Rising damp

Rising damp is when water migrates in porous building materials, e.g. masonry, from below ground level, into and through the construction, and up above ground level. The pore structure and porosity allow these materials to transport liquid moisture. Moisture accumulation in walls due to rising damp can appear visibly by discoloration (darkening) of the wall to a certain level above the ground. Examples of rising damp in external façades are seen in Figure 4–18.

Figure 4–18: Examples of rising damp in external masonry, appears as darkened/moist areas (Left and center), and an example also showing salt efflorescence (right). Source (center): CSC Sárl https://conservation-science.ch/

Accumulation of moisture in masonry facades can yield several deterioration mechanisms, why it is undesirable (Section 0). Moisture accumulation in the wall can cause damage to surface treatments; peeling of paint, damaged render, deterioration of brick and mortar due to frost damage, or the appearance of salt efflorescence. If moisture accumulates behind a diffusion open paint, the evaporation of moisture is inhibited, and in time will cause the paint to bubble and peel off. Examples of damage from moisture accumulation in walls can be seen in Figure 4–19.

Figure 4–19: Examples of rising damp and damages caused by moisture in the walls; left: darkening of masonry and salt efflorescence, middle: frost damage in masonry, right: peeling of paint due to moisture accumulation and too diffusion tight paint (Source: Morten Hjorslev Hansen).

Rising damp can appear in both external and internal walls. Internally the problems will most likely be noticeable in the basements. Damp walls and moisture in general are always considered problematic and issues to be addressed. Moisture in the façade is undesirable as it can lead to e.g. frost damage, mould growth and general deterioration, especially of embedded wood.

When adding internal insulation, the risk deterioration processes increases, as the existing wall becomes colder, and furthermore the drying potential is reduced. Table 4‑10 summarizes where one should pay special attention when visually inspecting the building, and what to look for.

Table 4‑10: What to look for, and where with regard to rising damp.

With regard to rising damp, there are several possible moisture sources, including ground water, infiltrating surface water, damp soil, defective piping and surface water combined with terrain sloped towards the building. Some of the possible moisture sources in basements are illustrated in Figure 4–20. Walls affected by rising damp, can even become more susceptible to moisture, as the risen ground water contains salts that furthermore absorb moisture from the surrounding air (osmosis). Rising damp can sometimes be mitigated according to the moisture source; e.g. repair of defective piping and wrongly sloped terrain may alleviate the moisture source, while moisture from damp soil can be reduced by installation of a drainage system on the external side of the walls below ground. To prevent moisture from migrating upwards via porous materials in the construction, physical damp-proof moisture barriers can be installed in the perimeter of the wall.

Moist areas in façades due to rising damp can appear when the porous masonry is in contact with moisture, and absorbs water by capillary forces and due to water pressure. There are several sources from which the water in a moist façade can originate, including;

• Ground water, infiltrating water (seepage of surface water through the ground) or damp soil surrounding the foundation or basement walls

• Defective piping (either underground plumbing or external drainage systems)

• Surface water (precipitation in the case of the terrain being sloped towards the building)

Remedial actions if damage is identified

The typical remedial action, to insert horizontal moisture barriers in the external wall, is complicated and requires experts.

Initially the moisture source should be identified, and the cause of moisture appearance should be addressed. Depending on the moisture source, there are several options for remedying the moisture accumulation due to rising damp. Obvious physical damage and problems, e.g. in the case of leaky drainage or piping, these systems should be repaired properly, or if surface water appears to be an issue due to the terrain being sloped towards the building, the slope should be reversed if possible. A slope of minimum 1:40 away from the building for the first 3 m next to the building is preferred (Brandt, 2013). Heating of the basement is generally a good idea, but especially if the internal moisture sources are high, as it improves the drying potential of damp walls. Furthermore, to not impede drying of a moist façade, diffusion open paint (e.g. silicate paint) is advisable for the internal surface. This allows more drying than in cases of diffusion tight paint and can prevent peeling of paint. Ventilation of the basement will remove moisture in the winter, however, in the summer ventilation with hot moist outdoor air may cause high relative humidity or even condensation on the colder basement walls.

With regard to moisture penetration from damp soil, it may be beneficial to implement a drainage system around the building, which can relieve some of the moisture load on the wall. The drainage system can be combined with external thermal insulation of the basement wall; a warmer basement will improve the drying potential. If the risk of moisture penetration from the outside is small, insulation with hydrophobic mineral wool or loose aggregates (coated) can be used, thereby the external surface will be diffusion open, allowing the wall to dry to the outside. If the risk of moisture penetration from the outside is high, the thermal insulation could typically be a lining of plastic sheets combined with a cellular foam with drainage grooves (Brandt, 2013). These plates are mounted and shaped in a manner that carries the water away from the construction to the drainage system. Another option is to roughcast the external basement walls, including two layers of asphalt to ensure water tightness.

Traditionally rising damp in basement walls is inhibited by damp-proof courses, which are continuous moisture barriers. Physical moisture barriers can be of steel plates that are vibrated into mortar joints in the depth of the masonry construction if the joints are continuous. It is also possible to saw through the wall thickness, a section at a time, and place either steel sheets or reinforced roofing felt as moisture barrier. Examples of these measures are seen in Figure 4–21.

Figure 4–21: Examples of continuous moisture barriers (Source: Morten Hjorslev Hansen (left and middle), (BYG-ERFA, 1997) (right)).

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