there's going to be insulation under the slab? interesting. not sure I know or understand what's the purpose is.
Well...the purpose is to minimize heat loss with the slab.
With a slab, in the winter, the cold from the outer wall and ground will come up and make the slab cold, so you end up heating the room that much more.
It may not be common in warmer areas when building, but it's apparently the norm (not sure how many years now) in colder winter climates.
Up here, there's a need to put the footings 4' down to avoid the frost depth during the more severe period of the winter. So with a couple of feet of frozen ground around your slab...the cold will come in.
TBH, I wasn't even thinking about it, and at first I thought they were just going to place foam against the cement wall, which they did...but then I realized it was going to be under the whole slab...at least, I'm expecting it to be. They just got started today, so we'll see tomorrow.
It has nothing to do with soundproofing insulation...strictly for heat insulation.
When my floor goes in...we're putting another layer of poly on the top of the cement slab, and then building a 2'x4' sub-floor frame, on the flat...and there will be additional insulation placed in the cavities of the frame...then 3/4" plywood...then the hardwood floor.
So in the end, my floor will be well insulated...and with the room-within-room build, it will be a pretty efficient room AFA heating and cooling. I actually don't think it will take much to heat it up and then keep it warm considering all the layers and double room build. I think once warmed up, my audio gear and lighting will probably maintain the heat even in winter.
Cooling will also be much more efficient, since the outside heat will not come in as much...though with the gear going, a studio usually needs a bit more cooling than your typical living room/bedroom, all other things being equal...but the extra insulation makes a big difference.