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Sound proofing our floors

7 years ago
I've bought a three unit building (one unit per floor) with family, and we're all moving in together. There will be young kids on the top two floors. We're doing a big renovation. The current flooring is a subfloor of maybe 6" wide planks, covered with hardwood. It does a great job of transmitting noise between floors :-/

The house is brick and block exterior with wooden joists. We are ripping up the floors to the joists on the top two floors, and I've come up with a subfloor design to try to minimize noise between floors, especially footsteps.

First, Roxul between the joists. Then the following layers on top of the joists:
- 3/4" AdvanTech ply
- green glue noiseproofing compound
- 1/4" cement board (probably HardieBacker), screwed down
- green glue noiseproofing compound
- 1/4" cement board, screwed down
- 1/4" rubber underlayment, glued down
- 0.31" CoreTec Plus flooring (this isn't part of the sound proofing... this is just the flooring that was chosen)

This raises the existing floor height by about 1/3".

We are also going to use green glue noiseproofing sealant around the perimeters of the walls.

Here were my primary references for the design:

The NRC article says:
"In such simple joist floors, the most important factor influencing the impact sound attenuation is the total mass of the subfloor and the ceiling layers."

So the cement board is mostly to add mass to the subfloor, and to act as a sandwich for the damping compound. The low frequency noises (footsteps) are reduced mostly through adding mass into the subfloor (if I'm reading the article correctly). The damping compound also helps.

The dense material (cement board) will help transmit higher frequencies - i.e. mostly airborne voices, music, TV, etc. I'm not as worried about these noises, but even so, the recycled rubber underlayment (resilient layer) above the cement board and beneath the flooring should help counteract the higher frequency transmission through the cement board.

Another quote from the NRC article:
"Adding a resilient topping on top of the concrete layer reduces noise levels at high frequencies and counteracts the effect of the concrete there. The combination of the two elements significantly increases the IIC (Figure 4). In other words, a soft floor covering combined with the additional layer of concrete improves impact sound attenuation at all frequencies, providing an IIC of 50 or higher."

Does this seem like a good subfloor design to reduce noise between floors, especially footsteps? Any suggestions?

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