Soundproofing in between floors

[Chris Fiore]

About a year ago I purchased a large brick 75,000 sqft building from the 1800's that was separated into apartment units. The previous owner just separated the units, threw cheap carpet down, incorrect electrical, drop ceilings, ect. some units weren't even painted. The building has 3 floors and 43 units. It was previously a silk mill before he had purchased it. Since I have taken over ownership, as people move out I re-renovate the units. I do a full gut and remodel including a better layout, new electrical, new plumbing, new lighting, mostly all new sheetrock other than pre-existing walls, laminate in the living room, carpet in bedrooms and tile in the bathrooms, kitchens, and showers. All new kitchen with new cabinets, countertops, SS appliances, backsplash, ect. I take out all of the drop ceilings to expose the 16 ft ceilings with Steel I-beams through the ceilings. My biggest issue is soundproofing the units. I have done 6 units this past year and can't seem to get it right. The first 2 I did I didn't do anything for the sound, not realizing it would be an issue. After I had received feedback from the new tenants I started to double 5/8 the walls with Roxul Safe-n-sound insulation in between. It has helped tremendously. I still get complaints daily from tenants that they can hear the tenants above them talking, walking, music, tv, ect. When I renovate these units, they usually need the floor to be leveled. I use OSB board atop 2x4's that are trimmed to be level. When I started getting complains I started to stick the Roxul under the OSB board. On top of the OSB I put the floor mentioned above with the correct under layments. Underneath all of that Is the original floors that are 4x6 floorboards. (also the ceiling to the unit below) and you can still hear almost everything between units. Anybody who has been in a similar situation please feel free to chime in with any solutions you have found to help! It would be greatly appreciated. Thanks!!