Module 1 · Moisture, Air and Materials

1.4 Hygroscopic Salts and the Moisture They Hold

10 min read

A wall that has been wet for years contains salts, and those salts change what 'damp' means in that wall. Nitrates and chlorides are strongly hygroscopic: they pull moisture from room air and hold it, so the plaster stays measurably damp long after the original source has been cut off. Failing to identify this is the most common reason a correctly executed repair is followed by a complaint that the damp 'came back' — it never left, because the remaining problem was never liquid water in the first place.

Learning objectives

By the end of this lesson you will be able to:

Explain how hygroscopic salts hold moisture independently of any active water source.
Identify which salts indicate which origin, and what each implies for remediation.
Distinguish a hygroscopic moisture problem from an active ingress problem by measurement.
Specify remediation that addresses salts rather than repeating a failed repair.

Key terms

Hygroscopic: Able to absorb moisture from the surrounding air. A hygroscopic salt reaches equilibrium with the air's relative humidity rather than with any liquid source.
Deliquescent: So strongly hygroscopic that the salt absorbs enough moisture to dissolve into solution. Calcium chloride and calcium nitrate do this at ordinary indoor humidities.
Efflorescence: Salt crystallising on the surface as water evaporates. Usually cosmetic, and an indicator that evaporation is happening at that face.
Cryptoflorescence: Salt crystallising below the surface, within the pore structure. Damaging, because crystal growth generates pressure that spalls the face off.

Water moving through masonry carries dissolved salts. When it evaporates at a face, the water leaves and the salts do not — so they accumulate at the evaporation zone, concentrating over years into a band that often marks the historic high point of wetting.

Those salts then behave according to their own equilibrium with the air. Sodium chloride begins absorbing at about 75% RH. Calcium nitrate absorbs well below 50%. So in an ordinary heated room at 55–60% RH, a nitrate-contaminated plaster will sit permanently moist, will read high on any meter, and will feel cold and damp to the touch — with no water arriving from anywhere.

The diagnostic consequence is sharp: total moisture content tells you how wet the material is, and hygroscopic moisture content tells you how much of that is the salts' doing. The difference between them is the part an actual water source has to explain.

What each salt tells you about origin

Nitrates — almost always from organic decomposition: historic animal husbandry, cesspits, agricultural ground, long-term urine contamination. Strongly hygroscopic. Their presence indicates ground-sourced water over a long period.
Chlorides — de-icing salt, coastal exposure, some historic building practices, and occasionally contaminated fill. Hygroscopic above about 75% RH.
Sulphates — commonly from the masonry itself, from cement or from flue gas contamination in chimney breasts. Not usefully hygroscopic, but expansive, and the cause of sulphate attack on cement.
A chimney breast staining that is heavy in sulphates with a tarry smell is flue condensate, not rising damp, and a chemical injection will do nothing for it.

Salt analysis is cheap — a semi-quantitative test strip kit costs very little and takes minutes on site — and it is the single most informative test relative to its cost in the whole toolkit. Module 4 covers running it properly.

Telling hygroscopic moisture from active ingress

This is the measurement that decides the specification, and it is done gravimetrically on drilled samples. The principle: measure the moisture the sample actually holds, then measure the moisture it will hold from air alone, and compare.

Take drillings at depth and seal them immediately — a sample exposed to room air for ten minutes has already begun equilibrating and is worthless.
Weigh wet, oven-dry, and weigh again. The difference is the total moisture content.
Re-expose the dried sample to a controlled humidity — conventionally around 75% RH — until it stabilises, then weigh again. That gain is the hygroscopic moisture content.
Where the hygroscopic figure accounts for most of the total, the wall is holding moisture because of salts. Where the total substantially exceeds it, water is genuinely arriving and you still have a source to find.

This procedure is what turns 'the wall is damp' into a statement about mechanism, and it is why gravimetric sampling is not optional at practitioner level. It is also, bluntly, why a great deal of injected damp-proofing has been sold into walls that had no active rising water at all.

What salts mean for the repair

If the moisture is hygroscopic, cutting off a water source achieves nothing, because there is no water source. The salts are in the plaster and in the outer face of the masonry, and they will keep doing this indefinitely.

Remove the contaminated plaster, generally to a defined margin beyond the visible salt band — the contamination extends past what you can see.
Allow the masonry to dry, and re-test rather than assuming a duration. A wall dries at roughly a month per 25mm of thickness in reasonable conditions, but only under conditions that permit evaporation.
Re-plaster with a system chosen for the situation: a salt-resistant render where the substrate remains contaminated, or a sacrificial poultice-and-lime approach where the aim is to draw salts out over time. The choice is a conservation decision as much as a technical one, and Module 7 develops it.
Never re-plaster with a dense cement render over contaminated masonry and treat it as solved. It relocates the evaporation, and the salts reappear above the line.

Say this in the report explicitly. A client told 'the damp was treated' and then meeting a salt band six months later has a complaint; a client told 'the wall holds salts and the plaster is being replaced to a margin because of it' has an explanation they can hold you to and which will turn out to be true.

Common mistakes to avoid

Reading a persistently high meter reading as ongoing ingress without testing for salts. Hygroscopic plaster reads high forever.
Taking salt samples from the surface only. Surface efflorescence tells you evaporation is happening there; contamination through the depth is the question that matters.
Leaving drilled samples in the open before sealing them. They begin equilibrating with room air immediately, and the total moisture figure is then meaningless.
Treating any salt band as evidence of rising damp. Sulphates on a chimney breast are flue condensate, and nitrates high on a wall may mark a historic leak.
Re-plastering to the visible salt line. The contamination extends beyond it and will re-emerge just above the new work.

Lesson summary

Salts deposited by years of evaporation make a wall hygroscopic, so it holds moisture from room air with no source to find — which is why a technically correct repair is so often followed by a complaint that the damp returned. Comparing total moisture content with hygroscopic moisture content on properly sealed depth samples separates the two, and the salt species points at origin: nitrates at long-term ground contact, chlorides at de-icing or coastal exposure, sulphates at the masonry or a flue. The repair follows from that, and re-plastering to the visible line is not it.

Next: sorption isotherms and equilibrium moisture content — what 'dry' means for a material, and when a wall is actually finished drying.