‘Rising Damp’ must be the most argued over type of damp ever. A lot has been written about the causes and treatment of “Rising Damp”. Simply put, damp proofing companies maintain that ‘rising damp’ is a type of damp that rises from the ground due to a ‘broken’ or ‘missing’ DPC. The moisture travels up the wall through capillary action within the wall structure to a maximum height of around 900mm (depending on pore size). It then stops at this point as this is where the capillaries in the brick cease moving moisture upwards due to gravity and natural evaporation. Some would question the existing of ‘Rising Damp’.
Misdiagnosis of ‘Rising Damp’ has also become synonymous with a diagnosis of a lack of an effective DPC requiring an 'injected chemical damp-proof course'. Although this has been very good business for the damp proofing companies, it has often resulted in a waste of the clients' money and resources; original plasters and finishes have been destroyed in the process of installation, and unnecessary damage has been caused to original walls by the drilling of irrigation holes. In addition, money that might have been spent on more cost-effective maintenance or repair works has been wasted.
It should be understood that 'Rising Damp' (or ‘capillary damp’) is only one of many ways that cause high levels of moisture in the lower sections of the walls. It is rarely the primary source of the excess moisture and only occurs when there are other factors present. I should point out from the offset that ‘rising damp’ is at the bottom of the list when it comes to the causes of excessive damp in our homes.
The mechanisms involved in the transfer of moisture are complex, particularly so for what I would prefer to call ‘capillary damp’. A capillary does not necessarily have to be a tubular, closed shape, but can be any confined space with respect to its surroundings. There are so many variables involved with ‘capillary damp’ that I can only skim through the issues in a blog such as this.
Interstitial condensation
Interstitial condensation happens within the thickness of a building’s elements, for example inside the floors, walls or roof. The warm air vapour passes through the surface and cools down within the structure, leading to condensation.
When vapour in the air meets a cold surface, it condenses and forms moisture droplets. This cooling point is known as the dew point temperature. If dew point temperature is reached within the fabric of a property such as roof structure, walls, and floor cavities, it will deposit water within these components and cause interstitial condensation. This also reduces the thermal resistance of the surface, lowering the surface temperature which further increases the risk of surface condensation. If the moisture withing the wall is restricted or prevented from evaporating then it can lead to capillary damp. Condensation is the big one and maybe I will write a Blog on the causes of excessive moisture in the home! We’ll see.
Indirect and direct capillary action
In scenarios where there is a normal water table, capillary damp can be caused by indirect capillary action as a result of excess vapour accumulation. Commonly, the water table is situated much lower than the base of the walls. This makes direct capillary action or suction impossible due to the absence of liquid water. Above the water table we have a layer of damp but drained soil (depicted in orange on the sketch below) which evaporates its moisture content slowly upwards in the form of vapours. The moisture content of the soil, even if well drained, is always vapour-saturated, being at 100% RH. This makes the wall capillaries near the ground also vapour saturated, leading to moisture accumulation.
Where there is a high water table, capillary damp is caused by direct capillary action or capillary suction, the known "wicking effect". This only occurs during flooding or very high water table scenarios, when the water table is touching the base of the wall, the wall being in contact with the water table. Although this can happen, it is a relatively rare occurrence, and should be regarded as a special-case scenario rather than being the everyday norm.
Where does the moisture come from?
Where does the water come from that causes capillary damp? The majority of the mainstream answers would be that it comes from the ground. True ‘Rising Damp’ is driven by the ongoing evaporation of groundwater from the wall under the ‘missing’ or ‘broken’ DPC.
Condensation
One source of internal moisture is condensation. The wetting cycle starts with the deposition of tiny water vapours onto solid surfaces. The ongoing accumulation of vapours in multiple layers eventually leads to liquid water accumulation (known as condensation) or to capillary flow. Under the right circumstances vapour accumulation alone can lead to significant wetting. The average family can release over 15 litres of water a day into the air, just by breathing, cooking, showering and washing. This is enough to raise the air moisture level, known as the relative humidity, inside the home to over 70%, which can result in condensation and mould growth.
Damp conditions at the base of walls may be greatly increased by surface condensation. The main reason behind condensation is the temperature difference between the wall surface and the room. This occurs when warm moisture-laden air cools to dew point (the temperature at which moisture condenses) against a cold surface. Such cold surfaces commonly occur when the insulation value of the external wall is reduced by water penetration, as described above. The wetter the wall gets, the colder it gets. For every 1% increase in the moisture content of a wall, there is a 5% reduction in its thermal efficiency.
Intermittent occupancy with intermittent heating provides the conditions for condensation of further water on these cold damp surfaces. These phenomena are the main causes of damp in the base of walls rather than 'rising damp' alone.
In materials science and biology, capillary condensation is the "process by which multilayer adsorption from the vapour [phase] into a porous medium, proceeds to the point at which pore spaces become filled with condensed liquid from the vapor [phase]." Schramm, L.L The Language of Colloid & Interface Science 1993, ACS Professional Reference Book, ACS: Washington, D.C.
In my long experience of surveying buildings and undertaking renovation works to old buildings, I have never come across a damp issue where a ‘broken’ or ‘missing’ DPC was the sole cause of the damp problem.
The likely source of the moisture from the external environment, seen in misdiagnosed cases of ‘rising damp’, is damp is from broken or cracked drains, leaking water pipes, high ground water levels, high ground levels, faulty guttering and downpipes etc. This damp should rightly be called ‘capillary damp’ (I am putting ‘penetrating damp’ aside for the moment as this Blog is about ‘rising damp’ or ‘capillary damp’.
Water Vapour Pathways
I have found that the biggest source of moisture for capillary damp internally is from high vapour content under suspended timber floors and lifestyle. Inadequate underfloor ventilation leads to water vapour condensing on the base of the wall which then rises above the skirting height in the room above as it seeks to equalise with the drier air above. The source of this moisture is from leaking water pipes, leaking drain pipes, high ground levels, high water tables etc.
As water vapour, moisture moves from a centre of high concentration to one of low concentration. For example, if there is 85% relative humidity under a suspended floor and 50% relative humidity in the habitable space directly above, then moisture will move into the habitable space. This is not dependent on air flow. The moisture in the air is trying to reach Equilibrium via ‘Vapour Pressure’.
A relatively common example of misdiagnosed ‘rising damp’ is when a suspended timber floor has been replaced with a concrete floor. Before the installation of the concrete floor, moisture vapour would have been able to evaporate from the ground under the suspended floor without affecting internal finishes. However, the new impermeable concrete floor causes the moisture to remain trapped beneath, forcing it to the sides of the room and into the base of the walls. The resulting damp problems are then often used as justification for the injection of a chemical DPC and the removal and replacement of plaster with impervious cement render.
Vapour movement is the most fundamental moisture transport mechanism because it can take place anywhere. Wherever there is air, there are vapours and thus humidity present. As such, vapour movement affects both the wetting and drying process of a wall as both rely on vapour movement. The rate of diffusion through the structure will depend on its vapour resistance and the vapour pressure differential.
Generally speaking, I have found that capillary damp in walls it is a manmade problem caused by the restriction of vapour movement. In particular, where Damp Proofing Companies have installed waterproof cementitious renders over damp walls. These actions simply trap moisture in the wall and drive it higher up the wall via capillary action which only makes the wall wetter by trapping the moisture within the wall, preventing evaporation of the water vapour.
When you see "breathable" on labels for paints, plasters, or insulation, be cautious if the claim only pertains to water vapour,. "Breathable but waterproof" is a huge red flag, as these products might trap moisture inside the walls, leading to dampness and structural damage.
The lack of vapour movement is where capillary damp (as well as most dampness problems) eventually starts. Many problems - condensation, mould, cold and unhealthy buildings, persistent dampness issues - can be traced back to lack of ‘breathability’ - the use of non-vapour-permeable materials. True breathability involves supporting the building's ability to handle water in all forms—liquid and vapour. Materials that are capillary active, like non-hydraulic lime, allow walls to naturally transport water from the interior to the exterior, preventing moisture build-up. Hygroscopic materials (which absorb and release moisture from the air), help balance indoor humidity.
Concentrations of hygroscopic salts, which are often found in masonry, can also absorb moisture from the air, especially at relative humilities above 75 per cent. In a room that is sometimes unoccupied, with fluctuating relative humidity levels, this can result in the regular appearance of salt blooms on the surface (‘cyclical efflorescence and deliquescence’), resulting in damage to vulnerable materials, and giving the appearance of rising damp.
Materials that are capillary active, like non-hydraulic lime, allow walls to naturally transport water from the interior to the exterior, preventing moisture build-up. Furthermore, hygroscopic materials, which absorb and release moisture from the air, help balance indoor humidity. This natural regulation is vital for old buildings which are sensitive to moisture and humidity changes. Once vapour movement is blocked, hidden moisture sources are created that can (and often will) lead to various forms of dampness problems.
The rising moisture will eventually evaporate unless there are other factors that can prevent evaporation.
Evaporation
Evaporation is an important factor in controlling capillary damp. The factors controlling evaporation include: Temperature, Humidity, Air movement and Surface condition, for example has the wall been painted with an impervious coating? Has it been pointed or rendered with impervious sand and cement render or mortar? Has free air circulation been blocked by objects or plants etc that have been placed against the wall?
The slow process of absorption of water into the structure with subsequent evaporation leads to the gradual deposit of salts in the wall. The masonry acts as a filter system for impure water as the various soluble salts are drawn into the wall and then left behind. What will happen in these situations is that the water during its capillary ascension will carry dissolved salts to higher levels. When the surface of the wall is reached, the water evaporates, the salts crystallize with increase of volume, are deposited there, progressively decreasing the size of the pores. This decrease in pore size will hinder the evaporation of water and a higher capillary rise will occur. The size of the pores influences the progression of rising damp, being higher the smaller the pore diameter.
Moisture content is increased in the mortar from the hygroscopic (i.e. tending to absorb moisture from the air) nature of the salts with the possibility of attracting further moisture into the wall. This contribution would be relatively small in comparison with capillary moisture. However, this can have a bearing on the ‘tide mark’ seen on the inside of a damp wall.
Rising moisture will eventually evaporate unless there are other factors that can prevent evaporation. Anything which restricts the rate of evaporation or prevents evaporation will contribute to the saturation level of the wall.
How to treat the effects of ‘capillary damp’
One of the most important things to do is to first remove the source of the water vapour, be it from internal condensation, leaking water pipes, broken drain pipes, faulty guttering etc. (I am excluding the effects of high water tables here as this is uncommon and requires very specific treatment).
Lower high external ground levels. This may include the provision of adequate ground drainage around the building to minimise water penetration to the foundations, and the re-detailing of surface drainage so as to ensure surface water is drained clear of the foot of the walls. A perforated plastic land drain can be laid to falls in a trench lined with geo-textile and back filled with ‘beach cobbles” or large diameter hard core. Proprietary external ‘drained cavity systems are also available.
Facilitate evaporation. Ventilation of the base of the wall should be the starting point. Wall base ventilation increases evaporation and leads to a reduction in the damp level. This of course is possible only when the groundwater is lower than the base of the wall.
If there are suspended timber floors to the ground floor then its imperative that there is adequate underfloor ventilation. This will prevent base-of-wall condensation (as well as protect the flooring timbers). When we hamper evaporation by applying impervious coatings, the level of ‘rising damp’ will increases. Rising damp increases when drying conditions are hampered by using more waterproof coatings. Faster ‘rising damp’ is always observed on the wall side where evaporation conditions have been hampered.
So, impervious coatings should be removed from the wall - brickwork pointed with sand and cement mortar should be replaced with lime mortar - excessive internal moisture vapour should be managed by installing humidistat controlled mechanical extractor fans - high external ground levels should be lowered.
Increasing the temperature of the air within the building will allow the air to carry more moisture vapour and increase the surface temperature of the wall surfaces thereby lowering the dew point and facilitate evaporation from the wall. The climatic conditions of the environment affect the drying process and influences the level reached by the ‘capillary damp’. In places with high relative humidity, evaporation will take place with greater difficulty and consequently there will be a greater degree of ‘capillary damp’. On the contrary, when the relative humidity is low, evaporation will be maximum and the level achieved by the damp will be lower.
The system works well so long as routes for absorption and evaporation remain unimpeded. Problems arise when modern materials are introduced to the system – vapour barriers, damp proof membranes, impervious plasters, renders and paints – and the walls can no longer ‘breathe’ and naturally allow moisture to diffuse through the wall. The introduction of such modern materials breaks the system by disrupting routes for absorption and evaporation and this can result in serious damp problems.
The health of an old building lies in its walls' ability to manage moisture effectively. Prevent moisture issues by selecting materials that are capillary active and hygroscopic, ensuring good ventilation, fixing leaks promptly, and maintaining proper drainage around the property. Regular inspections can also help identify and address moisture problems early.
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