Soundproofing a floor: How about cement backer board w/ Green Glue and mass-loaded vinyl?

Question

I've been researching soundproof underlayments for floors, but the selection seems to be very paltry. It appears that you're pretty much stuck with mass-loaded vinyl (MLV), QuietWood, and a few other sheet-type materials. The bad part is that these are not as effective as materials used for soundproofing walls and are certainly no good at blocking lower frequencies, so I came across an idea: What about MLV on my subfloor, followed by Green Glue (GG) then cement backer board (BB)?

I think this would be a very effective solution. Putting the MLV down first would put a protective layer over the subfloor so I don't get it all gooey with GG. Instead, the GG would go between the MVL and BB. After I seal all the BB seams with accoustical caulk and tape, I would then lay my carpet over that.

If BB is too over-the-top, what if I replaced that with simple 1/4" plywood?

Answer 1

The reason the floor materials aren't as effective is because this problem is best solved with an air-gap and a flexible connection spanning that air-gap. But you have a hard time supporting a floor with that air-gap. Therefore, solving this problem via the ceiling below this floor is better, using resilient or sound attenuation channel there.

That said, on your floor, carpeting over Homasote, QuietWood, or some other material will be as good as you'll get on the floor. Since I don't have experience with MLV or GG, I'll defer to others on that. However, the cement backer board seems like the weak link here since that will transmit sound rather than absorb it.

Answer 2

Answering my own question here with an email response from a Green Glue associate. In response to me basically asking him the exact same thing as in my original question:

This application will certainly work. We would recommend using the heaviest & thickest material that you can...1/4 plywood is going to be a bit too light & thin. If height is what prevents you from using 1/2" plywood or greater, consider using 1/4" cement board (Hard Backer) instead of the plywood. The cement board is heavier and stiffer than plywood and will offer better performance.

What I am going to do first is lay down 2-lb MLV, acoustically caulk it, and tape it. I will then turn up my subwoofer, run down to my garage directly beneath it, and, if the MLV doesn't appear to be performing to my expectations, I will start my plans to put down cement board with Green Glue in between.

Note that the cement board is not for soundproofing in and of itself, but to provide a second firm surface to allow the GG to perform at its best.

I'll post details here in a few weeks to report my progress and also post pics.

Answer 3

I don't know what Green Glue is other than it appears to form a gasket of sorts.

My thoughts would be to try insulation along with layering some dis-similar materials to avoid conductive transfer.

I'd suggest a sandwich of XPS foam, topped with cork underlayment, then your finish floor (which, ideally, could be dry-core subfloor panels topped with carpet).

BUT...note that I am not an acoustical engineer of any sort and the above is just pure speculation. :)

That said, to justify it:

• the XPS, when taped and sealed, would be an additional air barrier, which should help with transference. It also completely docouples your finish floor with the subfloor in terms of any direct fasteners (helps reduce transference)
• the cork is fairly cheap, and does the same as the XPS but as a different material, should help reduce sound transference. It will also help pad the XPS from the drycore
• the Drycore is maybe where this gets a bit over-engineered, but it does provide a form of an air gap underneath.

All that said...you state that the sound is transferring into your neighbors condo which is 'diagonal' from your current room. Two theories there.

1. The sound is coming through the shared wall in your garage. As such, it may be a whole lot easier to soundproof that wall, rather than your floor.
2. The sound is actually transferring through the building structure itself...which could be the case if the shared firewall is stick framed. Ideally, the shared firewall would be masonry but that's not always the case. If that's the situation, your solution might be to invest in headphones.

Answer 4

Green glue is a damping compound that is based on hysteresis principle. (Memory and recovery to the original shape...sort of like a ball bouncing) GG will not work if not applied against any material other than a hard surface material. Soft materials defeat that purpose and do not work. forget about MLV! That only works on high frequencies.Does nothing for below 200 Hz. Forget about screwing or glueing and think Floating floors. Yes you could use minimal amount of screws as long as you cover each screw with a sealant. Even a smallest opening the size of the screw will let the noise back in. Think about your room as if it was a fish tank, and you are trying to stop the water from leaking.