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

[Jeremy]

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, and improving guttering to the rear of the house but that's a way off for now.

 

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.

Here's a visual showing the two different foundations in the room (seems likely there was originally a wall and the pad represents an extension added some decades ago, or a reeconfiguration of the room with an internal wall removed):

 

 

View from behind:

 

Edited May 19, 2021 by Jeremy

[markc]

The soil would have been whatever was at hand. I recently ripped out a kitchen floor in a victorian house. Under the pitch it was a randon mix of existing soil, stone rubble and what looked like boiler ash - presumably from the nearby railway. No mixing or thought went into it, simply levelled (very roughly and capped with pitch before the tiles

[Jeremy]

7 minutes ago, markc said:

The soil would have been whatever was at hand. I recently ripped out a kitchen floor in a victorian house. Under the pitch it was a randon mix of existing soil, stone rubble and what looked like boiler ash - presumably from the nearby railway. No mixing or thought went into it, simply levelled (very roughly and capped with pitch before the tiles

 

Lovely. I've even heard reports of subfloor made of a mix that included small bones!

 

Were there any consequences of these random mixes that I might need to attend to? I'm planning to sift out earthworms and insects (though I suppose stone walls and quarry tile will provide a natural barrier) and then mix with clean sharp sand, but otherwise seems like the earth will sort itself out.

 

Would love to hear from folks about what the best use of modern materials for a "cap" might be. Lime screed? Extra sand blind? Simply smooth it all out with a 2x4?

[markc]

Because the soil will be `in the dark` no weeds, no grass, no moss etc. .. never seen a mushroom patch under a floor.

nothing growing means no food so no worms etc.

come to think of it, ive never seen any fungi or growth of any kind under a floor

[George]

While I appreciate the intention, this would fall foul of Building Regulations due to a lack of insulation. 

 

I am not convinced that breathability is important for floors. The primary cause of damp problems sounds like the external ground level.

 

I would install an insulated slab and lay tiles onto this. If you are concerned about breathability of the floor, you could look at a limecrete floor with foam glass aggregate. These can be made to comply with Building Regulations. 

 

In my house I did used a layer of foam glass aggregate (glaspor or similar), before laying a DPM + conventional insulation. This way, there is a capillary break below the DPM, which should reduce any moisture 'moving' towards the base of the walls. However, in hindsight I think this was not necessary. 

 

The mechanical ventilation system you are installing will mean you won't get any damp issues anyway.

[Temp]

What are your plans for insulating the walls? Solid brick walls and high level of ventilation needed to stop damp, doesnt sound like a great match for ASHP.

 

 

[Mr Punter]

Why not insulate the floor with some polystyrene?  It will make the rest of the room much warmer and dryer.  We have come a fair way since the house was built.