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

One of my guys here swears by his 60 cm thick sandstone walls. Insert [Schwedenofen], he doesn't care if one more cubic meter of wood goes through or not. He says it's nice and warm in winter, nice and cool in summer, he doesn't notice the outside weather. There's still an old clay plaster on it.

A 100% reliable horizontal barrier can no longer be installed with the wall thickness and irregular natural stone masonry. Drilling and liquid silicification may work; my parents did this in their house, I would say with about 98% success.

It is probably better to create the indoor climate in the long term through ventilation and heating. I don't think much of consciously drying the walls, as the capillaries will then draw up new moisture again.