The amount of water inside damp walls - and weather a wall is getting damper or drier at any point - is determined by the balance of two factors:
- Water in: how much water being absorbed into the building fabric how quickly. A number of factors - such as the moisture content of the soil or of the surrounding air, the porosity of the wall, the salts content of the fabric - affect the water intake, determining how wet the fabric gets.
- Water out: how much water is being released from the building fabric how quickly. A number of factors - such as evaporation, the quality of the drainage, the breathability of the plaster, the temperature of the surrounding air etc. - all affect the drying-out process of the masonry, determining how dry the masonry gets.
This is an important, but very simple concept.
Doing certain actions that reduce the moisture content of walls results in a drier fabric. Doing other actions that increase their moisture content (even if slowly), results in a damper fabric.
Old houses are built onto the soil, subject to the ongoing evaporation from the soil. The soil is an ongoing source of moisture that over time can add up significantly. It was surprising for us to see and measure during our lab tests the amount of humidity build-up into the bricks from an even mildly damp soil.
Over time this slow moisture build-up, coupled with the combined effect of ground salts, will result in rising damp even in perfectly well maintained buildings.
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