Hi,
I'm looking to set up our side room as a nursery and the current timber subfloor has gaps between the planks. I was thinking about laying some plywood over the top to cover the gaps/stop the draft coming through and then adding a vinyl roll over the top. Likely adding a vapor barrier in between as well.
The current planks are 20mm. Should/can I use 7mm or 9mm plywood with these:
https://www.bunnings.com.au/paslode-25-x-2-0mm-500g-bright-steel-annular-underlay-nails-650-pack_p2360396
Or does Bunnings sell 20mm underlay nails like this if I wanted to use 7mm plywood? I wanted to avoid going any thicker with the ply as that would mean the flooring will be higher than the rest of the house which is not ideal.
timber floors
Will I need to fill these holes with anything particular?

Any suggestions and recommendations would be appreciated.
Thanks!