Floor structure and substructure in the attic with filling?

Hi KlaRa,

thank you for the prompt reply. I am also happy to wait until next week for my subsequent statement

A few more points:

1:
2x OSB (if yes, which thickness?) vs. 2x18mm gypsum fiberboard BRIO 18:
--> Which of the two would you personally prefer? OSB is, for example, significantly cheaper, so what advantage would BRIO have?

2:
I would definitely like to use the 10mm softwood fiberboard as the first layer on the joists (since fleece strips with a buildup of 30mm would increase my total buildup by 20mm, causing a step).
In your opinion, would I then have to nail/screw this softwood fiberboard directly to the joists instead of laying it floating? Would it still be ensured that the board can "work" (i.e. expand and contract over the years without any disadvantage to the floor)?

3.
Regarding your second suggestion:
(Quote)
"Nail fleece strips to the joist layer,
place a layer of OSB or BRIO on top,
then apply the REGUPOL mat floating on that,
then again the second layer of BRIO or OSB, glued tongue and groove."
- Could I use the 10mm softwood fiberboard instead of the fleece strips?
- The first layer of OSB or BRIO laid floating?
- What is the thickness of the REGUPOL mat?
- Is the second BRIO or OSB also laid floating (tongue and groove) on the REGUPOL mat?

Regarding 1) Lower OSB board 15mm, upper board 22 mm thick. Advantages BRIO: none. Regarding 2) Wood fiberboard crumbles quickly under load: not a good idea! Suggestion: "Knauf Insulation." Cut into strips and nail it hidden on the support beams. They are available in thicknesses of 15mm, 20mm, and 25mm. 20mm would be recommended. To avoid height problems, now lay the two OSB boards directly on top of each other with offset board edges, floating on the insulation strips, but screwed together. Tongue and groove should be well glued with D3 glue! Regarding 3) Not applicable with the above procedure. If necessary, also 2 pieces of 15mm boards (instead of 15+22) can be used. Height saving: 7mm. Regards: KlaRa

1)
What, then, speaks against OSB and for, for example, Knauf BRIO? After all, OSB boards are significantly lighter and less than half the price of Knauf BRIO.

2.1)
Regarding the crumbling of the Knauf BRIO board.
In the Knauf video, exactly this is recommended for such purposes (see from minute 1:35)
[MEDIA=YouTube]-fOyo44p6uc[/MEDIA]

2.2)
If, for example, I were to nail "Knauf Insulation" in 20mm strip-shaped only onto the joists, then my height on the joists would be 20mm higher than between the individual joists where the mineral wool ends at joist height. In other words, I have a small hollow space there if the boards (BRIO or OSB) are laid floating on top. Isn’t that suboptimal, since the boards might possibly bend slightly there over time under heavy load?

Hello Markus.
Regarding 1) I had already responded.
Regarding 2): the technical layperson can only infer from product advertisements. For that, one must have understanding.
Experts in the field, on the other hand, must also consider the background information, which inevitably remains inaccessible to the layperson.
The soft fiberboards are load-bearing across their surface, as shown in the video.
However, when laid as strips above wooden beams, they will fail due to the high point loads. Aside from the fact that the impact sound insulation will then only be marginal.
We need a springy underlay to achieve sufficient decoupling. We cannot achieve that with the soft fiberboards on load-bearing beams!
Therefore, my variant represents the best possible result in terms of impact sound insulation in timber construction.
The filling of the compartments will logically only take place once the substructure of the raised floor has been completed.
Then there are no cavities. Even if smaller cavities exist in the compartments, the negative impact is marginal. The resulting resonance space is negligible!
As a metaphor:
Regarding the maximum permissible total weight of your car, it is irrelevant whether the passengers also wear a hat.
It is similar with cavities...
In public forums, we can only give generally valid advice.
I am currently working on and supervising specific projects in Frankfurt and Berlin, where the load-bearing capacity of all floor slabs is only limited, but still must carry a high area load of up to 5 kN/m2.
I must check these situations on site, as I am responsible for my construction specifications.
With local conditions unknown to me, which were only reported to me, this is of course not possible. In this respect, the responsibility lies solely with the executing party! Thus, it lies with you.
Here you also recognize the great danger if every supposed "expert" provides their non-binding (and often not technically substantiated) recommendations.
What is often overlooked:
Anyone who undertakes a trade must not only know the technical regulations but also comply with them.
Accordingly, every builder who acts as a craftsman later bears the risk of success and failure. Installation videos can be quite useful, but they do not replace the necessary background knowledge.
Regards, KlaRa

Hi KlaRa,

Regarding 1:
It is unfortunately not clear to me yet. You said there are no disadvantages of OSB compared to Knauf BRIO. If that were the case, everyone would just buy OSB since it is lighter and significantly cheaper. So why does someone still buy Knauf BRIO, for example?

Regarding 2:
We will definitely lay mineral wool between the joists instead of using loose fill.

Did I understand you correctly that I should not lay the wood fiberboards in strips, but I can lay them as boards floating over the entire surface (including on the mineral wool between the joists)? And then put my precast screed on top of that?

So wood fiberboards are not recommended only in strips in this case, but they are recommended as boards over the entire surface?

Regards,
Markus

Post deleted, initial situation overlooked.