Temperature changes play a very important role in nature, thus heating plays a very important role in evaporation, and as such, in the dehydration of old buildings.
Let's look at some of the fundamental differences between warm and cold air.
- Warm air expands, becomes lighter and thus rises. The pressure of light "fluffy" air is also less.
- Cold air, on the other hand, is denser and heavier and as a result, sinks. The pressure of the dense cold air is higher.
Thus we can establish a direct relationship between temperature and pressure: warm light air has less pressure, cold dense air has more pressure. Changes in temperature thus translate to changes in pressure.
The temperature of the walls will also vary due to heating, the cooling effect of the ground different environmental factors, and horizontal and vertical temperature gradients will form.
Generally speaking, the base of the walls will be cooler than their top, and the internal face of the walls will be warmer than the outer face, due to heating.
The temperature gradient of the walls also affects the capillaries, creating a temperature difference between the upper and lower part of capillaries - the bottom of the capillaries being cooler than the top.
Temperature differences translate to pressure differences, resulting in higher pressure at the base of the capillaries and lower pressure on the top. This pressure difference creates a lifting effect inside the capillaries.
If the capillaries are empty, then due to the pressure difference, air will move upwards, resulting in ventilation and the dehydration of the capillaries or of the building fabric. However, if the capillaries are partially or fully filled with salt water, the pressure difference will move the water column upwards, resulting in the upward movement of water inside the capillaries.
As the walls capillaries rest on the soil and thus - in lack of a damp proof course - the capillaries are connected with the soil and to an unlimited supply of water through the water table, the rise of the moisture inside the capillaries will become an ongoing process, and will become one of the "pumping" mechanism of rising damp.
Why Heating Alone Will not Dry a Building from Rising Damp?
Although drying a building with heating is a logical idea, there is one major oversight here: the base of the building stays connected to the soil and water table with a technically unlimited supply of water.
If the water table would be disconnected from the wall capillaries, heating indeed would work, as the limited amount of water from the wall fabric would gradually evaporate and the wall would dry out.
However, because the base of the walls is connected to an unlimited supply of water, the pressure difference created by heating just draws more and more water and salts upwards, resulting in the breakdown of the building fabric by salt crystallization. Thus in itself, heating accelerates the breakdown of the building fabric.
To make heating work you MUST break the permanent connection of the walls from the water table which involves the additional installation of a damp proof course (DPC). Without the presence of a workable DPC which would disconnect the building from the water table, heating alone can not dry out the walls, only the surface.
There is a hidden element here: the APPARENT workability of heating. Often when people move into an old building they turn on the heating AND replaster the walls in the same time. The redecoration itself HIDES the problem for a few years, creating the impression that heating alone has worked and solved the problem of rising damp.
This is just a temporary affair, an apparency. Behind the plaster the rise of water and salts continue and after the breakdown of the plaster, in a few years, the rising damp problem returns.