Replastering Old Damp Walls

Older vs. Newer Buildings

Older and newer buildings have been built very differently. Older buildings have been designed and built to be water permeable. Being built from breathable materials, it allows for a large percentage of the moisture to freely evaporate. Newer buildings, on the other hand, have been built watertight, using modern non-permeable materials (including plastics), designed to keep moisture out.

Mixing old and new materials, often has a detrimental effect on old buildings, leading to the rapid decay of the historic fabric, for the following reasons:

• Moisture: the fabric of old walls often contains significant amounts of moisture. These can originate from multiple sources such as the ground (rising damp, water splashback) or the air (rain, driving rain, sea-spray etc.).
• Salts: water dillutes existing salts in the environment and moves them into the building fabric. When moisture evaporates salts crystallize and expand in volume (5-10 times), leading to the cracking and crumbling of the masonry. Most common sources of salts are rising damp (salts absorbed from the ground), leaks/water ingress (trickling water moves salts around, often to the surface), chimney soot (which contains very high amounts of salts) as well as the air (in form of sea spray, salty winds, or pollution from cars or nearby factories – these salts are washed by rain into the wall fabric).
• Non-breathable materials: the natural degradation of the old building fabric due to moisture and salts is further aggravated by the use of modern non-breathable materials (e.g. cement plasters, foams, plastic paints etc.) primarily designed for newer buildings. These, in older buildings hinder evaporation and the accumulating moisture severely damages the wall fabric long-term. Removing these does not necessarily solve all dampness problems, but restoring breathability often results in a significant improvement.

As a result, the use of breathable materials is recommended, such as lime plasters, that prevent the accumulation of moisture in the building fabric. 

The Common Solution

The ideal solution for old, damp and salty walls is a lime plastering system that satisfies the following requirements:

• Breathable: to let water vapours pass through freely preventing the accumulation of moisture.
• Not too hard: so it won’t damage the underlying softer building fabric but resistant enough to withstand some water pressure.
• Resistant to salts: salts are the primary reason behind the premature breakdown of lime plasters. A good lime plaster should be able to cope with all salts without being damaged by them.
• Long lasting: have a long service life.

Commonly, lime plastering an old wall involves the application of a main lime coat and a lime finish. This checks the first two points above: breathability and softness.

But there is more to lime plastering than just breathability – the problem of crystallizing salts. Crystallizing salts are the leading cause behind the ongoing decay of masonry and the premature breakdown of plastering. The main reason why old masonry crumbles and requires constant attention is because of the presence of salts, not water. 

Salts cause in masonry several important permanent and irreversible changes, reducing the breathability of the fabric. Salts also act as a moisture barrier having a similar effect to cement. These factors must be well understood and taken into account in order to create long-lasting renovations.

The Improved, Right Solution

The right solution for old, damp and salty walls is a breathable plastering system that also takes into account and deals with the damaging effect of crystallizing salts.

Here are at the components of the lime plastering system that checks all 4 points above.

1. Salt-Resistant Lime Base Coat

To mitigate the problem of dampness and salts,

The problem of dampness and salts is mitigated with a traditional lime plaster of which origins stretch back to ancient Rome. Being outstanding architects and builders, the Romans have discovered that by adding certain volcanic soils and other minerals to lime, they can significantly alter its properties, including its mechanical strength and water resistance, while retaining its breathability.

and thus to prolong the life expectancy of the thermal insulation, we always recommend the application of a special lime base coat under the insulation.

Such a volcanic lime plaster performs several important functions:

• Acts as a selective moisture barrier that stops liquid water while allows the masonry to breathe. 
• Acts a salt barrier, preventing the crystallization of salts into the thermal plaster. Salt crystallization is the main cause of crumbling and degradation of plasters and historic masonry - not moisture!
• Consolidates the frail, crumbly masonry, providing you with a dry, solid surface to build on. 
• By stopping the liquid water helps reducing the internal humidity, contributing to a drier, more comfortable environment.

These traditional pozzolanic Roman mortars differ significantly from today’s NHL (Natural hydraulic lime) mortars. NHL mortars are made of lime and clay. For the clay to react with the lime, the mix must be fired at higher temperatures (at about 1200 °C) than lime alone (which is fired at around 850-900 °C). The higher temperatures close off some of the pores, making the NHL plasters less breathable than normal limes. Thus, NHL plasters trade breathability for improved water resistance.

Natural pozzolanic lime mortars are made of lime and volcanic ashes / sands. Because the volcanic ingredients have already been pre-burned by the volcano, these are incredibly breathable and they readily react with the lime at low firing temperatures (850-900 °C), resulting in a very breathable final product.

Various volcanic lime mortars have been extensively used by the Romans in very demanding environments including sewers, ports, spas and aqueducts and they have survived for many centuries. They have also been widely used in Venice, well suited to the humid and aggressive environment of the Venetian lagoon.

2. Main Lime Coat

Once the problem of salts have been taken care of, the renovation goes on as normal, and the "common" solution is applied. A good quality breathable lime plaster is applied next.

For external or cold walls, where improving the thermal insulation is also desirable, the main lime coat can be swapped with a thermal lime plaster.      

3. Lime Finish

The lime plastering job is finished with a good quality lime plaster.

Recommended Materials

Here is a list of high-quality materials recommended for the plastering of old walls.

1. Rinzaffo MGN – Salt resistant protective lime base coat
2. Calcina Bianca MGN – High quality main lime plaster 
3. Calcina Fine MGN – High quality fine lime plaster

Key Application Points

Here are some key points about the application of each plaster coat.

1. Rinzaffo MGN – Salt Resistant Protective Lime Base Coat.

• For above ground level walls one single coat is recommended, in not less than 10 mm thickness. 
• For underground walls or for walls subject to sideways water pressure two coats should be applied at not less than 20 mm combined.
• Extra material should be allowed for uneven walls, which take up extra material.
• Attention should be paid to compress the base coat and close off all pores.
• The material dries quite quickly, typically in 1-2 days, depending on ambient conditions. 

2. Calcina Bianca MGN – High Quality Main Lime Plaster 

• The plaster is applied in not more than 10 mm thickness.
• Typical application thickness is 12-15 mm. 

3. Calcina Fine MGN – High Quality Fine Lime Finish

• For smoother finish the finishing coat is recommended to be applied in 2 x 2 mm coats, with a fine 4-5 mm fibreglass mesh to prevent surface shrinking and fine cracks. 
• A lime finish should be painted with a breathable mineral paint. Plastic, film-forming emulsion paints should be avoided.