A few years ago, we completely renovated the first floor of a house built in 1965. In that process, the wooden joist ceiling to the attic had already been completely stripped and filled with 8 cm of insulation (rock wool) + about 6 cm of gravel as filler between the joists. Coir strips (about 8mm) were placed on the joists first for decoupling, and then 25mm OSB (glued together, floating on the joists/coir strips). Now the question is how to proceed in the most sensible way, especially to properly address the impact sound issue. The floor has to be 90% vinyl.
My idea is as follows:
— A second layer of 12.5 or 15mm thick OSB that is bolted to the first layer and glued over the entire surface (half offset during installation). This will reliably close the joints of the first layer, and the stiffness of the board as a whole will be even better.
— Then the vinyl is laid on top with a separate shock-proof soundproofing.
Does this procedure make sense? Currently, it still clearly feels like the 25mm OSB is "floating" on the coir strips and, depending on the load, giving in a bit. Will this snap vinyl joint hold up or will it become a problem? Alternatively, you could glue the vinyl down, but if you don't need it, I wouldn't do it.
We have experienced something similar and the OSB panels also floated. I would have already screwed that layer of OSB to the joists, otherwise the top layer would float again.
My building expert at the time said to use sand or gravel in between, and I still think that's great because it's simple, but probably not state-of-the-art. We used cork filler in between, but rock wool is fine too. Cork strips also make sense when screwed in.