I'm not actually building a home theater, but in doing a lot of searches on sound insulation products, some of the most useful information I've found is here on the AVS forums. So I was hoping someone here might be able to help answer some questions.

I'm remodeling a second-floor condo before I move in, and it currently has no sound insulation at all in the floor -- it's just a plywood subfloor and normal drywall on the ceiling below. As part of my remodeling, I'd like to fix that, and I'm mostly concerned about airborne noise rather than impact noise. I want to be able to watch movies without forcing my downstairs neighbors to listen to them, too.

I'm planning to install nail-down hardwood flooring, and the existing subfloor isn't especially flat. The recommendation from my contractor is to spread something like DAP Flexible Floor Patch and Leveler to level the floor, then nail down an additional layer of plywood, then nail the flooring to that [1]. The reasons for using a latex-based leveler are so that it won't crack when nails are put through it and because the existing subfloor flexes a decent amount.

So my question is: in this situation, what's the best sound insulation product to use? The main options I'm considering are green glue or some sort of pad, either of which would go in between the two layers of plywood subfloor.

My initial thought was green glue, because this seems like a perfect application for it -- I'd have two layers of plywood subfloor anyway, and it could go in between. However, I'm concerned that the latex-based leveler would reduce the effectiveness of the green glue. It would be a sandwich of plywood <-> latex leveler <-> green glue <-> plywood <-> hardwood floor.

Would green glue still be effective in that situation? Alternately, are there any sound-insulating pads/membranes/underlayments that would work better than green glue?



[1] Actually, his recommendation is to use a floating floor, but there's a nail-down-only floor that I love, and I've searched for weeks and just can't find a floating floor that I like nearly as much.

It all comes down to 'how flat is that floor?'

If you put a new layer of OSB over the existing layer without the leveling compound, would it be unacceptably non-flat? In other words, does the subfloor have local variations (localized bumps and dips on a 1-2' scale) or is it wavy across the whole floor (the middle 1/4" lower than the edges or something).

If it's the former, I'd skip the levelling compound altogether. Green glue should fill the small gaps, and if the two floors are not in contact at all points it wouldn't seem to hurt much.

If it's the latter (large scale height variations), then the question is how 'hard' is the latex stuff when it dries? I think the gg is designed to grip both pieces of wood or drywall, stretch and compress as the two surfaces vibrate, and absorb the vibration by heating the gg.

If that's the case, if one side is connected to the top floor, and the other side is firmly connected to a hard layer of latex that is firmly connected to the bottom floor, you would be fine.

If the latex stuff stays soft, however, it may not let the green glue bridge the gap the way it needs to.

Since this is an unusual situation, I'd consider getting advice directly from the green glue company. The impression I've gotten is that they are good at answering this kind of question, if they know.

It all comes down to 'how flat is that floor?'

If you put a new layer of OSB over the existing layer without the leveling compound, would it be unacceptably non-flat? In other words, does the subfloor have local variations (localized bumps and dips on a 1-2' scale) or is it wavy across the whole floor (the middle 1/4" lower than the edges or something).

If it's the former, I'd skip the levelling compound altogether. Green glue should fill the small gaps, and if the two floors are not in contact at all points it wouldn't seem to hurt much.
It's bumps and dips, but they're wide enough that nailing more plywood down without some kind of leveling wouldn't make the floor sufficiently flat for the requirements of the nail-down floor.

One of the other options besides a leveling compound was to run layers of cardboard strips along the joists and build them up to level, and then nail through that. That would allow green glue to fill in the gaps between the joists, but that would take a lot of green glue, and it's also my understanding that green glue works best when squeezed tightly between two layers, rather than as a gap filler.

If that's the case, if one side is connected to the top floor, and the other side is firmly connected to a hard layer of latex that is firmly connected to the bottom floor, you would be fine.
Unfortunately, I don't know quite how hard the latex stuff gets. Their installation guide says that you can tell it's dry when you can press a coin into it and not make an impression, so it's not *that* soft, but I haven't seen it in person.

Since this is an unusual situation, I'd consider getting advice directly from the green glue company. The impression I've gotten is that they are good at answering this kind of question, if they know.
I did actually call them, and I was told that yes, green glue will still work in my situation, but I couldn't get a good answer about whether or not the performance would be degraded. Unsurprisingly, it sounded like they hadn't tested this particular type of setup, so I was hoping someone here with field experience might have an idea of how it would perform.

One interesting thing that came up in the conversation was "Green Glue Stage 2", which the rep said is a new product they've been selling for, I believe, new construction of multi-family buildings, and they haven't put anything about it up online yet. He ended up deciding that it wouldn't be appropriate for my situation, but now that I've been doing all this research into sound insulation products, I'm curious what it is.

I would not attempt to fill the whole floor with green glue. Waste of money and unlikely to be better than over the latex. Green glue doesn't work that way.

If I were in your spot, I'd probably put the green glue in over the latex, or just go with the latex. Can't really help you more than the gg folks have.