ᐅ New Floor Construction: What Would You Recommend?

In the hallway of the attic in our old building, I noticed that the floor is sagging and no longer level. I checked underneath. The floor structure consists of 20mm (about ¾ inch) polystyrene insulation (now more like 17mm (⅔ inch)) and 20mm (¾ inch) chipboard. Some of the chipboards were apparently installed without tongue and groove joints, so over the years they have pressed into the polystyrene. There is carpet laid directly on top.

The question now is how to level the floor again. I assume that the old structure needs to be removed since it is no longer usable. The new structure should be roughly the same thickness as the old one, around 40mm (1 ½ inches). I was thinking about using 20mm (¾ inch) polystyrene with 18mm (¾ inch) or 22mm (⅞ inch) OSB boards (of course with tongue and groove). Does this make sense? Would 18mm (¾ inch) OSB be enough, or would it definitely need to be 22mm (⅞ inch)? Or am I completely off track, and is there a better solution?

For stability reasons, use 22mm (7/8 inch) tongue and groove panels. Check with your building materials supplier about dry screed. These panels also have tongue and groove edges and insulation directly underneath (so everything is combined in one panel). They are screwed and glued together.

Hello questioner,
An adequate answer to your question can really only be given if the overall construction and context are known. You described them well, but there are aspects we do not know here in the forum.
The old polystyrene, for example, was probably a makeshift solution—for instance, to compensate for unevenness in the subfloor (which is not allowed through polystyrene). As impact sound insulation, polystyrene is generally unsuitable when used with wood chipboard panels.
The new assembly depends on the existing subfloor.
It is likely that height adjustments are needed in several areas, which should be done using a dry fill. On top of this, a separation layer, such as loosely laid MDF panels, and then gypsum fiber, OSB, or wood chipboard panels of sufficient thickness, tongue and groove bonded, should be installed.
Depending on the use (heavy point loads or active foot traffic), it may be necessary to lay the top layer double, rotated 90° and glued together.
These are all details we do not have information on.
But from the basic structure, you should now have an idea of how it could proceed.
Good luck and enjoyment with your renovation:
KlaRa

Why insulate inside the house? What kind of ceiling is that? For wooden beam ceilings, the thickness of the OSB board depends on the spacing of the beams. Then, use Styrofoam or something similarly pressure-resistant.

@ "Land register":
At no point in the entire post was thermal insulation mentioned. So why refer to it now?
Furthermore, if polystyrene was chosen as a layer for the old wood chipboards, then this certainly does not involve (historical) installation on sleepers. Therefore, spacing between supports does not need to be considered.
It is also contradictory to first ask, "why thermal insulation inside the house?"—a legitimate question—and then ultimately recommend "Styrodur or something similarly pressure-resistant."
Styrodur is a thermal insulation material. Thermal insulation is not required between floor ceilings.
And "something pressure-resistant" is not suitable as a height adjustment.
Replies should therefore include a valid, logically consistent conclusion based on the construction conditions and the corresponding requirements.
Otherwise, they are not useful for the questioner, who is seeking help in this forum.
Regards, KlaRa

What kind of surface do you want for the floor?
You want the same level as before – so the new surface needs to be taken into account – unless you want to keep the carpet.