Rising damp or hygroscopic salts?
Channel: Simon Hichens
Date: 11/01/2021

To Steve Hodgson PCA, Property Care Association May 20

The trigger for my email was the PCA claim that a 2.5m high sloping band of hygroscopic salts were an example of rising damp. There are no scientific studies proving groundwater can cause a sloping salt band or that rising damp can be as high as 2.5M. I believe the cause of the salt band is hygroscopic salts probably from a chimney, or maybe manure splashing onto the wall and that rain and air flow has cause the redistribution into a 2.5M high sloping band. But I only have circumstantial proof and have not investigated the wall.

See the way water can bounce up a wall, potentially splashing up manure. And see the way rain can cause a skin of saturated bricks, creating pathway for the redistribution of hygroscopic salts. The claim and lack of evidence goes to the heart of the rising damp debate. If surveyors, builders and damp proofers are told to believe that rising damp can be up to 2.5M and that unsightly hygroscopic salts are caused by rising damp, then traditional buildings will continue to be damaged with inappropriate treatment. I would estimate that unsightly internal damp patches from hygroscopic salts accounts for about 30% of dampness found during surveys outside of the condensation season, at least in London, with costs of treatment and associated damage and disruption in the order of £100M.

I have zoned in on hygroscopic salts from chimney breasts as these are perhaps easier to understand. However, it is also true of hygroscopic salts from manure, which can be harder to differentiate from rising damp. But the point stands for both, that there have been no scientific papers to describe the redistribution of salts, on walls or in the alternative, no studies demonstrating that water can rise up a brick wall 2.5M as claimed. 

Hygroscopic salts in brickwork, often become unsightly when the plaster around affected walls are replastered. About 1M chimney breasts are altered every year. The damp patch looks like it was created by hygroscopic salts, probably from a chimney breast. The consequence of misdiagnosis is damaging and often ineffective treatment. I find damp patches from these salts in about 30% of my surveys in summer, including in two properties so far this week. See the inside and then outside of some examples, all of these properties were misdiagnosed as rising damp by another surveyor.

Hygroscopic salts are relatively easy to diagnose, as water at normal levels of humidity, evaporates leaving a cold patch. Whereas damp from hygroscopic salts, only evaporates below the critical relative humidity, which is about 50% for calcium nitrate, the most common hygroscopic salt found in homes. It therefore does not show up as distinctly through an infrared camera as normal damp. Nitrates can be easily identified using tablets. The solution is to cover these damp salts in a non-porous medium with substantial overlap. Which can be as simple as coat of gloss paint, followed by emulsion for small damp spots. Removing plaster and covering hygroscopic salts in slurry, as is the standard treatment for rising damp, causes salts to dissolve, drawing them to the new surface, as can be seen here.

I believe salts on this wall, which probably came from a chimney, but might have come from manure, were redistributing through rain, airflow and evaporation. Rainwater can bounce much higher than the standard 150mm gap below a damp proof course. It can cause calcium nitrate from manure to splatter a wall, which can then become drawn up by rain saturated brickwork. The salts, distribute themselves in a sort of goldilocks evaporation band, depending on various factors such as temperature and rain. Look at some salts bands which could not be explained by groundwater alone. Horizontal salt bands are very common. So are vertical bands. Sometimes both together. Sometimes from chimneys, sometimes from manure.

This property is interesting in that the internal hygroscopic damp spots are above the window and doorway, which cannot be explained by rising damp.
Looking outside, we can see both a long horizontal band and faint vertical band above the doorway. The neighbour is an army barracks, so I guess there were stables. Then looking at Google maps over 10 year, we can see that these white salts, found in the centre of the dark band are recent and also found above the door compared to 10 years ago.

Looking through a thermal image you can see heat loss through the short section of wall which is acting as a thermal fin. Paula joined me on a survey a few years ago. Our last property had a good example of a short section of wall acting as a thermal fin, increasing the risk of mould in the corner. The salt band on this wall is both horizontal and vertical. There is a thermal fin, but apparently no chimney stack. I think the fundamental misunderstanding of hygroscopic salts, is significance, and should be investigated and considered in any survey of traditional properties.