Question 1

What are hygroscopic salts in walls?

Accepted Answer

Hygroscopic salts are soluble mineral salts (primarily nitrates, chlorides, and sulphates) that are present in old masonry and that absorb water vapour directly from the air. The term 'hygroscopic' refers to this ability to attract and retain moisture from the atmosphere. In walls, these salts originate from groundwater (carrying dissolved salts from soil and subsoil), from contamination by old fireplaces, farm buildings or stables (nitrate-rich ground), from sea spray (chlorides), or from cement and mortar components (sulphates). Once deposited in the masonry fabric during a period of wetting, the salts do not wash out when the wall dries - they remain behind in the masonry pores and mortar joints. In humid conditions (typically above 60-75% relative humidity, depending on salt type), the salts absorb moisture from the air and wet the surrounding masonry, causing damp patches.

Question 2

How do hygroscopic salts differ from efflorescence?

Accepted Answer

Efflorescence and hygroscopic salt deposits both involve soluble salts in masonry, but they are different phenomena. Efflorescence occurs when water carrying dissolved salts migrates to the wall surface and evaporates, depositing the salts as white crystalline staining on the face of the brickwork or render. The salts are visible and the staining can usually be brushed off. Hygroscopic salt deposits remain within the masonry (in the body of the brick, mortar, or plaster) rather than on the surface. They do not produce visible white staining; instead, they cause recurring damp patches in the plaster or wallcovering because the salts at the masonry surface or in the plaster draw moisture from the air. The damp patch may appear without any visible white deposit, particularly once the plaster is painted.

Question 3

Why does my wall stay damp after the damp course treatment?

Accepted Answer

A common scenario: rising damp is diagnosed, a chemical damp-proof course is injected, and new plasterwork is applied - but the wall continues to show damp patches months later. This often happens because hygroscopic salts have been left in the wall or incorporated into the new plaster. If the old contaminated plaster was not fully removed (or was only removed to a certain height while salts extended higher), or if sand with a high natural salt content was used in the new render, the salts will cause recurring damp patches regardless of the effectiveness of the new DPC. The remedy is to use a renovation plaster system formulated with a low water/cement ratio and salt-blocking additives (sometimes called 'waterproof render' or 'salt-resistant render'), or to apply a salt-resistant backing coat before the finish plaster.

Question 4

What is the treatment for hygroscopic salt damp?

Accepted Answer

The treatment for hygroscopic salt damp depends on the severity and extent of salt contamination. Where the salt content is moderate, applying a renovation plaster system (formulated to be tolerant of salt-contaminated backgrounds) over the existing masonry after removing all contaminated plaster can be effective. Where contamination is severe (particularly nitrate-rich walls in old stables or farm buildings), the contaminated masonry surface may need to be treated with a proprietary salt neutraliser or anti-salt barrier, or physically isolated from the new plasterwork using a waterproof render coat or tanking membrane. In all cases, the source of the original moisture that deposited the salts should be identified and cured before replastering - new plaster applied to an actively wet background will fail regardless of salt resistance. Identifying hygroscopic salt damp rather than active penetrating or rising damp requires a moisture meter survey combined with salt analysis of the masonry.

Hygroscopic Salts

Just so you know...