Part 1 of 2: Understanding the problem
Everyone knows that water damages buildings from above. The failed gutter, the cracked roof, the missing flashing — these are the visible culprits, the ones that get attention, the ones that get fixed.
What gets far less attention is what happens from below.
Rising damp is not dramatic. It does not announce itself with a sudden leak or a visible failure. It works slowly, invisibly, and over decades — and by the time the damage becomes obvious, the process has usually been underway for a very long time. In many historic buildings across Europe, the damage that is visible today began not last year, or ten years ago, but in the postwar decades when a combination of urbanisation, pollution, and well-intentioned but deeply misguided repairs created conditions that have been destroying historic masonry ever since.
Understanding how this works is the first step toward addressing it. And addressing it begins with understanding that the problem is almost never simple, and almost never the same from one building to the next.
How old walls actually manage water
Historic masonry walls — brick, stone, or a combination — are not waterproof. They were never intended to be. They are designed to be breathable: to absorb moisture, to hold it, and to release it through evaporation when conditions allow. The system works as long as the wall can breathe freely in all directions.
This is what is meant by the water balance of an old wall (waterhuishouding). Moisture enters — from ground water rising by capillary action, from rain penetrating the surface, from condensation. Moisture leaves — through evaporation from the wall surface, both inside and outside. When the rate of entry and the rate of departure are roughly in balance, the wall performs as designed. When that balance is disrupted — when moisture is entering faster than it can leave, or when the pathways for evaporation areblocked — problems begin.
The critical point is this: in a breathing wall, the moisture moves. And as it moves, it carries dissolved minerals with it. When the moisture reaches the surface and evaporates, those minerals are left behind — deposited in the pores of the masonry or on its surface. This is the beginning of salt damage.
The three salt families — and why they matter differently
Not all salts are equally destructive. Understanding the difference matters for diagnosis and for treatment.
Chlorides are the salts of seawater and road de-icing salt. Buildings near the coast, or on busy roads that are heavily salted in winter, accumulate chlorides over time. Chlorides are hygroscopic — they attract and hold moisture even in relatively dry conditions — which means a wall contaminated with chlorides is almost always damp, even when the visible source of moisture has been addressed. Chloride damage is chronic and difficult to resolve without addressing the source.
Nitrates are associated with organic matter — agricultural land, animal waste, decaying organic material in the ground. Buildings in rural areas, particularly those surrounded by farmland, often show elevated nitrate levels in the masonry. Nitrate salts are also hygroscopic and contribute to persistent damp conditions.
Sulphates are, for historic buildings, the most destructive of the three groups. They form needle-like crystals as they precipitate out of solution — crystals that expand as they grow, exerting pressure within the pores of the masonry from the inside. This is the mechanism behind the powdering and spalling of historic brick and stone that is visible at the base of so many buildings: the crystals grow, the pore walls fracture, the surface disaggregates.
Sulphates come from multiple sources. They occur naturally in some groundwaters. They wash down from atmospheric pollution — sulphur dioxide from combustion combines with moisture to form sulphuric acid, which then reacts with masonry to form sulphate salts. And, critically, they are present in Portland cement.
This last point deserves to be stated clearly: cement mortar is itself a source of sulphate contamination in historic masonry. Every cement repointing job on a historic brick or stone facade has introduced sulphates directly into the wall. Over time, those sulphates migrate into the adjacent masonry, where they contribute to exactly the salt crystallisation damage that the repointing was supposed to prevent.
Identifying salts — the practical approach
Two of the most common salt deposits have distinctive appearances that allow a preliminary identification on site.
Efflorescence is the white, powdery or crystalline deposit that appears on the surface of masonry as salts are carried to the surface by evaporating moisture. It is typically white or cream, sometimes fluffy in texture, and wipes away easily. Efflorescence itself is not structurally damaging — the salts have already left the masonry — but it is a clear indicator that salt-laden moisture is moving through the wall.
A simple field test: dampen the deposit with water. Efflorescence typically dissolves readily. Calcium carbonate deposits — which look similar but have a different origin — do not dissolve and may fizz slightly when exposed to a weak acid.
Cryptoflorescence is more serious. This is salt crystallisation that occurs just below the surface rather than on it. The crystals grow within the pore structure of the masonry, and the pressure they exert causes the surface layer to detach and fall away — the characteristic powdering and spalling at the base of affected walls. By the time cryptoflorescence is visible, the damage is already done to the surface layer.
The urban context — a century of accumulated damage
For historic buildings in cities and on busy roads, there is a specific and underappreciated source of sulphate contamination that has been accumulating for decades.
The postwar period — roughly from the 1950s through the 1990s — was a period of intense atmospheric pollution in most European cities. Vehicle exhaust, industrial emissions, and domestic coal burning produced sulphur dioxide at levels that are difficult to imagine from the cleaner air of today. That pollution settled on every surface it touched, including the facades of historic buildings, they can be seen as the lungs of the city. It reacted with moisture in the masonry to form sulphate salts. It acidified rainwater that then penetrated the masonry surface.
Simultaneously, the ground around historic buildings in cities was being progressively paved and sealed. Impermeable surfaces — concrete, tarmac, paving — prevented rainwater from soaking into the ground and blocked the natural pathways by which groundwater had previously been able to disperse. The water table rose in some areas. Groundwater accumulated against building foundations without the natural drainage that had previously allowed it to move.
And then came the cement repairs. Well-intentioned, professionally applied, and catastrophic in their long-term effects. Cement repointing sealed the surface of the wall against evaporation — exactly when the wall needed to breathe more, not less. Cement renders trapped moisture inside walls that had no way to dry out. And the cement itself introduced sulphates directly into the masonry.
The result, for buildings that went through this period with cement repairs and impermeable ground treatments, is a masonry that is contaminated with multiple salt types, unable to breathe, and still — decades later — working through the consequences.
The result, for buildings that went through this period with cement repairs and impermeable ground treatments, is a masonry that is contaminated with multiple salt types, unable to breathe, and still — decades later — working through the consequences.
The aluminium foil glued to interior walls behind radiators in the 1970s and 1980s — a popular energy-saving tip at the time — belongs in the same category. With a µ-value of 4,000, aluminium foil is essentially impermeable to vapour. Applied to an interior wall surface, it blocks the evaporation pathway that the wall depends on, trapping moisture and salt within the masonry.
The µ-value — understanding breathability
The µ-value (moisture resistance factor or vapour diffusion resistance) is a measure of how resistant a material is to the passage of water vapour. A value of 1 means the material offers no more resistance than still air. Higher values mean greater resistance to vapour movement.
For historic masonry, the µ-value of any material applied to the wall — mortar, render, paint, coating — determines whether the wall can continue to breathe or whether its evaporation pathway is blocked.
The contrast between traditional and modern materials is stark:
| Material | µ-value |
| Air | 1 |
| Lime putty plaster (air lime) | 7-10 |
| NHL 2 | 15 |
| NHL 5 | 25 |
| Clay bricks | 20-35 |
| Limestone | 20-35 |
| Sandstone | 25-60 |
| Plasterboard | 10 |
| Gloss Paint | 40-200 |
| Cement Plaster | 75-250 |
| Polystyrene foam | 100–750 |
| Aluminium foil | 4,000 |
Air lime plaster, with a µ-value of 7 to 10, is almost as permeable as still air. It allows moisture to move through it freely, maintaining the wall’s ability to breathe. Cement plaster, at 75 to 250, is between seven and twenty-five times more resistant. Gloss paint can exceed 200. Polystyrene foam can reach 750. And aluminium foil, at 4,000, is effectively a vapour barrier.
Every time a low-µ material on a historic wall was replaced with a high-µ one — lime render replaced with cement, limewash replaced with gloss paint, open joints repointed with cement mortar — the wall’s ability to manage moisture was reduced. The moisture that could no longer leave through the surface stayed in the wall, carrying its salts, feeding its crystals, doing its damage.
Every building is different
It would be convenient if there were a standard diagnosis and a standard treatment for rising damp and salt damage in historic buildings. There is not.
The specific situation of each building — its materials, its history of repairs, its ground conditions, its orientation, the chemistry of its local groundwater, its proximity to roads or agricultural land — determines what is happening and what needs to be done. A building on a busy urban road has a different salt profile from one in a rural agricultural setting. A building that was repointed in cement in 1975 has a different set of problems from one that has been left entirely untouched. A building with a solid blue limestone plinth has different ground-level moisture dynamics from one where brick runs straight to the pavement.
Part 2 of this series will look at how to approach diagnosis, what can be done to address the problem, and — critically — what has been lost and what can still be saved.
RestoreFacade Guild — for the craftspeople and owners who understand that the damage you cannot see is often the damage that matters most.
Part 2: The approach — diagnosis, treatment, and what breathable materials actually mean in practice.