Rising Damp
Misdiagnosis of rising damp in existing buildings is widespread, often based on incorrect interpretations of visual evidence and moisture meter readings. This misdiagnosis has become synonymous with a lack of an ‘injected chemical damp-proof course’ due to a successful sales campaign by specialist remedial contractors over the past 40 years. While this misdiagnosis has been profitable for these contractors, it has resulted in a waste of clients’ resources. Original plasters and finishes have been destroyed during the installation process, and unnecessary damage has been inflicted on the original structures through drilling of irrigation holes. Additionally, money that could have been allocated for more cost-effective maintenance or repairs has been squandered.
Injected chemical damp-proof courses may offer some protection for certain structures when properly specified. However, their general application is not often the most cost-effective solution for controlling damp problems in buildings and may be wrongly specified, rendering them ineffective. Water-based products, in particular, may only form an effective ‘hydrophobic band’ when applied to a dry wall after it has dried out, which makes their installation in damp walls problematic.
Rising damp refers to the upward movement of moisture through permeable building materials via capillary action. It becomes problematic when the moisture penetrates vulnerable materials or finishes, especially in occupied areas of a building. This moisture dissolves soluble salts from the building materials and may carry additional soluble salts from its source. When the moisture evaporates through a permeable surface, these salts are left behind, forming deposits on or within the surface. In areas with extensive evaporation, salt crystals form a harmless flour-like dusting. However, when evaporation is restricted to localised areas, such as defects in an impermeable paint finish, concentrated salt deposition occurs, resulting in thick crystalline deposits resembling small flowers, known as ‘efflorescence.’ If evaporation takes place within the material, salts can be deposited within its pores, leading to the formation of fractures and surface spalling. This type of decay is often observed in porous brickwork or masonry.
When moisture penetration has been an ongoing issue, evaporation at the edge of the damp area leaves a distinct ‘tide mark’ due to salt deposition. This tide mark is frequently mistaken as a typical diagnostic feature of ‘rising damp.’ However, these salt accumulations can persist even after the water penetration that initially caused them has ceased. Similarly, water penetration may have occurred due to factors other than ‘rising damp.’
The primary source of moisture at the base of building walls is usually defective ground and surface drainage, which exists to some extent in nearly every building due to rising ground levels, drainage system failures, and inadequate consideration of drainage slopes when using concrete or finishes around buildings.
The presence of ‘moisture reservoirs’ in sub-floor areas can also result from chronic plumbing leaks or flooding caused by catastrophic plumbing or drainage defects.
Damp conditions at the base of walls can be exacerbated by condensation. This occurs when warm, moisture-laden air cools to its dew point against a cold surface. Such cold surfaces often arise when water penetration reduces the insulation value of external walls. Intermittent occupancy with intermittent heating provides ideal conditions for condensation and the accumulation of further water on these cold, damp surfaces, particularly in ground floor bedrooms. These phenomena are the primary causes of dampness at the base of walls, rather than solely ‘rising damp.’
Hygroscopic salts, commonly found in masonry, can also absorb moisture from the air, especially when relative humidity exceeds 75 percent. In rooms with fluctuating relative humidity levels, such as occasionally unoccupied spaces, this can lead to the regular appearance of salt blooms on the surface, known as ‘cyclical efflorescence and deliquescence,’ which damages vulnerable materials.
When addressing issues related to high moisture levels in the base of walls, it is important to recognise that “rising damp” is just one of several factors contributing to such problems. Even when it is a significant factor, it is rarely the primary source of moisture. Therefore, effective management of high moisture levels requires accurate identification of the moisture source and the underlying defect before determining the most cost-effective solution.
There are several measures that can be taken to control dampness and its effects:
Addressing moisture sources: Providing suitable moisture sinks to dissipate the moisture at its source without causing structural or occupant issues is crucial. This involves repairing any contributing defects that act as moisture sources, such as broken pipes.
Implementing moisture barriers: Introducing physical barriers, like damp-proof membranes or hydrophobic materials, can be effective in creating a “damp-proof course” to prevent moisture infiltration. However, it is important to note that using relatively impermeable materials as moisture barriers may not always be cost-effective and can even have counterproductive consequences. These materials restrict moisture movement and hinder drying, potentially leading to long-term moisture retention and chronic issues. They can also trap moisture in permeable materials, resulting in localised areas of increased dampness or even causing moisture to migrate to previously unaffected surfaces, leading to salt efflorescence. Careful consideration and appropriate ventilation measures should be taken when using moisture barriers.
Isolating vulnerable materials: To protect materials such as timber and interior finishes from dampness, it is advisable to isolate them from moisture-prone areas. This can be achieved through the use of damp-proof membranes or by creating ventilated air gaps.
It is worth noting that damp-proof barriers are susceptible to local failures, concentrating moisture and damp problems at those points. Impermeable materials, including those used in tanking systems, tend to exhibit this characteristic. Traditional building techniques, on the other hand, rely on the use of permeable materials and ventilation details to dissipate moisture and prevent it from coming into contact with vulnerable materials or interiors.
The application of “chemical damp-proofing” may provide a temporary solution or a nominal damp-proof course, particularly in walls of uniform construction like sound brickwork laid with strong cement mortar. However, it is important to consider the potential for gaps or deterioration over time, as these can accelerate decay. Chemical damp-proofing is less reliable for walls constructed with natural stone, as the injected hydrophobic material may not distribute evenly where needed due to variations in permeability and the presence of cavities.
To prevent damp problems effectively and affordably, minimising moisture sources and providing adequate passive moisture sinks are key. This starts with ensuring proper ground drainage around the building to minimise water penetration into the foundations. Surface drainage should also be re-evaluated and improved to ensure effective removal of surface water away from the base of the walls.
While “French drains” have gained popularity, they are often poorly specified and can become problematic in UK conditions, turning into “French Moat.” A more traditional and effective approach in the UK involves creating a ventilated and drained “dry area” around the base of the wall. This area is typically covered with York stone slabs to prevent debris accumulation and minimise maintenance requirements. Alternatively, a perforated plastic land drain can be installed in a trench lined with geo-textile and backfilled with materials like “beach cobbles” or large diameter hard core. There are also proprietary external drained cavity systems available as options.
Wall Construction: Addressing Damp Problems and Ventilation
The use of impermeable finishes like sand/cement renders around the base of external walls is a common cause of damp issues. These finishes hinder moisture evaporation from the wall’s base, resulting in moisture seeping into the interiors. Over time, impermeable materials tend to crack, allowing water to penetrate the wall’s base but preventing proper drying. To overcome these problems, it is recommended to opt for traditional breathable lime mortar renders. Additionally, correct detailing of renders to direct water away from the wall’s base and prevent bridging of existing damp-proof courses is essential.
Cavity wall construction, when properly ventilated, can help dissipate moisture and prevent its penetration into the building. However, this ventilation can be compromised by debris or the improper injection of insulation foams. These defects can also bridge existing damp-proof courses, leading to water penetration into interior finishes.
Failures in existing damp-proof courses typically occur due to bridging caused by inappropriate repairs, alterations, raised ground levels, or localised damage from structural movement or poor construction practices. If a damp-proof course is a crucial design element to control moisture movement within the structure, localised repairs are usually necessary. The preferred approach involves “cutting in” a new layer of damp-proof material specifically where needed, instead of injecting hydrophobic solutions into the masonry to create a blanket moisture barrier.
Ventilation is crucial for traditional buildings constructed in damp or potentially damp areas. These buildings often included through-ventilated sub-floor cavities, cellars, or basements, which allowed moisture to evaporate and dissipate before reaching occupied areas or vulnerable finishes. Restoring ventilation, rather than applying additional damp-proof materials, is the solution if ventilation in a basement, cellar, or sub-floor cavity has been restricted. This can occur due to blockage of air bricks by earth and plants or the use of relatively impermeable materials. Re-establishing ventilation helps prevent moisture build-up and the penetration of moisture into vulnerable structures.
As mentioned earlier, reinstating a through-ventilated suspended floor is generally preferable to replacing it with a concrete slab. While moisture dissipation remains important, basements and cellars can still be utilised as occupied areas. However, it is crucial to keep the walls ventilated rather than sealing them. Through-ventilated dry lining systems, instead of impermeable finishes or tanking materials, can achieve this. Traditionally, timber panelling with battens or lath and plaster has been used for dry lining, ensuring proper ventilation behind the panels. Ventilation at the top and bottom allows for moisture dissipation, preventing dampness and decay issues. Similar results can be achieved using modern materials and techniques, with various products available on the market for cost-effective through-ventilated dry lining systems. These include specialist plasterboard systems and studded plastic membranes that create vertical damp-proof course details behind the dry lining. However, a professional survey should be undertaken to determine the right solution for yourself prior to any works being carried out.
CONCLUSION
By understanding sources of moisture found within a building leading to dampness at the base of walls can be easily avoided, despite the loss of traditional skills and the complexities introduced by new materials and occupancy styles. In fact, new materials and techniques can be advantageous when their environmental properties are carefully analysed. On the other hand, misdiagnosing rising damp and applying specific products and techniques without considering the consequences can result in unnecessary waste of money and upheaval for the customer. A more coherent approach to diagnosing damp problems in buildings is a fundamental to our independent damp surveys.
