Thank you.

We are planning on putting carpet on the second floor. I don't believe it was required for fire code. We were only told that it helps reduce sound transmission. It is necessary/recommended for such an application (to be used on 2nd floor, under carpet, when it is not required for fire code and no radiant floor heating)?

Gypcrete is common in low-rise condos (4 stories) as it is rather light-weight (which is why it is often referred to as light-weight concrete). It adds bulk/thickness as well as a low-cost way (labour) of 'flattening' subfloors. Yes it *helps with acoustics...but then again so does carpet.

The PAIN IN THE @SS with gypcrete is it is VERY hard to 'fix' at a later date. Let's play the "pretend" game. Let's pretend you want to add a rigid floor (like hardwood) at a later date. Gypcrete has a LONG history of crumbling. Once it crumbles it is SUPER difficult to patch (read: expensive). You can't nail through it/into because it crumbles. You can't DIY it unless you USED to be a flooring installer....etc. If you ever need to deal with the gypcrete at a later date, you will be finding new swear words in the English language. It could get quite exciting (tongue firmly in cheek)!

A well built subfloor system (proper thickness of subfloor; proper joist spacing; proper MATERIALS used like subfloor grade plywood as the substrate; proper ceiling insulation; etc) will create the SAME type of noise abatement as gypcrete without the FUTURE horrors of trying to work around that particular product.

The carpet underpad is the acoustic insulation that is needed. A high-priced underpad with a nicely insulated floor/ceiling construction (it doesn't have to be super-duper insulated...just nicely insulated) and you don't NEED gypcrete.

An upstairs that is 'ready' for hardwood/upgrade is what you are going for (ideally). The carpet over the subfloor is going to get you 10 years of service before you decide whether or not you want a rigid floor at that time (many people want hardwood but can't afford it at the time of build).

Personally I would skip it and pay for the 'upgrade' for the subfloor substrate (which is the higher grade plywood) and be done with it. And then I would check to see how s/he designed the insulation for the floor/ceiling assembly.

Thank you so much for the detailed explanation. It has been most helpful!