Floor structure and substructure in the attic with filling?

Hello dear forum members

As part of a modernization, we are currently renovating our attic. The new roof is now on, and now we are moving on to the interior construction, starting with the floor. Below the attic is an inhabited apartment. Various rooms are to be created later in the attic (office, bathroom, living room, etc.).

We have already laid 20cm high beams. Under the beams there is a clay layer from earlier, which we want to keep as it is. (see picture 2)
The next step is now to "fill" the space between the beams so that we can then build the floor structure (OSB, etc.).

1st question:
And this is exactly where we are unsure what we should use to "fill" the space?
- Loose fill?
- Mineral wool?
- Something else?

As mentioned, a height of 20cm (beam height) needs to be filled. The entire attic area is 60 sqm (8 m x 7.50 m) and the distance between the individual beams is 55 cm.
What would you recommend to us here and why?

2nd question:
How thick would you make the floor structure?
We want to screw OSB boards onto the beams, then apply impact sound insulation and then the actual floor covering (e.g. vinyl, parquet, tiles in the bathroom, etc.).

- How thick should the OSB boards be in the above situation? (they have to withstand considerable weight, e.g. a bathtub in the bathroom, etc.)
- How much impact sound insulation?

We are extremely grateful for any answers and recommendations.

Greetings from Würzburg,
Markus

Hello

I have a similar setup in my house. I filled the gaps with loose fill and placed a foil underneath as a filter protection. On top of that, I did not lay OSB boards but dry screed. This does not make any noise when walking. It was installed as a floating floor.
I had to fill about 10cm. In thicker areas, we were advised to insert Styrofoam. It is cheaper than loose fill when calculated by volume.

Regards

Thank you for the response.

Are there any other opinions/recommendations?

Yes, there are other opinions (even so close to Christmas ). 1. I like the relatively low spacing of the supporting beams! 2. A clay filling can certainly be left in the old infill panels. 3. It is a bad idea (nelly190) to put Styrofoam into the infill panels only for cost reasons. Styrofoam does not belong there; instead, either a mineral bulk fill or, for fire protection reasons, either mineral wool or glass wool. The latter only in the case of a completely wooden construction of the building. A mineral bulk fill on filter paper adds mass, which we absolutely need for sound insulation. 4. We must give up the idea that a wooden floor construction can achieve the same impact sound insulation as a solid construction. 5. In addition to the mineral filling of the infill panels, I would lay the impact sound insulation (as far as it deserves its name) on the supporting beams if no build-up height is available; and exactly that can be seen in the photos. Here, correspondingly thick felt underlays should be attached stripwise to the supporting beams (by nailing or gluing) before the at least 19mm thick OSB panels are installed floating and glued tongue-and-groove. 6. It becomes critical when using a bathtub!!! Here, you should definitely contact a carpentry company and have them advise you on where and how the bathtub will be installed. Under no circumstances install OSB, close your eyes, and just do it!! 7. Instead of OSB panels, gypsum fiber boards with tongue-and-groove edges suitable for bathrooms can also be used. That would be my favorite. Take a look under BRIO! 8. No bathroom in a wooden construction without full-surface sealing of the floor!! Also think about the transitions floor/wall and the tight execution at the inner and outer edges with corresponding prefabricated corner formations/molded parts. ---------------------------------------------------------------------------- What you intend to do on your own is a complex matter. One must know what he/she is doing and not only know the technical rules but also apply them. Motto: Whoever installs as a private person is subject to the same testing and duty of care obligations as a professional company. They must know and apply them. Otherwise, it becomes really expensive once furnished and installation-related weak points arise. Correcting these afterwards is not only associated with enormous costs but also always very time-consuming – if it is even still possible. Regards and good luck with the construction project: KlaRa

Regarding 3: Here I will probably use mineral wool. For mineral wool, I would probably take the cheapest one (wl040), since I don't need any special thermal insulation as there will be heated rooms below the ceiling and above.

Regarding 4: I am aware of that. What type of impact sound insulation would you suggest? And in what thickness?

Regarding 5: Only lay the OSB boards floating? Can such a sufficiently secure construction be achieved, or should I not screw the OSB boards for this purpose (of course with tongue and groove + glued)?

New points:

10. Should I use dry screed somewhere in the floor build-up? If yes, where and what purpose/benefit would it have?

11. Do I have the possibility to install underfloor heating at least for the bathroom in my construction? If yes, which type?

I had already given a note on impact sound insulation under point 5. Dry screed, this term technically falls under prefabricated screed elements.. OSB, wood chip or gypsum fiber boards, all fall under this technical term! If the supporting beams are widened by laterally screwed slats (this method is also excellent for compensating unevenness of the supporting beams), then you can also install monolithically on felt underlays. Every screw, every nail connection between prefabricated panels and supporting beams will have an unfavorable effect on structure-borne sound. Underfloor heating in timber construction is also possible. However, it then becomes very complicated and we need a greater build-up height. That would be possible with gypsum fiber boards and milled heating pipe layouts. Special screed with about 6mm thickness and a load-bearing, bend-stable substrate are necessary!! The layman will reach his limits at the latest here!