Greg wrote:
I think you misunderstand what I asked or have never used the stuff. It
will stick to the inner cavity, trust me. Any excess that expands
outward will be trimmed off before the drywall is installed and after
expansion has stopped. I already have it, so why should I spend money
on fiberglass and vapor barrier if this will work? I just want to know
if it will work or if there is something about it in this application
that I don't know. Part of the appeal is that this space is close to a
street corner and the noise abatement will be a plus.

Yeah, the low expansion stuff won't expand nearly as much as the
original flavor; this is what it's made for, more or less.
The only negative I can think of is that IIRC, polyurethane breaks down
to cyanide gas in a fire; but if it's behind a flame barrier like
drywall, that's probably much less of a problem than the polyurethane
foam inside all the furniture.