Rising damp is generally believed to be capillary action: the walls acting as a sponge wicking up liquid moisture from the ground making the walls wet. Although this is a good analogy to illustrate the concept of capillarity, real walls behave differently.

The reason why rising damp needs attention and must be addressed is the presence of ground salts rising damp draws up from the soil  into the masonry. The real problem is not the water or humidity but the salts accompanying rising damp.  

Contrary to common belief it's not moisture that damages the wall fabric or the plastering. It's the salts. High moisture content alone does not damage old buildings. Salts do.

Although water can cause damages to lime plasters through flooding, longstanding water ingress or frost, in the vast majority of cases, clean rainwater evaporates without a trace without causing any damages. Thus, dampness caused by clean fresh water is fully reversible.

The real problem, however, is when dampness is combined with minerals or salts. In the presence of salts everything changes. Salts are hard, solid minerals that in the presence of water temporarily go liquid. However, when humidity evaporates the salts re-crystallize and expand in volume 5-10 times. The crystallization or mechanical expansion of salt crystals is what makes old masonries or plasters crumble, salts being responsible for far more damages than water ingress and frost. This is a very important technical point that needs to be understood.

On real buildings the effect of salts can be very difficult to assess, as dissolved salts become invisible to the eye and touch, only dampness being obviously present. However, under controlled laboratory conditions the effect of salts can be easily demonstrated, measured and understood.

Here is a lab experiment: when two identical bricks - one salty, the other one not - have been subjected to high air humidity variations for an extended period of time, the salty brick has crumbled badly while the non-salty brick stayed completely intact in a high humidity environment.

So salts are the real problem, however it's water that carries them everywhere, contributing to their damaging effects.

Dealing with high humidity and the destructive effect of salts requires special lime plasters. These are ancient plaster mixes initially used by the Phoenicians and Greeks, then rediscovered and perfected by the Romans throughout the centuries. Being outstanding architects and builders, the Romans have observed that adding to lime certain volcanic sands and ashes (also known as natural pozzolans - highly porous and breathable lava materials), these can significantly alter the properties of lime while retaining its breathability.

Through the skilful use of different types of volcanic sands and ashes, the Romans have created many types of lime mortars with outstanding properties - some of them becoming waterproof or able to set underwater, or becoming highly resistant to salts resisting seawater.

These 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, as they perform extremely well in the humid and aggressive environment of the Venetian lagoon.

Traditional pozzolanic Roman mortars differ significantly from today’s modern NHL (Natural hydraulic lime) mortars.

NHL mortars - a mix of lime and clay - need to be fired at higher temperatures than normal (air) limes for the lime and clay to react. The higher firing temperatures and more heat partially closes off the pores of NHL plasters, reducing their breathability. Thus, NHL plasters trade-off breathability for improved water resistance.

Roman mortars use lime and volcanic materials. These are mixed at room temperature without heat as the volcanic sands and ashes have already been pre-burnt by the volcano. This keeps the pores structure open resulting in high breathability. The volcanic ingredients make the plaster highly resistant to dampness and salts, making these Roman mortars perform extremely well in damp and salty environments.

Thus, the Rinzaffo MGN plaster is a lime plaster that acts as breathable water and salt filter. Due to its small pores it blocks liquid water. Due to its open pore structure allows the wall fabric to breathe. Having no salt content of its own, it does not introduce salts into the masonry. Being waterproof, it becomes impervious to salt penetration and crystallization, resulting in an extremely long service life even in very demanding and damp environments (e.g. basements).

The vast majority of insulating solutions on the marketplace, however, have been developed for newer buildings. They offer good thermal insulation, but modern materials – foam, plastic, metal foil, cement etc. – are non breathable, they trap moisture which can lead to future dampness problems.

Due to increasing energy costs, government pressure as well as the need for greater comfort, breathability is often sacrificed, leading to severe dampness problems in old building years or decades later.

Using breathable insulation materials in old buildings, such as lime thermal insulation, is very important for the long-term health and the integrity of the building fabric.