my garage floor has a bit of slope to it, so i didn't even have to build one in. but you can spray that "good stuff" in floor cracks and slope the floor so that it all runs off to one place where you can put in a drain.
just a tarp changes everything. especially in oregon. you're gonna get rained on all winter long. and autumn. and most of spring.
What is best for sounds isolation?
Out of curiosity what are you using?
https://en.m.wikipedia.org/wiki/Vapor_barrier
From my understanding R-value would be his number one priority. As far as sound goes Roxul Safe n Sound is a great fire retardant sound proofing bat insulation. Most sound proofing solutions have no R-value though.
Look at the acoustic lab data from your NRC in Montreal. If you parse thru the data, the worlds cheapest fiberglass insulation does better in the low frequency bass.
Grow room or a bunker?
that's sweet man