I'm renovating an 1880s Victorian house - it's solid brick masonry walls with (in most cases original) lime plaster on the interior, and in most places suspended timber floors over earth. It's a lovely breathable structure with some unwise modifications that have been added over time and have caused some problems, particularly for the mitigation of water vapour. I'm adding an air source heat pump with oversized radiators, to be powered eventually by solar PV to keep the house warm, and adding some individual mechanical ventilation with heat recovery devices to improve air circulation. There are air bricks in place which seem to be providing excellent ventilation to the subfloor where floors are suspended timber (no signs of rot after 100+ years).

My current project is to address some serious damp/mold problems in the back of the kitchen, which rotted out the bottom/back of the previous cabinets (all now removed / recycled). The brick wall is partially buried underground, but also for some reason a former occupant installed a plastic membrane and concrete slab, presumably as part of a renovation on the cheap. The rest of the bottom floor is quarry tiles on bare earth. I'd like to remove the slab and membrane and reinstate the earth floor that was previously in place with quarry tiles on top.

Here's my key question: what sort of soil should I install for the subfloor? I've been working my way through Crimmel and Thomson's Earthen Floors book to try and get a sense of things, and gather this should be a good mix of (mostly = 50-70%) sand, clay and soil (at 20% or less) and maybe fibres as well. I could easily excavate the soil necessary for this from outside the house and mix with sand, but wonder if there would be a need to address the biology in the soil? Also, for folks who have done this kind of flooring, what sorts of top layer have worked? I could do a thin lime screed on the new section, but the rest of the house is bare compacted earth, so am not sure if this is necessary.

There's also some long term potential here for removing soil which is against the house and replacing it with gravel and a french drain, but that's a way off for now.