my guess is that it isn't super effective as sound insolation as it isn't particularly dense, probably about the same as using pink R25 or something. It will stop a lot of high frequencies but not much below like... 400-500 Hz? Those numbers are straight from my ass. I'd guess most of your neighbor bleed is low end/mechanically transmitted. maybe the best thing you could do is mass loaded vinyl sandwiched between another layer of drywall, wish is a lot to do and not cheap, and probably wouldn't "fix" the situation.

Yeah rigid foam is going to mechanically transfer vibrations if it's pressing against the inner and outer structures. If it could be only applied to one surface that would be better, but if they're just injecting it into the voids then that's not an option. I would opt for blown cellulose insulation, or see if there is something denser but still squishy that they can blow in. Ground up tires would be cool

Edit: my bad , I didn't realize the difference between open and closed cell foam, and I'm also learning that there's sound absorption vs sound blocking, and those types of foam are respectively better at one vs the other.

Some of this gets at the root of my question, because I’ve read that open-cell foam works quite well for sound deadening but that closed-cell foam doesn’t (too rigid). Apparently neither is great for low-frequency sounds, but (happily) that’s not the concern.

Wood Goblin wrote: Tue Jun 10, 2025 2:21 pm Some of this gets at the root of my question, because I’ve read that open-cell foam works quite well for sound deadening but that closed-cell foam doesn’t (too rigid). Apparently neither is great for low-frequency sounds, but (happily) that’s not the concern.

Totally. Low frequency mitigation takes an industrial and obsessive/expensive effort. Conversation is much easier.

I scored a deal on a box of Pliteq Genie clips on ebay a few years ago, and used them to isolate a room in my garage. Basially you screw them to the frame, clip in hat-track (aka furring track/channel) and then screw drywall to that. So in a 2x4 framed room, I was able to insulate with 6" R21 fiberglass, sealed the seams with acoustic caulk (NOT GREEN GLUE) then taped like normal drywall seams. There was a window, which I replaced with a 1/2" glass coffee-table top that happened to be the same size, with some foam padding strips. I got another 5/16" glass table top I was going to attach to the floating drywall side, but never got around to installing that sheet.

It worked reeeally well. I live on a busy street a block from train tracks (not lite rail) and it dropped the noise floor and cut the rumble significantly. It wasn't complete soundproofing but it was pretty great. Then my wife convinced me to convert the garage into a rental so that work all went away.

I dug around a lot, found a studio design and acoustics forum online that I think doesn't exist anymore, and read a bit in the Master Handbook of Acoustics, and basically, for sound isolation (soundproofing) you want Mass-"spring"-Mass. The "spring" is air or fluffy insulation like typical fiberglass (unfaced), The Mass is your drywall and/or exterior siding/stucco, whatever. You want as much mass as possible on either side and you want the space between the Mass as big as possible. The Genie Clips/floating drywall thing kinda added to it by giving me another 2" between the mass, and decoupling the Drywall from the framing, and I pretend it acts as a low frequency resonant absorber too, which it may, to some negligible degree.

Not sure how spray foam would work instead of the fiberglass, but I had good results with all that^

There are horror stories of bad spray foam jobs— if we are talking about the 2- part polyurethane stuff…spraying into a wall cavity seems like rolling dice. AFAIK there is a maximum thickness of application for cure/outgass.
surrounding yourself with potentially unstable polymer concoctions is, part of life on this planet…don’t add to the problem?
It seals well but is porous and can hold moisture/condensation…i
rigidity may transmit sound rather than damp it
Sealing any gaps behind trim with acoustic sealant or flexible caulk and decoupled mass like has been mentioned is best bet imo