Neither open nor closed cell spray foams have much density. Hence, not much soundproofing value. I like spray foam but never use it around pipes for soundproofing. It's an awful mess to clear away if a leak develops. Mass-loaded vinyl wraps and other speciality products are effective, but likely no cheaper than cast.

Just How Much Quieter is Cast Iron than Plastic?

Best inherent sound insulating value is cast iron because of the dampening mass of the metal pipe. If I was concerned and the sound was important, I'd do cast iron.

Good luck on your project.

Or, as I was informed, a double layer of drywall. But my toilets don't go in until next week.

Is that true about the spray foam and its lack of density? When we did some walls (closed cell), the soundproofing effect in those rooms was pretty huge. But I can see the point about not wanting to irreversibly encapsulate plumbing pipes.

This unsourced posting on spray foam expert Mason Knowle's forum quotes a STC 38-42 for four inches of open cell foam; Knowles doesn't dispute it. As you can see, there are other sound deadening alternatives, such as doubling drywall on green glue and cellulose.

BTW, depending on quality, cast iron drains can last from 100-300 years.

This post was edited by worthy on Tue, Apr 9, 13 at 11:03

Spray foam is more effective at closing up passages for sound to 'leak' through.

Its very rigidity works against it in many cases though for stopping conducted sound transmission.

Isolating the pipe with flexible material from structural contact is a PITA of detail work, and a single error allowing hard contact can easily defeat all the other work.

A pipe touching a stud that then has drywall fastened to it is a recipe for spreading the sound.

Since most of the noise is relatively high frequency fiberglass often works well along with using soft materials between pipes and mounting brackets for isolation.

Using cast iron still remains the 'golden bullet' for the most part.

Its very high mass compared to the water means that it simply does not even create much vibration when the water moves through it.

Avoiding wet walls in the 'public rooms' on the first floor is often the lest expensive method.

Run the stacks in places that already are relatively noisy like a kitchen and not in the living room, dining room, or library walls.

Thanks all - we went with CI stacks and drops, and our plumber wasn't thrilled with the work, but enjoyed the upcharge, and ensured us that we'd be happier. I'll let you know.

stagger the studs also - that helps a lot.