The house build i'm starting work on very soon is set to have earthen floors with hydronic heating.
The exact type is not decided yet : maybe just cob or some rammed earth or Tataki.
Maybe some rooms will have one form and others a different type or maybe there will be bottom cob / RE layer and a harder top Tataki layer.
Anyway, my wife is really considering wooden floors.
And as hardwood is very $$$, softwood (spruce, pine, fir) will have to do.
Where the wife might decide she absolutely wants wooden floors, i reckon putting a floating floor of these T&G softwood boards on top of the leveled earthen floor will also make the hydronic part easier.
These boards are less than 4" (10cm) wide and about 3/4" (19mm) thick.
The only issue that could occur is for the floor to not be thoroughly dry which will warp the boards due to humidity entering the wood.
When I started building my house in 1994, hydronic heat here in Maine was not well received yet, so I simply built my concrete slab on grade without pex tubing running through the floor. After a few years went by and I decided in 2005 to add onto my house (double in size) I realized hydronic heat was the answer and installed pex within my concrete. This left my house with half radiant heat, and half without. When we decided to redo tha half of the house that did not have radiant heat, we rectified the situation.
Since I could not put pex inside the slab that was there, I nailed sleepers to the concrete floor and placed 10 inch wide White Pine Flooring (cut from my own wood lot and sawmill) onto the sleepers. Before I did that though, I put loops of pex between the sleepers, then placed sand around the pex...or so that sand is over the concrete, around my pex pipes for heat transfer, but under my wooden softwood floor. I hope that makes sense. Even I did not think it would work as great as it does. Sand is an excellent conductor of heat and allowed me to save a lot of work on mixing and pouring concrete.
So all that is to say my situation is similar to your situation.
Now, you will NOT have any issues with moisture, at least in the winter months because heat from the pex will dry the earth. In fact no matter what you have for a floor system this hold true with hydronic heat. In our home my wife puts delicate fabrics like pantyhose and towels on the floor to dry! And if there is a spill from one of the kids, it is soon dried up, and the same can be said for the area in front of the bath or shower. You do however have to have a thermal break between what your pex tubes runs through, and the ground. In most cases this is insulated foam because otherwise the earth...here in Maine at a constant 57 degrees, will wick away your heat. You need a thermal barrier to stop that or you will have an inefficient system.
In my home it goes like this: 2 inches of rigid Styrofoam Insulation, 6 inches of concrete. 3/4 of an inch of sand to surround the pex pipe, then 3/4 inch pine boards screwed to sleepers. In your case you would not have the concrete nor the sand, just the rammed earth or COB, BUT you would still need the insulation to make a thermal break from the ground. Without it, you are just pumping heat downward. Yes, heat rises and you would heat your living space, but not nearly as efficiently if all the heat rose into the living space.
I will use insulation under my floors, EPS typeII, about 4-6".
Cob would be very easy on the EPS but i'm not sure about RE as the tamping might affect the insulation.
Maybe a layer of cob as base coat and then RE since the cob will spread the load of tamping.
Regarding sand, it's a heat conductor but not a very good one (like stone, concrete, cob, RE, etc).
That's because it has air pockets between the grains.
On the other hand, it's only 3/4" so it's not a lot.
Gypsum has been used in this scenario to embed the PEX but it's a moisture trap, not good for the wood.
The "Heat Rises" metaphor is not quite true.
Heat cannot rise as it's a form of energy.
It tends to travel from warmer to colder bodies by radiation or conduction and between warmer bodies and colder fluids (air for example) by convection.
Hot Air does rise indeed and the phenomenon is called convection.
In your application, there's not much air between the sand grains and the temperature differential is too small for convection to have any significant percentage.
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