used indirectly via a floor covering!
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I would keep the chipboard substrate if it is still load-bearing and shows no bending moments when walked on.
Only the thermal insulation in the basement is important for sufficient floor warmth, for which the next top covering would have to provide.
Good luck: KlaRa

Thank you for the answer. I don't really like the solution, so I will probably pour 3-3.5cm of screed. FE50-Largo. Now a silly question. Since I am pouring screed on concrete (old leveling compound), do I need to put a barrier layer (foil) there? Currently, I have edge strips and a layer of acrylic primer (EcoPrimT PLUS) on it. It feels like rubber! Is that okay?

:
apparently you did not (properly) read my answer.
No one spoke of "pouring" 3-3.5 cm of leveling compound anywhere.
Rather, I spoke of applying a common surface leveling (which not only equalizes the existing screed surface but also provides a secure substrate for subsequent bonding) and filling the joints.
A foil is neither sensible nor possible, because—based on DIN 18560 Part 4—for separating layer screeds in residential construction, a minimum thickness of about 45 mm for cement screeds is necessary.
What is a foil supposed to achieve with a 3-3.5 mm thick layer of leveling compound, other than crumbling on the first walkover?
Edge strips of 5-8 mm thickness must be applied to wall surfaces in unheated screeds for sound insulation reasons.
Conclusion:
Anyone experimenting with any compound without the necessary background knowledge about why and what must be done should not be surprised if success does not follow afterward.
Of course, it is up to you to choose the path. But you should not ignore the warning advice of those who have been professionally dealing with the topic of "flooring technology" for decades .......
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Nevertheless: good luck: KlaRa

ok, but you said "the visible surface of the substrate is filled with about 3mm of putty" and "at the transitions to other rooms... (which are higher) it is ramped with a stable putty compound." I don't want that. Then I am at the final floor in the other rooms at least 3-3.5 cm lower and the floor at the transitions is not vague. Currently, the room only has the reinforced concrete with a solid layer of leveling compound. Therefore, I will now use flowing screed. It starts at 30 mm layer thickness. That is perfect. According to the instructions from Knauf, it works with and without foil. As far as I know, foil is only used if you have thermal and/or impact sound insulation underneath. I don't have that. I am doing this together with my drywall builder, who is a trained bricklayer. Thank you!