Air humidity

Air humidity may be a source of moisture sufficient to cause damage or to allow processes like salt crystallisation to take place.

Surface condensation & Damage mechanism due to hygroscopic behaviour salts

In all cases a thin moisture fi lm may appear on the surface, which points at the presence of condensation. Because of the nature of the condensation process, though, not always a film will be visible.

Surface condensation

Surface condensation can be considered the result of the combination of air humidity and a cold surface (thermal bridges, or heavy and inert construction, or not absorbing material) (fig. 1)

If the indoor climate is stable, condensation may be due to a thermal bridge or a high moisture production. Measurements are necessary to find out about thermal quality of construction and the indoor climate class. Surface condensation (and even a high RH at the construction surface) can be enough for moulds to thrive.

If the indoor climate is transient, cyclic patterns of condensation may take place. The properties of the material may be an important factor. When moist spots or a thin moisture film are visible, the explanation may lie in the fact that the construction is thermally inert or the surface is not absorbing.

Whether the climate is transient or stable depends on several factors, such as the continuous use of heating, peaks in moisture production, and the use of the room showing condensation (in bedrooms, bathrooms, kitchens the climate can be transient).

In case of doubt, the possibility of an external moisture source could be still investigated by sampling the wall behind.


Surface condensation can occur both indoors and outdoors.


Fig. 1 Surface condensation (outdoors) on Ledestone (Belgian sandy limestone)


Condensation in flues

Surface condensation due to high temperature and relevant high vapour pressure of combustion gasses may be severe and lead to moisture transport also in masoned flue structures (causing staining on the -opposite- surface).


Hygroscopic process (may occur indoor and outdoor)

The hygroscopic uptake of the salt contained in a wall (building part) may lead to severe damage due to salt crystallisation (surface change, staining, deposit of salt efflorescences, crypto-florescences, loss of cohesion inside the materials leading to crumbling, sanding, powdering, staining). Besides, moist spots / zones may be visibe.

MC and HMC profiles will provide a clue to control the presence of hygroscopic salts. Monitoring the RH and the temperature of the air as well as the temperature of the material surface will show whether changes in RH take place, explaining the salt activity (dissolution and crystallization) (fig. 2).

Monitoring of the climatological circumstances implies the use of thermocouples to be placed on the wall at different heights (e.g. 50, 100, 200, 300 cm) to measure the surface temperature in the course of time, and the monitoring of the air temperature and the RH by means of sensors applied in the interior and at the exterior of the building.

Fig. 2 Monitoring indoor climate conditions in a church with salt crystallisation problems