Floor on old wooden beam ceiling - how to build up

Hello forum community,

about years ago we completely renovated a single-family house built in 1965 on the ground floor. In this process, the wooden beam ceiling to the attic was already completely opened and filled with 8cm insulation (rock wool) + about 6cm gravel as loose fill between the beams. On the beams, first coconut strips (about 8mm) were laid for decoupling and on top of that 25mm OSB (glued together, floating on the beams/coconut strips). Now the question is how to proceed most sensibly, especially to achieve the best possible impact sound insulation. Vinyl is supposed to be used as flooring 90% of the time.

My idea is as follows:

- 2nd layer of OSB with 12.5 or 15mm, which is screwed to the first layer and glued over the entire surface (half offset in installation). This should definitely keep the joints of the first layer closed and also improve the stiffness of the panel as a whole.
- Then vinyl with separate impact sound insulation laid floating on top.

Is this approach sensible? Currently, you can still feel quite clearly that the 25mm OSB "floats" on the coconut strips and also gives way somewhat depending on the load. Can a click connection of the vinyl withstand this or will it become a problem? Alternatively, the vinyl could be glued, but if it’s not necessary, I would prefer to avoid that.

Thanks for your tips/opinions

Best regards
Michael

We experienced something similar in the area and the OSB boards also floated. I would already screw this layer of OSB down onto the joists, otherwise the one above will float again. The idea with sand or gravel in between was suggested by my construction expert at the time, I still think it's great because it's simple but obviously not state of the art. We used cork granulate in between before, but rock wool also works. The cork strips are also useful when screwing. I had such a floor for 30 years and screwed 28mm solid pine or larch planks directly onto the joists. Simply great! It's not for gap fetishists but I would do it that way immediately. Get inexpensive planks and you can treat them, for example, with colors from OSMO. Just roll it on. We had hard wax oil and later, after changing tastes, applied the decorative wax coating directly without sanding. I thought it was great and you will never wear it out, not even with a dog. Never floating, screw the boards onto the joists.

I am not a structural engineer, but what comes to mind spontaneously: Our house is 35 years older and I have just completed a similar project. Since you live in a single-family house and do not have any tenants, I consider the current setup sufficient in terms of impact sound insulation. Maybe I am too cautious, but one should not overlook the fact that there is already a considerable weight on it. 6 cm of gravel on the surface plus OSB... Then an additional layer of OSB seems too much to me, at least for what you expect. I think you can lay the vinyl directly now.