There are 2 major cycles occurring inside old walls:
1. The Water Cycle
Follows the following path: soil > wall fabric > finishing > air.
If the wall fabric and finish is breathable, most of the water can easily evaporate, resulting in a relatively steady moisture content of the walls.
If evaporation is impeded (e.g. by using non-breathable materials), dampness gradually accumulates into the building fabric, making the walls damper and damper over time.
The moisture content of a wall in depth varies with the distance from the surface. As a general rule, areas near the surface are drier due to the effect of heating and ventilation, while the core of the wall stays significantly damper.
2. The Salt Cycle
The salts cycle: follows the following path: soil > wall fabric > finishing.
Along with the water, diluted organic salts are also transported into the building fabric. Unlike water which evaporates through the pores, salts STAY and ACCUMULATE over time inside the building fabric - the older the wall the saltier it gets.
The impact of salts onto the building fabric is far more important than the effects of dampness and humidity. The sacrificial nature of lime as well as most physical damage in old buildings - such as crumbling, peeling, debonding of plaster etc. - can be attributed to the effect of crystallizing salts.
Even in professional circles the true effect of salts and the damages they can create onto the building fabric is to a large extent unknown or severely underestimated.