Moisture in the exterior wall of a 300-year-old house

Hello,

we have an old house that is about 300 years old and has exterior walls approximately 60 cm thick.

Now we have bought a moisture meter and found that one exterior wall in particular shows a reading of "100" up to a height of about 120. At these spots you can also feel that the plaster is somewhat hollow underneath and will probably flake off over time.

The other exterior walls are between 70 and 80.

Around the outside of the house we have now found a concrete ring about 50 cm wide all around. Apparently, this is not connected to the floor slab, and in the gap, that is between the floor slab and this base, you can see a kind of black sealing compound protruding a bit. Could it be that the previous owner installed a drainage there?

The house has not been inhabited for more than 10 years and thus has not been regularly heated. It used to be an old mill and is otherwise in good condition. In 2003 there was flooding of about 50 cm in the house. Could it be that it has never really dried out properly since then?

How should/could we get the water issue under control under the above circumstances? Or will it not fully work for such an old house in a flood-prone area anyway?

I need your help.

Thank you

Why are you doing a moisture measurement on exterior walls that are exposed to the weather every day? That is somehow illogical.

Sorry, maybe expressed incorrectly:

I measured inside the room (building exterior wall).

I used a Trotec BM31 measuring device (measures up to 4cm deep into the masonry).

Understand. There were no materials three hundred years ago that allowed for dense masonry. One must accept that. If this is the case, one must not try to seal such buildings today, because that will fail; moisture must be able to enter and exit, the house must breathe. Therefore, no dispersion paints inside, but Keim natural paints. Therefore, no lime-cement plaster inside, but renovation plasters with shell lime components; one must try to imitate the materials from that time. Then this house will stand for another three hundred years, but it will always have more humid air inside than new buildings, it will never win the [kfw schiessmichtot] prize, but it will have atmosphere. K.

Well, living in something like that means either accepting an unpleasant climate or spending a lot of money.

Well, living in something like that just means accepting unpleasant climate or spending a lot of money.

What exactly do you mean or how?