Hygroscopic salts

Roughly 15 – 20% of our damp surveys in London period properties, diagnose hygroscopic salts in residential buildings.

How to recognise hygroscopic salts

Chimney breasts

Most hygroscopic salts are found on or near chimneys. They typically show on the surface when a chimney is removed, re-plastered of after a leak, flood or extensive condensation.

Replastering or chimney removal often starts the process.

What is hygroscopic?

Literally – water loving.

Table salt is a hygroscopic salt

Pure sodium chloride, that is table salt, causes moisture to condense when the relative humidity exceeds 75%RH. The form of condensation is called deliquescence. For ease, any salt that causes condensation at or below 75%RH is considered to be hygroscopic.

Surveyor tips

  1. Once you’ve got your “eye in” you’ll start to see the tell tale stain on almost every street.
  2. You’ll start to recognise them roughly 20% of the time in period properties (at least in London).
  3. They are most common on or near ground floor chimney breasts, especially ones that have been opened up, re-plaster or removed.
  4. Understandably, they are often mistaken for rising damp, especially as groundwater also have hygroscopic salts. The way you can tell is rising damp would be even, horizontal and rarely seen above a few brick courses.
  5. You’ll find hygroscopic salts at the top of top floor chimney breasts – often mistaken for roof or chimney stack failure. The way you can tell is hygroscopic salts are colourless and vary according to humidity and temperature, not necessarily when it is raining.
  6. In towns and cities hygroscopic salts that has been on a wall for a long time often looks dirty from soot.
  7. Occasionally you’ll find hygroscopic salts where away from a chimney breast, where manure is likely to have been in Victorian times before cars replaced horses.
  8. It can be a challenge to identify missing chimney breasts, especially if the stacks have been removed. I will look at other similar constructed neighbouring properties and or consider where the chimney was likely to have been.
  9. Nitrate is easy to identify with Palintest (either from Protimeter or exactly the same test at a fraction of the price from Kusuri).
  10. Calcium chloride looks similar to calcium nitrate, so a test failure does not mean that you don’t have a hygroscopic salt.
  11. Hygroscopic salts are not a source of moisture. The amount of water in a property is not increasing. Technically their source is vapour (condensation).
  12. They are benign can’t cause mould or rot, but are considered unsightly.
  13. The easiest treatment is two coats of solvent based stain block primer such as Zinsser cover stain, with a very large overlap, 300mm for smaller spots.

Calcium nitrate

Soil is full of nitrate salts, including calcium nitrate a good fertiliser. Groundwater (the source of water in rising damp) and used in mains water in parts of the country is full of nitrates as is timber used for fires, and probably coal. Manure is full of calcium nitrates.

Critical relative humidity

The critical relative humidity of calcium nitrate is 46.7%RH at 30˚C. Salts become more hygroscopic as temperature increase. Therefore, the damp patches can look worse in summer.

Heat loss

Manure is the second most common source.

And on the hottest days in summer, water can appear to run down the wall.

Here is a selection of illustrative photos.

Looking at the outside can prepare you before a damp survey.

Finding hygroscopic salts

Despite no visible damp, the damp meter reading is off the scale 99.9WME.

On many occasions there is no visible damp.

A kettle can make the hygroscopic salts more viisble.
There is dampness just above the lip over the fireplace.
I managed to extract a reasonable amount of calcium nitrate to test for nitrates and experiment with.
Using the Palintest Nitrate test kit, I positively identified nitrates.

6 thoughts on “Hygroscopic salts”

  1. Interesting article and view. I can understand and appreciate the issue. Our Damp surveyor has suggested this as an issue in our fire place but he suggested the slurry solution. I’m unsure how a gloss paint would act as an effective barrier however and a slurry would not.

    1. Good question. The slurry and oil based primer act in similar ways, i.e. stop the unsightliness of hygroscopic salts.
      The differences are:
      1, Price; £10 – £50 for primer versus £2,000 – £10,000 for slurry.
      2, Primer is quick, easy and inexpensive to reapply if a second application is required.
      3, You only need to cover the patch of damp (with a decent overlap) not the whole wall.
      4, There is no builder’s mess, damage to walls and furniture.
      5, Decoration is easy.

      I’m sure there are plenty of other benefits.

  2. We have the telltale brown staining showing through on anaglypta wallpaper on a chimney breast (remedial roof works complete, increased airflow etc). Would you recommend the wallpaper is removed before using zinsser cover stain? Many thanks

    1. Yes I would, but the brown stain may come from a rainwater leak. Check during a heavy rainstorm. You may have both. For hygroscopic salts, you could take a sample save size of a coin, note colour, perhaps by taking photo against a similar piece of uncoloured wall. Dry it out, possibly with oven or microwave. Photo colour, leave in humid room (65%RH to 85%RH) and see if moisture returns, consider using a damp meter before and after. Hygroscopic salts become damp, a simple leak would not, unless you have a combination.

  3. Thanks for all this very helpful information Simon. I believe we have some sort of hygroscopic salt issue in the corners of the two upstairs bedrooms , below the chimney stack. The old stone chimney was open to the weather for years before we moved in and had it replaced in brick, so my theory is that the leaking rainwater possibly carried salts to these two adjoining corners. The leak was fixed with the new stack, but the high concentration of salts remained in the effected areas, the ‘damp’ spots on which have now been growing and spreading for a number of years, causing the old wallpaper to peel. As you say, they’ve been ‘weeping’ this summer. Today I’d had enough, so I took down 8 sackfuls of ancient plaster from 3 adjoining surfaces in the corner of one of the bedrooms (lath ceiling, lath stud, and stone back wall where chimney breasts are). (Do you think the salts mostly absorb moisture from inside the house (condensation/vapour)? Or could the cause also be down to condensation from unventilated flues etc?) Once I’ve removed a bit more contaminated plaster, I plan to paint on a salt neutraliser , then PVA as you suggest; and then possibly re-plaster with lime based product. Does that sound ok? Also, there no signs of salt patches downstairs – hence my theory that years of rainwater carried salts to the rooms’ corners as it percolated through the sooty stack/flue/chimney areas. If this is correct, what type of salts are they likely to be, and does this effect remedial action in turn? Should I get some contaminated plaster tested? Thanks a lot for your thoughts, best wishes, Jack

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