Key Takeaways
- Hygroscopic salts are soluble compounds in walls and masonry that absorb moisture from the air, often visible as white tide-marks.
- Their presence always signals an underlying moisture issue (e.g., rising damp, penetrating damp, leaks) rather than being the root issue themselves.
- These salts can lead to long-term damage: they draw in moisture repeatedly, degrade plaster/mortar, corrode metal elements and compromise structural materials.
- Accurate diagnosis involves identifying both the salt deposits and the source of moisture, typically via specialist tools, surveys and testing.
- Effective remediation requires a two-step strategy: eliminate the moisture source (repair leaks, improve drainage, install damp proofing) and treat or remove salt-contaminated materials and finishes.
- Ongoing prevention includes using breathable building materials, proper ventilation, regular inspections and maintenance to monitor moisture and salt ingress.
What Are Hygroscopic Salts?
Hygroscopic salts are compounds that absorb moisture from the surrounding environment. These soluble salts are often visible as tide marks on plaster, brickwork and exterior walls. These marks are caused as nitrates and chloride become concentrated where water evaporates out of your wall.
So, by their very nature, hygroscopic salts can only be present when there is some form of excess moisture. Older buildings may be prone to damp, but salt stains are certainly not exclusive to them. In certain circumstances, salt marks can appear on walls in any type of building that may have damp patches.
What Can Cause Salt Marks On A Damp Wall
Rising Damp
Rising damp occurs when groundwater or inadequate drainage from waterlogged gardens ascends through walls, especially in structures without proper damp proofing.
This can lead to the formation of hygroscopic salt deposits on interior walls.
Penetrating Damp
Penetrating damp occurs when rainwater or external groundwater permeates through inadequately protected surfaces, introducing salts into the building materials.
This can cause damp patches and salt deposits on walls.
Rainwater
Leaky guttering, cracks in brickwork or mortar combined with a lack of damp proofing can cause rainwater and salts to soak through the porous materials. This can again cause a damp wall and salt marks.
Additionally, new build houses take up to 12 months to ‘dry out’ after completion and often have salt marks on the exterior walls for a minimum of a year. This is commonly known and not to be concerned about
Faulty Plumbing
Problems with plumbing that leaks or causes flooding can lead to damp which can occur on any surface. If not allowed to dry out effectively, this can lead to hygroscopic salt creating stains on walls, floors and ceilings.
Humidity
An environment that is too humid can cause condensation and damp walls leading to paint and plaster damage and hygroscopic salt stains.
Identifying Hygroscopic Salts in Walls
Visible Signs and Testing
Visible signs of hygroscopic salts include white marks or stains on walls, damp patches, and efflorescence.
Most professional damp surveyors will be able to identify hygroscopic salts by sight and with a little further information on the history of the building and location. Testing with damp meters and plaster sampling can help determine the presence of hygroscopic salts by the presence of nitrates and chlorides.
Effects of Hygroscopic Salts on Building Materials
Damage and Deterioration
It is important to understand that the mere presence of hygroscopic salt on your wall indicates that there is already a pre-existing issue present. It is important to get the correct advice on potential issues such as rising damp, penetrating damp or any other persisting case of damp and excess moisture. Even without the salt deposits, these problems can cause structural damage and should be dealt with immediately.
Hygroscopic salts can compromise the structural integrity of building materials. They absorb even more of the moisture than damp alone and rise through the masonry, plaster and other porous surfaces leading to various forms of damage over time. This includes degrading mortar and plaster, corroding metal elements, and causing surface crumbling and spalling.
Properties of Hygroscopic Materials
Hygroscopic materials are substances that attract moisture and then trap it within. A great example is the silica gel packets placed in pockets of new clothing and handbags. They are specifically designed to prevent moisture contamination from damaging clothing and materials.
Other examples are honey, glycerin, ethanol, methanol, concentrated sulphuric acid, and concentrated sodium hydroxide.
How Hygroscopic Salts Impact Air Quality
Increased humidity, condensation and excess moisture can have an impact on the quality of the air in your home and therefore, prevention is key. Dampness can cause health problems such as respiratory issues and irritate allergies, particularly for those with underlying conditions that are triggered by humidity, such as asthma. You can read more about the health risks of damp and mould here.
However, it has been know that hygroscopic salts, when used correctly, can actually lessen humidity levels when used as a dehumidifier.
Damp Proofing and Removal of Salts
Eliminating Moisture Sources
Fixing plumbing leaks, improving drainage, treating rising damp and addressing condensation issues can help eliminate moisture sources that contribute to hygroscopic salts.
Installing or improving damp proofing can prevent the upward movement of moisture through walls.
Treating Salt Deposits
Brushing or vacuuming visible salt deposits can provide a temporary solution, but does not address the underlying issue. Professional interventions, such as applying salt-inhibiting coatings or replacing damaged building materials, may be necessary for a comprehensive solution.
Prevention and Maintenance
Improving Building Materials and Construction
Choosing building materials carefully and ensuring proper construction practices and damp proofing can help prevent hygroscopic salt issues.
Regular inspections and maintenance can help identify and address damp problems before they wreak havoc on a property.
Regular Inspections and Maintenance
Regular inspections can help identify signs of dampness and hygroscopic salts, allowing for prompt treatment and prevention of further damage.
The Property Care Association recommends regular inspections and maintenance to prevent damp problems.
Conclusion
By understanding the causes and effects of hygroscopic salts, and taking preventative measures, property owners can ensure the longevity and health of their buildings. Dealing with hygroscopic salts requires a comprehensive approach that addresses the root cause of the issue, rather than just the symptoms.
Contact Damp Surveyors Ltd today for a professional to detect signs of damp, such as rising damp, penetrating damp, and many other types of damp. The damp survey is performed by trained, experienced, approved and qualified damp surveyors to spot the early signs of a damp problem on any building.
Frequently Asked Questions
What are hygroscopic salts?
Hygroscopic salts are chemical compounds within masonry and plaster that attract and retain moisture from the air, often leading to visible salt deposits when the moisture evaporates.
What causes salt marks on a damp wall?
Salt marks are usually caused by moisture intrusion via rising damp, penetrating damp, groundwater, leaks, or high humidity. As moisture travels through masonry or plaster, it dissolves salts and later deposits them on surfaces as it evaporates.
How do I identify hygroscopic salts in walls?
Look out for white tide-marks, efflorescence (salt crystals), persistent damp patches, flaking or crumbling plaster. A professional damp surveyor may use moisture meters, plaster sampling and salt testing to confirm.
How do hygroscopic salts affect building materials and health?
Beyond being a cosmetic issue, these salts compromise the strength of render/plaster, cause surface deterioration, corrode metal fixings and can increase humidity indoors—potentially aggravating respiratory issues.
How do I treat salt-contaminated walls?
First fix the underlying moisture problem (e.g., install or repair a damp-proof course, fix leaks, improve ventilation). Then remove contaminated finishes, apply salt-inhibiting coatings or use breathable plaster systems, and reinstatement with suitable materials.
Can I just paint over salt-affected walls?
No. Simply painting will not stop the salts or the moisture behind them. The root moisture must be addressed and contamination removed, otherwise salt will migrate again and the finish will fail.
