You know sir, that is a great question that I will admit I haven't looked into. I know that the spray foam insulation is AWESOME for its thermal properties.
EDIT: Found this from
this site:
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8) How about Spray Foam Insulation. I heard that is a great soundproofing product?
Save your money and use cheap (about 3-4 times cheaper) fiberglass or cellulose insulation. Use the extra money on an extra layer of drywall and quality damping compound. Insulation absorbs some sound waves and that's it. The fiberglass will absorb as much if not more than the spray foam. This has been confirmed to us by many builders. As a side note one thing we do hear from contractors and we have noticed it as well during sound control installs that we have done is that contrary to "science soundproofing" mineral wool and cellulose are better at controlling sound than fiberglass. Either way if you are on a budget fiberglass will do the job.
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So it doesn't sound (no pun intended) like it would be baaaad per se, but just that it is too expensive to use solely for soundproofing. The biggest issue I could see with it is that it expands and can fill a wall cavity, and an air gap of a couple of inches (if possible) is actually a good sound insulator. So if a wall was filled with spray foam, then I would think that it would be worse as you are basically "coupling" the inside and the outside of the wall.
Not sure how much control you have either if you could spray and fill only about half of a wall cavity's depth, then I think you would be OK and certainly more air tight.
