|« Moist areas » seen on buildings are usually attributed to infiltration or capillary rise, phenomena which require the presence of large quantities of liquid water in the humid area. However, in most cases, hygroscopic salts (such as chlorides* or nitrates**) which are able to adsorb easily and abundantly water vapour from the ambient air play an important role.
|When these salts are present in building materials, they are able to modify the reflection of the light of the surface of the porous material that contains them, giving it a more dark or « wet » look. The quantities of water and salts needed to be present within the porous material to make these optical illusions observable depends on the nature of salts, but the amount of water vapour in the air, even under « dry » conditions, is often sufficient. Non-destructive humidity measuring devices are usually sensitive to salts and are rarely able to distinguish between these « harmless » cases – although aesthetically disturbing – and cases of actual infiltration or capillary rise, but some simple laboratory tests can quickly help to overcome the indeterminacy.
In conclusion, before beginning heavy interventions as e.g. installing a drainage system or establishing a « capillary barrier » of any kind, we strongly advise to measure the amount of water actually present in the walls and possibly to analyse the salts.
* Chlorides may be introduced e.g. by de-icing salts;