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.
Plaster dries slowly, sometimes never, despite no source of water or thermal bridge. Hygroscopic salts migrate away from the chimney often along a level.
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
- Once you’ve got your “eye in” you’ll start to see the tell tale stain on almost every street.
- You’ll start to recognise them roughly 20% of the time in period properties (at least in London).
- They are most common on or near ground floor chimney breasts, especially ones that have been opened up, re-plaster or removed.
- 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.
- 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.
- In towns and cities hygroscopic salts that has been on a wall for a long time often looks dirty from soot.
- 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.
- 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.
- Nitrate is easy to identify with Palintest (either from Protimeter or exactly the same test at a fraction of the price from Kusuri).
- Calcium chloride looks similar to calcium nitrate, so a test failure does not mean that you don’t have a hygroscopic salt.
- Hygroscopic salts are not a source of moisture. The amount of water in a property is not increasing. Technically their source is vapour (condensation).
- They are benign can’t cause mould or rot, but are considered unsightly.
- 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.
The chimney breasts was removed in this rear kitchen / recepotion The neighbours wall has similar spots. This is a close up with the sofa removed. A simple sand and cement, with bonding, was used. Unfortunately salts dissolve in water and move to the new surface. Thermal images help differentiate a source of water, constantly evaporating, causing heat loss. Hygroscpic salts are in dynamic equilibrium, with little heat loss.
Heat loss
There is a pilot study undertaken to improve understanding of salt movement. The left have upside down triable is just water, the right-hand triangle is diluted calcium nitrate. The same volumn was applied in a simiular manner with a paint brush.
Manure is the second most common source.
This inside wall conforms with the outside. Unusually there was no chimney breast, bt it is likely to have been near manure.
And on the hottest days in summer, water can appear to run down the wall.
This wall was weeping with water during a summer’s heat-wave 2019. The property is an old barn conversion near Birmmingham. Note how window has no affect on the band of nitrates. See how wet the wall is by the reflection of this Protimeter datalogger. The infra is intersting. This wall was treated for rising damp, then dry lined with water soluable adhevisive. The hygroscpic salts mitragted through the adhesive.
Here is a selection of illustrative photos.
This was the basement in a bank after a flood.
Most walls started to dry out, but for this chimney.Here is a top floor chimney breast. The dark colour can make it easier to see. Here the raditor is just below the hygroscopic salt patch. Another wall where the chimney breast was removed. Angain dribble marks after a warm spell. Contractors often recommend removing “so called” “contaminated” plaster. But plaster replacement uses an aquious slurry, causing salts to migrate to the new surface.
Zinsser is oil based primary better, cheaper, less disruptive and easy to reapply.Damp wall Damp wall
Looking at the outside can prepare you before a damp survey.
2 M up coner of a wall, with a slope. Horizontal band Horizontal and two vertical bands. Vertical and horizontal band Vertical and horizontal. Vertical and horintal – near of army barracks (stables)
Finding hygroscopic salts
On many occasions there is no visible damp.
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.
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.