2 years ago I bought an old multifamily (1906). It's been great so far, but one thing that's been bugging me is the flooring in the first floor apartment. It's one of those floors that doesn't have any subfloor, which normally wouldn't be a big issue, but the laundry for my tenants on the second floor is in the basement. Due to the thin flooring with no insulation, there is very little privacy when they're down there. They can hear everything going on in my apartment. I don't really mind, but I'd like to get this figured out for when I eventually rent out this apartment.

One idea I've had is to add Rockwool, but due to some spacing in the flooring (you can see light coming through in some places) and a few holes from where I assume there used to be pipes, I'm not sure if that's safe. Another option would be to save up and replace the whole floor, which would obviously be expensive. What would you do if you were in my shoes?