The Main Challenge - Salts
After the installation of a damp proof course, renovation is the second major phase of handling rising damp.
The main challenge in renovating after rising damp is the presence of GROUND SALTS.
Walls affected by rising damp contain a significant amount of ground salts. These salts have been drawn up by the rising damp and deposited into the wall fabric over many decades. When salts crystallize, they expand in volume, leading to the breakdown of the wall fabric. Crumbling, flaking and loss of building fabric are primarily caused by salts.
Why Plasters Break Down?
Here is the typical life cycle of a fresh coat of plaster applied onto an old wall. Here are the reasons and the mechanism of why plasters break down periodically.
- Salt contamination of fresh plaster: the old wall fabric under the plaster is salty. When fresh plaster is applied onto the wall, the water from the plaster dilutes these salts, which start migrating from the wall into the fresh plaster. Your fresh plaster thus becomes salts contaminated by dissolved salts.
- Internal expansion of salts: the drying of plaster results in the formation of expanding salt crystals. Because plasters have some porosity (e.g. free internal space), the crystallizing salts initially expand into the internal pore space of the plaster (see image above).
- Evaporation draws more salts into the plaster: heating and ongoing evaporation moves humidity and salts from the depth of the fabric towards the surface (especially in the case of breathable lime), resulting in a build-up of salts inside the plaster. As long as the pores of the plaster have enough internal space to deal with the expanding and crystallizing salts (known as subflorescence), the plaster stays visually intact.
- Mechanical breakdown: once the pores of the plaster are filled with salts, significant mechanical pressure start building up inside the plaster, leading to its cracking and crumbling, Salts can also crystallize on the surface, forming a white ,powdery coat, known as efflorescence.
Depending on the type of plaster used, the breakdown cycle can be fairly quick (months) or much slower (a few years), but if rising damp is ongoing in the wall, the problem will eventually reoccur. This leads to an ongoing, never-ending renovation and replastering cycle in old buildings, repeating every few years, costing time and money.
How Different Plasters Behave in the Presence of Moisture
Different type of plasters behave differently in the presence of moisture. Here is a quick summary of the main plaster types used for the renovation of old buildings:
Most old building owners know that lime plasters are breathable, and as such be used in old buildings for renovating after rising damp. This is true, but it's not the full picture. When mentioning lime plasters, we mean "regular" lime plasters made of hydrated or aerial lime. These plasters are indeed soft, breathable and they gradually harden (carbonize) in contact with the air.
The longevity of lime plasters in a dry, salt-free environment (e.g. upper floors or internal walls) is really long, they can last for decades.
However, in the presence of moisture and especially salts, the situation changes significantly. Having a weak pore structure, lime plasters are easily damaged by crystallizing salts, reducing their longevity to as short as a few months to a few years - depending on the intensity of water flow (rising or penetrating damp) and the amount of salts in the wall fabric.
As such, regular lime plasters are not suitable for renovating after rising damp, yet this is a common practice in the UK, due to the lack of knowledge about the nature and mechanism of salts. Applying a regular lime plaster on any wall surface of the ground floor of a building older than 100 - 150 years is not recommended as it opens the door to problems.
Natural Hydraulic Lime (NHL) Plasters
Natural hydraulic lime plasters (short: NHL plasters) are stronger lime plasters, more resilient to humidity and salts. This is achieved by using less pure lime stone, containing clay and other minerals (impurities). These impurities react with the lime and form water-resistant compounds, resulting in a stronger lime plaster. Because these plaster are able to harden (set) in the presence of water, they are called hydraulic lime plasters.
The amount of impurities used in the mix (5 - 30%) determines the hardness and water resistance of the plaster - the more impurities, the stronger the plaster. Thus we have:
- Weak (feebly hydraulic) NHL plasters [marked as NHL 2, with 5-10% impurities]: used for internal plastering
- Moderately hydraulic NHL plasters [NHL 3.5, with 11-20% impurities]: used for pointing and bedding
- Strong (eminently hydraulic) NHL plasters [NHL 5, with 21-30% impurities]: used for foundation works, flooring and chimneys - as a guideline.
Because NHL plasters are burnt in the kiln at higher temperatures (up to 1200 °C) than "normal" lime (around 850 - 900°C), the higher firing temperatures affect their breathability, resulting in less breathable plasters.
In summary: NHL plasters offer better water resistance and mechanical resilience than normal lime plasters, to the detriment of breathability, and they also don't provide protection from the harmful effects of crystallizing salts.
Cement plasters pushed up the percentages of impurities up to 40% and firing temperatures were raised up to 1500 °C, resulting in a complete melt-together of silica and alumina in a product called clinker.
Cement-based plasters or eminently hydraulic NHLs (namely cements), have a practically absent pore structure, so they are non-breathable and as a result they trap humidity, resulting in the degradation of the wall fabric.
Their high mechanical strength makes them 5 to 8 times more rigid than lime plasters. Temperature and humidity variations that generate high tensile stresses lead to the cracking and debonding of cement plasters from the surface of the walls. As such, for technical reasons, they are absolutely not compatible with older buildings built and plastered with lime.
To cope with the problem and dampness and salts, special plastered have been developed for the replastering of walls following a rising damp treatment. These are known as renovation plasters. Several types of such plasters exist, some of them act as water and salt blockers, others as salt buffers by absorbing the salts, these need to be periodically replaced,
The vast majority of renovation plasters are cement plasters, they are non breathable and as such not recommended for old or listed buildings. Lime-based renovation plasters also exist, but they contain cement and various chemical additives.
Gypsum plasters are primarily used as finishes over cement plasters. Their weak pore structure makes them suitable for nice and smooth finishes.
Gypsum plasters have the ability to bond moisture, and in the presence of moisture and salts they completely disintegrate, having an extremely short lifetime.
- There are many types of plaster on the marketplace, including lime, cement and gypsum based plasters. These are all used for the renovation of old buildings.
- Lime plasters are soft and breathable but they can't withstand moisture and humidity.
- Natural hydraulic lime (NHL) plasters are more durable, but their breathability is impaired and they are damaged by crystallizing salts.
- Cement plasters are not breathable and as such they should be avoided in old buildings under all circumstances.
- Most renovation plasters are cement based. The lime based on have added cement and based on practical feedback, they do not perform very well long term.
- Gypsum finishes, used on top of cement plasters are easily destroyed by humidity and salts.
None of these plasters is ideal for renovating after rising damp, all of them having trade-offs - they are either breathable but too weak, or stronger but less breathable.
The ideal plaster for replastering old, salty walls would be a plaster that it is:
- Breathable: like a normal lime plaster
- Water resistant or waterproof: so it can withstand humidity, without being damaged by it
- Salt resistant: to efficiently block the salts, protecting the finishes from salt crystallization
- Mechanically resistant, to but not too hard for the underlying weaker historic fabric
- Should have the desirable properties of lime: warm, anti-bacterial, flexible etc.
Yes, such an ideal plaster for renovating after rising damp, that ticks all the above boxes, exists. It is a historic lime plaster, its origins are leading back to ancient Rome.
Roman Historic Lime Plasters
Being outstanding architects and builders, the Romans have experimented extensively with lime, formulating many plaster mixes that stood the test of time. They have discovered that by adding volcanic soils and other minerals to lime, they can significantly alter its properties, especially its strength and water resistance, while retaining its breathability.
The most commonly added materials were pozzolans (volcanic soils or rock fragments) and cocciopesto (milled bricks or terracotta fragments). They reacted chemically with the free lime, forming water resistant compounds. Such mortars were able to harden quickly not only in the presence of water but even underwater in the total absence of air, and they are known as hydraulic mortars. They have been extremely popular in Venice, well suiting the humid and aggressive environment of the Venetian lagoon.
The Right Lime Plaster for Rising Damp
The study of historical writings and an extensive technical research program resulted in the development of a unique lime plaster for the problem of rising damp and salts. This plaster, Rinzaffo MGN, based on ancient Roman technology, is breathable, waterproof (stops liquid water but allows the passage of water vapors), salt resistant, while also possessing all important properties of lime plasters.
The Rinzaffo MGN base coat, due to its unique pore structure, acts as a salt-barrier, retaining the salts inside the wall fabric while allowing the passage of vapors to the subsequent macroporous lime evaporation layer. By regulating the evaporation, it prevents the rapid crystallization of salts responsible for the detachment of plaster and the deterioration of masonry. As a result, the surface of the finishing stays dry and decorations remain aesthetically pleasing.
Since its inception in 1980 Rinzaffo MGN has been used extensively in the Venice lagoons withstanding very demanding conditions and extreme saline aggression. After 40 years of service life, Rinzaffo plasters still perform extremely well without breakdown or structural damage despite of being periodically submerged into the “high waters” of the Venetian tides.
You can read more about the development and history of Rinzaffo here.
What Plaster Coats to Apply
To provide a long-lasting plastered finish to old walls affected by rising damp and salts, the following plastering coats are recommended:
- Lime base coat: RINZAFFO MGN [7-10 mm]: a microporous breathable lime waterproof and salt-resistant base coat. Its main role is to prevent the migration of salts into subsequent layers, which cause the premature breakdown of the plaster. Rinzaffo’s internal pore structural is formulated in a way to block (larger) liquid water and salt molecules, while letting (smaller) water vapors molecules through, facilitating evaporation.
- Lime main coat [15-20 mm]: a good quality lime plaster such as Calcina Bianca MGN, a premixed lime plaster based on the highest purity aerial lime (over 90% calcium hydroxide), a small quantity of natural hydraulic lime and washed river sands without impurities.
- Lime finishing coat: [3-4 mm]: a good quality lime finish such as Calcina Fine MGN is a premixed white lime finishing plaster based on the highest purity air lime (min. 90% calcium hydroxide).
- A breathable paint: such as lime wash or mineral paints.
The missing ingredient here that prevents the premature breakdown of the plaster is the salt- and water-resistant lime base coat - Rinzaffo MGN. As part of the Roman heritage, it is widely used throughout Italy for renovating after rising damp, but it is largely unknown in the UK.
The longevity of lime plasters in a humid, salty environment can be 10X with the use of Rinzaffo MGN base coat, saving unnecessary replastering and money long term.
An Alternative - Plasterboarding Done the Right Way
In addition to the above described plastered finish, plasterboarding is also an alternative for a more modern looking finish.
Plasterboarding is the modern version of the Victorian lath and plaster technology, and it is an easy, fast, cost-effective solution for renovating after rising damp, that can be applied on older building,
One important note here: dot and dab MUST be avoided. Dot and dab, also known as drylining, is a very popular low-cost plasterboarding technique that involves the gluing of the plasterboard directly onto wall surfaces. Walls finished in this way can be painted almost immediately, resulting in time savings for tradesmen.
Although a budget-friendly option, it should be avoided on any external walls (due to condensation and penetrating rain) or on any walls prone to damp and salts (e.g. ground floor walls, or walls with rising damp or following a rising damp treatment) as over time moisture and salts can migrate through the glue into the plasterboard, resulting in surface damp patches, ruining the finish.
The plasterboard MUST be mounted on battens instead, preferably galvanized steel battens. You can read more about plasterboarding here.