Soundproofing a Heating Pipe (Noisy Neighbors)

[thollandpe]

Fire stop caulk and foam is only "fire stop" because it contains an intumescent substance that scabs up when exposed to very high temperatures and resists catching on fire. Not the droids you're looking for.

Stuffing the gap with steel wool, fiberglass, or better yet rock wool may be all you need. Finish it off with an escutcheon because that may help keep the expansion and contraction from cleaning out the crap from the gap. At least on your side of the hole. I'd avoid putting spray foam directly on a steam pipe because that will stink. That crap is effective AF, but nasty enough when cold. Use sparingly, or only when you need the trifecta of insulation, air sealing, and water sealing.
The idea of filling the apartment below with foam is a winner. Intombescent?

[smontanaro]

If you decide to insulate the pipe, you can get foam pipe insulation for 4-inch pipes. It's not as cheap as the more common small stuff used to ship bikes, but should be readily available (on Amazon if nowhere else).

[Mark Kelly]

Here's a cheap as chips solution that might work: a yoga mat.

I buy $8 NBR yoga mats from a local discount supermarket and use them to insulate brewing vessels, they seem to have fairly good noise damping properties as well.