In old buildings, stains, mould, bacterial proliferation, or condensation of water on walls and floors can be found.
Occasionally the same problems occur in recent constructions. The real problem starts at the formation of this phenomena and is caused by excess moisture inside walls, the structure or the environment. To find a correct solution to these problems the cause needs to be addressed and not the result that these problems create. First the source of the moisture needs to be found and subsequently, after a careful analysis of the solutions available on the market, intervention to neutralize or limit the damage. For example, in the case of moisture due to capillary action within the walls, specific cycles of dehumidifying plaster can be evaluated as a solution and special renovation cycles for both indoors and outdoors.
The water present in the ground penetrates the walls and rises with variable values depending on the type of materials used in the wall subject to the phenomenon of rising damp. The ascent varies, as said, depending on the porosity of the materials, the salt concentration or when plaster rich cement binder, stone cladding, marble, tiles, paint or finishes extremely loaded with additives, prevent the evaporation of moisture.
Taking into account that, as already mentioned, with rising damp there is commonly also the transport phenomena of soluble salts (sulfates, nitrates and chlorides, etc.) that crystallize on the surface due to evaporation of water. The salts contained within a normal cement plaster, subject to rising damp, crystallize within small pores, increasing in volume and developing inside of micro-cavities of the wall, a pressure that causes breakage, disintegration and degeneration of the plaster. Yet it is possible that, in the presence of sulphates, these combine chemically with the plaster binder making expansive compounds such as ettringite.
The use of macroporous dehumidifying renders mortar or plasters that can facilitate the evaporation of moisture from the walls towards the external environment thanks to the high porosity due to the presence of cavities uniformly dispersed within the matrix binder, do not eliminate the phenomenon of rising damp in masonry, but facilitate the removal especially during hot and dry seasons.
The use of macroporous plasters however, if on the one hand allow for easy removal of rising damp, on the other hand they cannot prevent the entry of rainwater into the masonry.
The result is that you will gain a benefit, in terms of dehumidification of the walls, only if they are located in an area where dry and windy days prevail over those that are humid and rainy. On the other hand it should be noted that if a waterproof coating is applied to a macroporous render/plaster to prevent the entry of rainwater into the wall, it would be more difficult for the moisture present within the wall due to capillary action to evaporate.
To avoid this problem apply a hydrophobic mineral finish to macroporous render/plaster which will maintain high breathability of the underlying dehumidifying render/plaster, without changing the values of vapour diffusion and transmission.