The truth about Salts in the walls of old buildings
Rising Damp wallies sell something like £300 million pounds a year of unwarranted damp proofing treatments on the basis of 'damp meters' which actually measure salt content of walls, by way of conductivity.
They claim salts are present in the wall because 'rising damp' climbs the wall, bringing with it salts from the ground. Apparently, our soils are saturated in salts – quite where from, I'm not sure – they cite agricultural fertilisers as one source. Not too many of those in the middle of our cities. Broken drains ARE a source – so when we do 'damp' surveys, we are always on the lookout for broken foul drains with inevitable escape of urea – nitrates, into the surrounding soil.
Damp industry claims about why salts are present in walls are complete rubbish.
I hope the following explanation will go some way to helping people understand just what is REALLY happening out there. I have to thank my friend Carole Ryan, one of the founder members of IHBC, and Senior Lecturer at Bournemouth University for providing guidance on these issues. Her wonderful and insightful book on the subject is referenced below – it is essential reading for anyone serious about Sustainability in older buildings.
Why are our buildings full of Salts?
The very air we breathe is loaded with acidic gases. The most common is Carbon Dioxide, closely followed by Sulphur and Nitrogen based gases. All of these are constantly reacting with our buildings, degrading the fabric and producing salts.
Owners of older properties – solid walled buildings – will be more familiar with crumbling stone and plaster than owners of houses built more recently. This is not just because the materials are more prone to damage. The Victorian Era was dominated by industrial growth – thousands of factory chimneys belched toxic fumes – Contemporary accounts of Ironbridge Gorge in the grip of the Industrial Revolution graphically describe the thick, suphurous fumes that clogged the Gorge. It is probably true to say that during this period, the air was more loaded with acid gases than even now – and damage to old buildings greater then. Cars have now taken over as the major cause, and falling emissions are balanced by the ever growing number of cars and trucks spewing diesel exhaust.
The commonest acid is that formed by the action of rainwater on Carbon Dioxide – forming Carbonic Acid. It is this acid which is responsible for the formation of the vast network of caves under many of the limestone Districts of England. It dissolves calcium carbonate – the main component of limestone, to form Calcium Bicarbonate – actually better known as baking powder. The problem is, it's VERY soluble – so the moment it forms, it washes away. And so the stone gradually melts.
The problem is, it's not just limestone that is affected. It is ANY stone or brick with calcium carbonate as the main binder between the grains. So any 'carbonaceous' stone – a sandstone for example with calcite binding, will erode as the binder is dissolved. That's why you often see sandstone blocks at the base of walls just turning to sand – especially alongside busy roads.
When I worked in the gold mines of Australia, the ores were 'roasted' to break the rock down. Sulphur dioxide was the stuff that went up the stack – and we frequently got trapped in the opencut by clouds of fumes that made us choke and cough – our faces were burned raw. This is the same gas that reacts with water to form sulphuric acid – the stuff in car batteries. It's nasty.
This stuff comes out of car exhausts and chimneys, and reacts with carbonates to form gypsum – calcium sulphate, which is a bit less soluble than baking powder, but still washes away in the rain. It can form crusts and blisters on the stone, protecting the surface, but acid attack still goes on underneath, softening the stone further. The sulphate reactions are quite complex, but all you need to know is that when you see the lovely stone of a mellow old limestone building sculpted beneath the cornices and mouldings – it's traffic fumes that are probably the cause.
There's another pretty nasty acid out there – Nitric Acid. The air we breathe is full of nitrogen – and when it combines with oxygen it forms Nitrogen dioxide. This reacts with water to form nitric acid. A similar reaction then takes place to that of sulphuric and carbonic acids – attacking the weak carbonate structure of building materials – this time producing nitrates rather than sulphates.
All of these salts are very soluble, and easily washed away – so the stone just degrades. These wet solutions containing salts are able to invade the stone or brickwork. They are soaked into the masonry during times of wet or damp weather – travelling deep into the stonework through it's pores, cracks and capillaries. This means that your walls now contain a lot of salt – and it did not come from rising damp. It can travel down the walls, into them, as well as up them. A major source of salts is from chimneys – fires burning fossil fuels provide a cocktail of acids, which attack the binders of the mortar, and produce salts which then soak the masonry. Nearly every time we see a survey from one of the damp companies, PCA member or not, they run for the fireplace and stick their silly 'damp meter' into it. Of course it goes off the scale – its packed with conductive salts from the fires, and they will travel many metres either side of the chimney too.
Damage from salts crystallising in masonry.
Wet masonry dries out. Salts are formed by acidic rain. It stops eventually and the stone and brick dry out. The salts stay there – they cannot evaporate. NOW we have a problem.
As the stone dries, these salts start to crystallise within the pore spaces of the stone. If they don't fill the pore the first time, next time it rains, more solution is fed to the growing crystal, and it gets bigger – eventually filling the pore completely. A bit like growing copper sulphate crystals in the lab at school when you were a kid. The trouble with these crystals is that they are VERY powerful, and start to 'jack' the stone – as they expand and grow, the stone literally explodes, and crumbles.
Hygroscopic salts in masonry
If you've come to this site because you've got those powdery fluffy deposits all over your plasterwork inside (that the idiot from the Property Care Association calls rising damp) then you now know it's nothing to do with rising anything – its the action of acid water on masonry – internally the water being provided by us – from condensation originally. There is another twist to the tale here – these salts attract water. Remember when you put salt on the kitchen table? It gets wet. Well so do the salts in your wall. If the air in the room is warm and humid, the salts in the wall go 'yippee' and soak up water and the wall looks wet. If the walls are plastered with lime, the effect of salts is a lot less, and takes many years to do any damage – if kept dry, walls can be soaked in salt, but still function perfectly. If plastered with gypsum, it just blows, peels, bubbles and flakes, and falls off. Gypsum is horrible stuff when used in old houses, and just doesn't work.
