Reinstating earth floor under fired-clay (quarry) tiles?

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.

Any and all comments welcome!

Just for fun - photos here:

There is possibly a good reason somebody changed the floor.
I had a friend in a similar house, illness all the time in the family.
I detected dampness everywhere, bad spouting, ground water and suggested he look at fixing the spouting for a tart.
That helped and then we concreted one room to see the effect. It was stunning, mould, dampness and illness all stopped, so the whole house was done.

You may reintroduce those same issues if you rip up the floor.

Thanks for this suggestion John. I'd wondered about this myself, but the current scenario is pretty bad - the back wall is quite damp and was producing mold (wet to touch at any given time) and I can't imagine the hydrostatic pressure from the current setup is helping that. My thinking is that if I open up the floor, it will at least allow for some additional evaporation which will be taken out by the active fan ventilation and by extension take some pressure off the back wall. I totally agree there will be more moisture in the air in the kitchen, but I think I've addressed this with the introduction of more active ventilation. Are there other possible problems that I'm not thinking of?

Worth noting that I am planning to fix a membrane to the back wall, with 1" battens, woodwool boards and lime plaster skim as well to keep moisture away from the new cabinets I'm installing.

John C Daley wrote:There is possibly a good reason somebody changed the floor.
I had a friend in a similar house, illness all the time in the family.
I detected dampness everywhere, bad spouting, ground water and suggested he look at fixing the spouting for a tart.
That helped and then we concreted one room to see the effect. IUt was stunning, mould, dampness illness all stopped so the whole house was done.

You may reintroduce those same issues if you rip up the floor.

Actually, you also raise a really good question here: re rain water removal which I hadn't thought about.

Here's a photo of the back wall - the last window before the fence is the kitchen window, so you can see how the ground level slopes upwards along the rear exterior wall:

I believe the discolouration on the bricks is from ivy that was growing up the wall which has since been removed (a few years ago). I gather it was growing quite vigorously as I've found a few roots that made it all the way through the wall into the pantry shown in the photo above. But as you can see there isn't any guttering or much overhand. This might be due to the fact that the house is protected by hillside behind and roof is sloped towards the front of the house. Will need to check and see.

I see many issues in that photo.
- grey line may be a drainage line for water from further afield
- drive may be draining to the wall

Without doing any research about English conditions I would consider earth works that;
- trap water coming past the gate with a grate across the drive and diverting it into a pipe underground that goes past the house.
- digging down against the kitchen wall, installing a slotted pipe in a sock at the bottom, below the floor level.
- line the wall with an appropriate membrane
- fill trench with gravel up to close near the surface
- try and seal the surface in some way to reduce moisture entering, maybe clay.

Uk causes of damp

- leaking gutters, vegetation on walls
- leaking taps or pipes
- ground built up over time
- old drains uphill destroyed
- replacing stone floors with concrete sometimes stops moisture evaporating from the floor safely
- earth against wall not smart

'Historically, most dampness was caused by the lack of maintenance leading to leaks.
Nowadays, the majority of cases of damp in period houses are caused by inappropriate building work that stops moisture escaping.

With older solid walls it’s advisable to remove incompatible modern materials such as cement render or mortar pointing that trap damp.
Replace with traditional lime-based breathable materials that allow moisture to evaporate.
Allow time for walls to dry out and keep the house heated.'

Society for the Preservation of Ancient Buildings

Most pre-c1919 buildings were built without damp-proof courses (DPCs) or damp-proof
membranes (DPMs) to act as barriers to moisture in walls and floors respectively. Moisture will rise to some degree as it is drawn by capillary action into
the pores of permeable materials, such as brick or stone, that are in contact with damp soil. This is usually not a problem where the construction can ‘breathe’, allowing evaporation.
The absorbed moisture will rise in a wall to a height at which there is a balance between the forces of
capillary rise and that of gravity and evaporation.
This height will vary somewhat with the time of year, wall thickness, pore size and the level of the
water table in the ground. Flagstone or brick floors used to be laid directly upon the bare earth and
the moisture that rose through these floors would be carried away by ventilation (see figure 4).
Additionally, the hygroscopic nature of many traditional building materials means they absorb
small quantities of moisture from the air when it is humid and rerelease it under drier conditions.