Understanding damp – the effect of salts
Many of the problems associated with dampness are really caused by salts, not the moisture itself. In fact, many cases of apparently damp walls are due to a build up of salts and the pores of the wall are themselves air dry (as described in the importance of pores).
The following are typical of the sorts of problems caused by salts (most of the walls shown in the accompanying photographs were air dry):
- brown or yellow staining – some salts discolour the decorations when
they reach the surface. This generally begins as a pale yellow stain on
the wall, making it barely noticeable except from certain angles or in certain
lights. As the salts continue to build up, the pale yellow stains turn into
bold brown stains and grow larger, making them impossible to miss, as shown
Salt stains beside an external door
Salt stains on a chimney breast
- persistent damp patches that vary in intensity with the
time of year or weather – when the salts are more hygroscopic than
the surrounding plaster, they draw moisture from the plaster and air to create
damp patches. This is often called salt damp. The intensity of the
damp patches varies according to the humidity on any given day.
When first seen, the damp patches can be mistaken for condensation that has soaked into the decorations. Unlike condensation though, no droplets will ever be seen glistening on the surface, and it will never (or very rarely) dry as the humidity in the UK is not normally low enough for long enough, to dry out the salts.
The damp caused by hygroscopic salts is often either mistaken for rising damp, or cited as proof of rising damp. However, as can be seen in the two photographs below, the salts have produced identical damp patterns despite one wall being a ground floor wall, while the other wall is on the first floor!
Salt damp - ground floor
Salt damp - first floor
Random salt damp patches part way up an internal wall
- white fluffy outgrowths from plaster – when the relative humidity
next to a wall is very high or the wall is very damp, salt laden moisture
reaches the surface more quickly than it can escape. This causes salts to
crystallise on the plaster surface and form delicate feathery outgrowths,
called efflorescence, as shown below. These are the most benign form of
salts – they can be easily vacuumed off and the salts themselves cause
salts growing out from plaster
- crumbling bricks or plaster – the reverse scenario to the fluffy
outgrowths above, is when the relative humidity next to the wall is very
low, or the wall isn't particularly damp... you would think this is near
ideal. In this scenario though, moisture can escape from the surface much
more quickly than fresh salt laden moisture can arrive. This causes the
salts to crystallise within the pores of the wall, this is called subflorescence
or sometimes cryptoflorescence. Salt crystals in the pores can be extremely
damaging because as the salt crystals grow, they burst the sides of the
pores, destroying the structure of the wall material so it crumbles away
(taking the salts with it).
When the salts cause individual bricks or stones on the outside of your home to crumble, as shown in the two photographs below, the affected bricks or stones will eventually crumble away completely. They will therefore have to be replaced to stop large holes appearing.
External brickwork crumbling due to salts
Bricks in lintel crumbling due to salts
Plaster surface crumbling due to salts
Plaster completely crumbling due to salts
Where do the salts come from?
Innumerable salts are already present in most things in your daily environment – even your own body as they are essential to all life on earth. Many of the materials your walls are built of contain naturally occurring salts. These are supplemented by additional salts that enter from the building's environment throughout its life. The following non-exhaustive list of salt sources gives you an idea of the variety of ways additional salts can enter the pores of building materials:
- soil – all soils contain salts. When soil comes into direct contact with a wall, salts can migrate from the soil into the wall;
- soot and tar deposits in chimney flues – solid fuels can contain huge amounts of salts. When these are burnt, many of the salts are given off in the smoke only to stick to the sides of the flues as the smoke forms soot or tar;
- products applied to the walls – many commonly applied products contain various salts (cement renders and cement mortars are some of the worst offenders), while some contain chemicals that can react with things like lime mortar to damage the mortar and create salts (many damp proofing fluids fall into this category). Every time these products are applied or injected therefore, you are loading the walls up with more salts;
- things accidentally splashed onto the walls – typically things like fertilisers and weedkillers, or if you live next to a road, de-icers. When these are splashed onto the walls, the salts are already in solution so the pores can immediately absorb them;
- atmospheric pollution from industry and vehicle exhausts – this is similar to soot and tar deposits in chimney flues. The range of salts is greater than for soot and tar though, and they are absorbed through the entire exterior wall face of the building, including the faces of the roof tiles or slates;
- rain – all rain contains salts to a greater or lesser degree. If the clouds or rain droplets have mixed with air that has passed over an erupting volcano (e.g. thunderstorms that have formed over southern Europe, dragging in air from Italy before drifting across the channel), or they have mixed with heavily polluted air (e.g. showers formed when Atlantic air hits air from eastern Europe), the salt content of the rain can be much higher. As with things splashed onto the walls, the salts are immediately absorbed by the pores of the walls and roof tiles/slates along with the rain;
- airborne sea spray – you don't have to be able to directly see the sea for this to be a problem. It has been found that airborne sea spray generally travels up to a mile inland, affecting all buildings as it does so (depending on local weather conditions, it can even cause problems hundreds of miles inland). The swirling air around buildings, means the spray is as likely to settle on 'sheltered' walls as those facing towards the sea;
- bird droppings, animal urine and manure – the walls of converted farm buildings that previously housed animals, or the upper storeys of buildings in towns that were previously infested by pigeons, can be severely contaminated with salts from these sources. Leaking soil pipes from bathrooms can also cause the same problems.
How can salts move through walls to cause problems in some places but not others?
As can be seen from the above salt sources, many of the salts that can affect your home have been airborne. It should therefore be no surprise to learn that water vapour can carry salts through the pores like liquid water does. The more moisture is present, the greater the salt load that can potentially be carried, hence more salts can be transported by liquid water than by water vapour. This affects both where salt problems occur and how quickly they appear.
Thus if water vapour regularly travels through a wall into a room, it can cause salts to gradually build up in the plaster as it leaves the confines of the pores. However, unless the wall already contains a lot of salts, it will take an extremely long time for the salts in the plaster to cause a problem – 150 years would not be unreasonable.
Salt problems will arise more quickly if water vapour condenses within the pores of the wall (as described in the importance of pores), because you've then got liquid water available to carry the salts.
The salt problems caused by condensation within the pores are made even worse if a typical 'on-off' central heating schedule is used within the building. If some parts of a wall are warm while other parts are cool, salts will migrate through the walls from cool to warm areas, while the water moves the other way. With a typical 'on-off' heating schedule, the long 'off' periods encourage condensation within the pores (particularly just above the level of the ground floor, where the walls are coolest). The subsequent short intense 'on' periods then draw the dissolved salts into the plaster and drive the water to the outer sections of the wall. This is a very common cause of problems on the outside ground floor walls of Victorian and Edwardian houses. It used to take around 50-75 years for the salts caused by condensation to become a problem, but with modern heating systems, it could now take as little as 20 years.
A special category of salt movement is presented by salts that are so strongly hygroscopic, they dissolve in the moisture they absorb from their surroundings. By drawing in enough moisture to form their own 'pools of water', they can migrate through the pores of an air dry wall in same sort of time as if the wall was extremely damp.
What can be done about the salts?
Contrary to what some people will have you believe, there is currently no effective way to artificially remove salts from walls. You will see references to 'poulticing' whereby a highly absorbent wet material is applied to 'draw' salts from the wall, and you will see references to 'washing' the walls with pure water, with advocates of both methods claiming to be successful. However, longer term monitoring following such treatments have shown that these methods have carried salts from the surface to deep inside the wall, making the long term problems much worse.
Many builders will attempt to persuade you to use their magic 'salt neutraliser'. In reality, this is just white vinegar, which being a weak acid will react with the plaster and mortar. It also reacts with the salts at the surface to form new versions that permanently crystallise within the pores. These new salts block the pores, stopping the moisture behind from escaping, so forcing moisture and salts higher up the wall, again making the overall problem worse in the long run.
To date, the only way to successfully deal with salts in the longer term, is the way it has been done for many hundreds of years. This means deliberately drawing them into relatively soft, renewable surface finishes, which are designed to harmlessly crumble when salts build up in them, automatically shedding the salts from the walls. When these start to crumble, it is the signal that the pores of the material have now 'filled up' with salts and it is time to replace it – which is easy to do because it is soft and crumbly.
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