In the ACI guide I have, ACI is reccomending 15MIL, I can only assume for installation survivability...

Yes, survivability is always the issue with under-slab vapor barrier so thicker is better if economics allow. Again, why not place the vapor barrier above the insulation as the article FBBP provided clearly explains why this is preferred and why vapor barrier is not degraded by staple holes? Does your inspector consider pea gravel to be an acceptable crushed stone for this application? Sounds like another case of a contractor just doing what they have always done and not learning to improve their game.

OK,
Not sure why you would want to poke 10,000 holes in your vapor barrier with staples,
I will leave that for smarter people.
Pea gravel is a good choice, it is considered compacted when placed.
Edges of peas are soft so minimal damage if any to barrier.
It drains well so minimal issues under slab as far as upward water movement unless you are building in wet soil below grade, then there might be water issues.
If that's the case you have bigger issues and might look at a perimeter drain tile and sump. Again pea gravel will assist in water flow.
6 mill under 2" foam is a good choice. Assuming you place the foam tightly there will be minimal cracks to conduct heat to the pea gravel (a non issue).
Fiber mesh works well, you will still want to consider control joints on larger areas. These can be placed with a simple corner screed when concrete is wet or cut in next day.
why the acrylic additive, you will lock water evaporation some what.
As soon as you turn on the heat the slab will dry.
6 sack is fine.
7 1/2 sack is prefered if a 2" or less slab.
Dan

I think the only reason to use pea gravel is if you need to leave a capillary break to pass water, perhaps to a perimeter drain? Other materials definitely pack better...

My engineer originally specced from bottom to top: vapor barrier, 2" pea gravel or sand(capilary break), R10 insulation and concrete with woven wire mesh. The capillary break didn't make much sense to me as there is no sump to feed any collected moisture to anyway, so any that got in there would just hang out below the taped insulation for a very long time. The Cal Portland rep who dropped off my blocks and my local concrete guy took a look at my plans and both zeroed right in on that detail. They both agreed that is a "older" way to do that. The Cal Portland rep was kind enough to send me an ACI reference that gave some examples of current best practice of where the vapor barrier should go. Sent that reference to my engineer and he agreed with it. I think he had added that floor stack spec, because that was the way he had always done it... No feedback good or bad = doing it the same way next time... After the reference and the concrete pro's input we changed our floor makup plan. Soil at my footing depth here is clear sand, so no real drainage issues within the footing. I am now planning on(bottom to top) Taped vapor barrier, insulation(taped) then concrete. the main reason IMO to put insulation on top is to protect the vapor barrier during the pour. I also think with the insulation on top to protect it, a 6 mil barrier will be sufficient...

Yea, I know the math and study supports it, but something just dosn't seem right about punching staples thru a vapor barrier I just got done laying out and sealing/tapeing all the edges on

Pea gravel has its place.
Can be dumped on site by truck or placed with cement truck (as dry gravel).
The later is particularly good if you have access issues, can place pea gravel with the cement shoot.
Pea gravel does not need additional compacting, is considered engineered fill as is.
It is easily raked level on site.
I have used it a lot and found it to be a good to bring up a few inches under slab to level as well as capillary break.
Good to have in the option deck.
Dan

Pea gravel is fine where it is accepted. Pea gravel isn’t as stable or as strong as crushed stone. So your seismic location and soil conditions may play a role in determining its suitability and acceptability.

As the article FBBP provided clearly explained, lots of holes in the vapor barrier will not significantly effect the vapor barrier performance. Of course, some of us place the PEX in the center of the slab so as to get both the maximum emitter heat transfer and the maximum vapor barrier performance.

As far as the article from FBBP, I find it very practical as well as helpful. That being said, it doesn't seem opposed to the use of vapor barrier below the insulation, its against the use of blotter sand between the concrete and vapor barrier. Which makes sense.... lots of moisture in there. Helps with the concrete curing, but its going to be emitting moisture for awhile, causing some serious problems if you top your slab with something that mold/mildew likes.

Per his article "If it rains after the rigid foam is installed but before the concrete is placed, the polyethylene can hold hidden puddles." - this is not applicable if the site is enclosed - no puddles are forming

Ive seen the argument that concrete may weep down, and cause a similar situation as the blotter sand as it will take longer for the trapped moisture to get outta there.... but the water volume is likely much less than sand, and with the foam taped, nothing much should get in there.

As far as pea gravel goes....it drains well. It self compacts. It is not as stable. Crushed rock - more stable.... compacts. However it also is more apt to move and shift over time if not fully compacted. The pea gravel isn't shifting anywhere, although granted it may not be as 'strong' - (here is one would insert the common reference of two kids of concrete - cracked and will crack, yadda yadda)

Now there is arguments over vapor permeability loss if the poly is trampled over stone, staples, etc. Some seem to think its negligible. Well, being careful, using a thicker poly, and taping foam seams covers that.... with the vapor on the bottom of the foam.

Seems to me like pea gravel may be a regional preference as well. Im actually in the healthcare field. You would be surprised at the regional preferences for procedures.... people get good at doing things a certain way and produce similar outcomes. Ive called both local gravel pits, multiple contractors, a few heating/hydronic installers, and a big area builder in my town.... all say pea gravel off the bat, don't even mention rock. Stuff isn't falling down here.

I guess my question was more around why you would request what is best practice and than choose to not follow the best advice and this specific statement in the article:

”According to most building scientists, here’s how the layers should go, from the bottom up: crushed stone; rigid foam; polyethylene; concrete.”

If pea gravel is acceptable and is a preference in your area, you won’t have any inspector surprises or future problems.

The article is very informative and goes into detail of why the layers are recommended as you state.

I am trying to rationalize for myself through education as to what would best serve my own application, hearing pros and cons of differing setups.

Im not arguing that "According to most building scientists, here’s how the layers should go, from the bottom up: crushed stone; rigid foam; polyethylene; concrete.” his reasoning seems very valid.

However, my application has several variables not addressed in the article- access to place the gravel, the fact it is already compacted earth beneath, and an indoor/dry climate.

Seriously though, I do appreciate and value all input from you and the others, and as you know there isn't necessarily a consensus on all aspects of what is 'bad' versus 'well, it doesn't really matter'

Thank you for the explanation. You appear to be very thoughtful and diligent so I was just curious what caused you to deviate from the recommendations. I also wanted to help you avoid any unpleasant surprises. Anyhow, now that I more fully understand your unique situation and thought process, your approach makes perfect sense.