Hygroscopic salts

Hygroscopic salts are commonly found in buildings, particularly older structures, and can be a cause for concern due to their ability to attract and hold moisture from the surrounding environment. Understanding their nature and impact is crucial in addressing damp issues in properties.

Definition of Hygroscopic

The term “hygroscopic” refers to substances that are ‘water-loving’. Salts like calcium nitrate and sodium chloride (table salt) are hygroscopic, meaning they can absorb moisture from the air. This property is utilized in various applications; however, when it comes to buildings, it’s often associated with damp-related issues. Sodium chloride, for instance, causes moisture to condense when the relative humidity exceeds 75%RH, a process known as deliquescence. For practical purposes, any salt causing condensation at or below 75%RH is considered hygroscopic.

Common Occurrence Near Chimneys

One common occurrence of hygroscopic salts is around or near chimneys. Historical use of coal or even equine urine in proximity can lead to the formation of these salts. The removal of chimney breasts or re-plastering of walls can release hygroscopic salts, which then migrate away from the chimney often along a level. Most of these salts become apparent on surfaces after events like a leak, flood, or extensive condensation, especially when areas are re-plastered or when chimneys are removed.

Calcium Nitrate and Diagnosis

Calcium nitrate is a prevalent hygroscopic salt found in older buildings. It’s estimated that roughly 15 – 20% of damp issues in residential buildings are diagnosed as resulting from hygroscopic salts. However, it’s crucial to note that these salts are not a source of moisture; instead, they absorb and release moisture from the air, maintaining a state of dynamic equilibrium with their environment. Therefore, the dampness associated with hygroscopic salts is often misdiagnosed as rising damp, especially since groundwater also contains hygroscopic salts.

Seasonal Variation

The manifestation of these salts is usually more pronounced during the summer as the critical relative humidity of calcium nitrate is 46.7%RH at 30˚C, and salts become more hygroscopic as temperature increases. This condition can cause damp patches to appear worse in warmer weather.

Identification and Misdiagnosis

Spotting the presence of hygroscopic salts can become second nature with experience – a tell-tale stain on walls, especially around ground floor chimney breasts or at the top of top floor chimney breasts, is a common indicator. These stains are often mistaken for rising damp or roof/chimney stack failure, yet a distinguishing factor is that hygroscopic salts are colourless and their appearance varies with humidity and temperature, not necessarily with rainfall.

Testing for Hygroscopic Salts

Testing for hygroscopic salts can be conducted using tools like Palintest. While calcium chloride looks similar to calcium nitrate, a test failure doesn’t rule out the presence of hygroscopic salts.

Treatment Options

Treatment for areas affected by hygroscopic salts involves creating a barrier that prevents moisture absorption. Applying two coats of a solvent-based stain block primer, such as Zinsser cover stain, with a significant overlap, is a simple yet effective solution to mitigate the unsightly appearance caused by hygroscopic salts.


In addressing damp issues in buildings, especially historical or older structures, a clear understanding of hygroscopic salts, their identification, and effective treatment methods are essential steps towards preserving the structural integrity and aesthetic appeal of the property.

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