back fill converted garage concrete slab with natural methods?

So I'm digging and digging for answers, thoughts, opinions on best possible options to fill in over the top of the standard concrete slab.  It has a drainage slant to it from 4.5 to 2" on a 12x24'.  Now I have experience living off grid and have been obsessed with natural building for a good 20 years but find myself living currently in a very much building code/get a permit area and I'm wanting to put in a RMH in this same space.  My thinking is currently to fill in the floor to 1" below level with a peat/perlite/portland mix and then run pex for radiant floor to encase in cobb that would have the RMH on top of that.

I tried building a wood deck to bring it up to level and then had MHW over the top of that but if it isn't perfect, the boards will shift and just one little bump threw the whole room.  Good thing it did though, as my on demand water heater flooded my floors several times before I found the solution to my incredibly hard water/no pretreatment and tankless woes.  I finally got that figured out despite the plumber offering me a hammer when he couldn't figure it out. (I'll never hire out plumbing again).

I think I can sneak past the code guys about the floor infill by then putting the MHW over the top that runs the length of the house.  

So the questions are:  do I need a vapor barrier between the existing concrete and whatever filler ends up being best?  I'm in Ohio currently and it's moist.

Is there a better insulation/filler I should consider like maybe just straight cobb on the floor would be okay?  I don't think papercrete would be relevant here.  Maybe foamcrete?  It's a room that's been converted to a living room so I'm not overly concerned about compressive strength.  There is no scenario where I'd consider pouring on more concrete even if I am in suburbia.  

I did watch the video with Paul and Uncle Mudd on codes and insurance companies but that's now 3 years ago.  Anyone heard of updated restrictions/codes being adopted?

as for the radiant floor, I'm doing an experiment with passive solar.  I'd like to try to hot water storage tank in the actual garage.  I have good southern exposure and very easy access to this floor through the new garage wall.  We'll see about all of that later.  

Thanks for any feedback I can get.  For now, I'm in permaculture for my day job and I have to get back to process of converting suburban Cincinnati estates into decorative food forests for my clients!

Life is good.
Jen

Hi Jen,

Welcome to Permies.

What us "MHW " please?
It sounds like you are using a garage as a living space.
Yes a vapour barrier is important, to prevent mould growing.What is the issue with permits and codes, I live off grid on an isolated farm, I generally build without permits and build to basic codes because they
are a good guide to functional building.
I dont build 'normal' structures at the same time.

@John C Daley:

MHW is manufactured hardwood, sorry.  As to codes and permits, I live in an area where the local municipality wants to believe we're in a suburb of Cincinnati.  By that, I mean I've gotten code violations for having more than 4, yes 4! ducks, I had pallets propped up in the front yard to keep my dogs in that they decided was a fence, and then made me take down my greenhouse with a bare ground floor for not getting a permit.  They wanted $600 a day for every day of violation.  So I'm not playing with them anymore.  I use my own yard as my plant nursery for my clients and my personal food forest/testing ground.  I don't want to take any chances of popping up on their radar again.

So, I try to live by example whereby any renovations I do to this particular footprint should be sustainable/green in nature if possible.  I'm seeking ideas for how to fill in this floor up to level as subflooring level to be relaid with MHW.  Pretty much all monies from my clients gets reinvested into the gardens and I've already had to have the roof replaced from a tornado that went through. I'd love to do this a cheaply as possible.  

I have found, over the years, that sometimes a solution is right in front of my face logically, but I just take a couple of years to find it.  I'm open to any thoughts even if they don't run straight to a solution.  Thanks in advance
Jen

You have a nice story, tricking the local 'snitches' is a game I love to play.
I take it you have a cold climate and dont want to live on a sloping floor?

Jen said, " I'm wanting to put in a RMH in this same space.  My thinking is currently to fill in the floor to 1" below level with a peat/perlite/portland mix and then run pex for radiant floor to encase in cobb that would have the RMH on top of that.

Since you are wanting to do a RMH, why not incorporate the radient heat floor into part of the RMH?

I know there are some threads though I don't have time to find them now.

The picture in the thread might give you an idea for the natural subfloor:

https://permies.com/t/29781/RMH-autopsy-redesign-modifications#233550

Maybe asking your question in the RMH forum might get more replies if you are wanting a RMH.

Here are some threads that explain about using a RMH:

https://permies.com/t/155232/RMH-Radiant-floor-heat-water

https://permies.com/t/96067/RMH-Floor-Heat

https://permies.com/t/37845/RMH-floor-mass