Hygroscopic Salts in Walls: What They Are and Why They’re Misdiagnosed as Rising Damp
Salts in walls are one of the most misunderstood aspects of damp surveying — and they’re responsible for a significant amount of unnecessary treatment being recommended and carried out. Understanding what wall salts are, where they come from, and what they mean is essential for anyone dealing with damp in an older property.
As an independent damp surveyor covering Derby, Nottingham, and Leicester, I regularly see properties where persistent high moisture readings and white efflorescence are attributed to active rising damp when the real culprit is old salt contamination from decades-old treatments or historic moisture events.
What Are Hygroscopic Salts?
Salts in building materials come from several sources: groundwater carries dissolved mineral salts upward through masonry; combustion in flue gases deposits salts in chimney breasts; previous chemical DPC treatments leave silicone residues and associated salts in the wall; and seawater or de-icing road salts in exposed coastal or roadside properties add to the mix.
These salts become problematic because many of them are hygroscopic — they absorb moisture from the air. Chlorides and nitrates, common in ground-affected masonry, are particularly hygroscopic. A wall with high salt contamination will show elevated moisture meter readings and visible damp patches at moderate to high relative humidity — not because there’s active water ingress, but because the salts are drawing atmospheric moisture into the plaster.
Why This Matters for Damp Diagnosis
This is a critical distinction. A damp proofing company with a basic moisture meter reads a high value at the base of an external wall and concludes rising damp. A proper independent investigation asks: is this active moisture movement from below, or is it salt-contaminated plaster absorbing atmospheric moisture?
The two have completely different causes, different solutions, and vastly different costs. Active rising damp (if it’s genuine) requires addressing the source of ground moisture. Salt-contaminated plaster requires removing the contaminated plaster and replastering with a salt-resistant render — but no chemical injection, no membrane system, and no DPC treatment of any kind.
The way to distinguish them is through calcium carbide testing (gravimetric moisture analysis, as set out in BRE Digest 245) or detailed salt analysis of plaster samples. Calcium carbide testing measures actual moisture content in the material rather than its electrical conductivity — salts affect conductivity but not actual water content, so a calcium carbide reading in genuinely dry salt-contaminated plaster will be low even when the moisture meter reads high.
Common Sources of Salt Contamination in Midlands Properties
Previous chemical DPC treatments: the silicone-based chemicals used in injection treatments can leave residues that alter the conductivity of the wall. More significantly, if the injection was carried out on a wall that already had a bridged or absent DPC, the ground moisture problem may have continued for years after treatment, depositing additional salts.
Old lime plaster on chimney breasts: years of combustion deposit hygroscopic sulphates in chimney breast masonry and plaster. A decommissioned fireplace in a property that hasn’t been used for decades can still show elevated moisture readings from salt contamination, particularly in humid weather.
Victorian and Edwardian basement and cellar walls: brick built in contact with ground, often without any DPC, has typically absorbed ground salts over a century or more. The plaster in these areas often shows tide marks and white crystalline deposits that look like active rising damp but are the legacy of historic moisture events.
Road salt exposure: properties immediately adjacent to roads that are heavily gritted in winter can show chloride contamination in the lower sections of external walls from splash and spray over many years.
What Salt Analysis Involves
A proper salt analysis takes a sample of plaster or mortar from the affected area, dissolves it in water, and tests the solution for the presence and concentration of chlorides, nitrates, and sulphates. The results indicate both whether salts are present and their likely source — groundwater, combustion, or sea/road salt.
This isn’t something most damp proofing companies do routinely because it adds cost and, in many cases, would undermine the diagnosis they’re about to sell treatment for. It’s a standard part of a thorough independent investigation.
Get Expert Assessment
Salt analysis and calcium carbide testing available as part of independent damp surveys across Derby, Nottingham, Leicester, and the wider Midlands.
📞 07983 550 662
✉️ richard.bull@dampdetectives.co.uk
Book a Survey →
Richard Bull MISSE, ACIEH — Independent & Unbiased — No Sales Pressure