Using natural earthen floor in updating an old building

I live in a 1935 barn, converted to a house in 1992, bought by me in 2014. I've been slowly updating things, and here is my current problem.

The basement floor is a very rough and sloping concrete slab. I want to level the slab and maybe put a recycled wood floor on it, or maybe just  real earthen floor.

My current plan begins with waterproof paint on floor and walls (no mold since I started this paint, so I will continue), then level the floor, then unsure. But what to level the floor with? I've looked at papercrete, earthen floors (but no recipe), cob floors - and there must be hundreds more options. Does aanyone have experience or advice?

I'm thinking that whatever I use, I'll spread it thinly and allow each layer to dry or mostly-dry before adding the next. I'm not sure how to make a good connection with the slab, especially since it will be painted.

Another possibility would be to simply use lumber to level the floor (lots of cutting there, jigsaw work) and add insulation under plywood, then the recycled oak floor.  In that case, suggestions for natural insulation?

I do have chemical sensitivities, but have been coping with conventional building materials including foam insulation, foamboard, and the like. Have wanted to go natural for 30 years, and wondering how much I can do in this existing structure.

The room space is 10x13x7' high.

You could consider a floating floor, since you're thinking of doing something with recycled timber. Then the leveling layer doesn't need to bond with either the concrete or the stringers that the flooring gets attached to. Sand or earth would both work in this context.

I'm planning to do a floating floor over finely crushed biochar on top of a rather ugly slab (at least it's level...that's about all it's got going for it). The idea is to put down a thin layer, maybe 2-3 mm, dampen it with a wheat flour slurry to stop dusting, then lay stringers on top. Next I'll backfill with biochar to the level of the stringers, then do another round of starch slurry and nail the floorboards down to the timber. The motivating factors are to get the insulative and moisture control properties of the biochar, and to have the floor not be acoustically coupled to the slab to limit sound transmission.

Hey, thanks. We make biochar but I never thought of it in building. Of course my floor is very much not level. .

Phil Stevens wrote:You could consider a floating floor, since you're thinking of doing something with recycled timber. Then the leveling layer doesn't need to bond with either the concrete or the stringers that the flooring gets attached to. Sand or earth would both work in this context.

I'm planning to do a floating floor over finely crushed biochar on top of a rather ugly slab (at least it's level...that's about all it's got going for it). The idea is to put down a thin layer, maybe 2-3 mm, dampen it with a wheat flour slurry to stop dusting, then lay stringers on top. Next I'll backfill with biochar to the level of the stringers, then do another round of starch slurry and nail the floorboards down to the timber. The motivating factors are to get the insulative and moisture control properties of the biochar, and to have the floor not be acoustically coupled to the slab to limit sound transmission.