In a previous section we have looked in more detail into some of the mechanisms and factors that control breathability. We have concluded, that the main factors that affect the breathability of porous masonries are:
As the physical properties of a building material are given and little can be done to change them, in this section we would like to look in more details at the last 2 points: the effect of electro-chemical properties and of added plasters and renders to the movement of moisture in porous materials.
Old masonries are often plastered, rendered or painted for functional or decorative reasons. These materials come in a wide variety of choices - however most of them have been developed for newer buildings.
Older buildings have been designed and built to be water permeable. Breathable materials allow a large percentage of the moisture to evaporate.
Newer buildings have been built watertight, using modern non-permeable materials (including plastics), designed to keep moisture out.
As a result of these fundamental differences, mixing old and new-style materials on old buildings can have very detrimental effects, leading to the rapid decay of the weaker, older fabric for the following reasons:
Removing these moisture barriers is highly recommended, however their removal does not always returns the masonry to its previous state as some damages caused by salt are irreversible. However, removing the moisture barriers and restoring breathability reduces the moisture content of the masonry, which in itself often results in a significant improvement.
To maintain the compatibility of plasters with the old building fabric, the use of lime plasters is recommended. Cement plasters should be avoided as lime alternatives exist for virtually every situation.
The main reason moisture bonds to solid surfaces are electrochemical charges, also known as surface energies or surface charges. These are small electrochemical charges present on the surface of all solid materials, which can attract and trap small particles, such as specs of dust or water vapours.
The higher these surface charges, the stronger a surface can attract moisture. The lower the surface charges, the weaker the attraction, the easier can moisture evaporate, the better the breathability. Thus service charges also determine the breathability of the fabric.
As old masonries can also contain a significant amount of salts, salt ions can act as an important catalyst, the electrical charge of salt ions increasing the surface charges of the masonry. Thus, non-salty bricks have lower surface charges, while salty bricks have (much) higher surface charges.
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, significant differences has been observed between them, how they handle moisture.
Experimental measurements have determined a significant difference between the surface charges of salty and non-salty masonries . Surface charges in the masonry can be measured with multimeters or data loggers and expressed as a small voltage, typically varying between ±500 mV.
Here are some comparative measurements between two identical bricks - one salty, the other one not - under the exact same wetting/drying conditions. We can observe that the surface energy of a non-salty brick is low, varying between 0 - 40 mV (green-blue lines), depending on the brick's moisture content. A similar salty brick has a much higher surface energy, varying between 150 - 250 mV (red-purple lines), being about 4-6 times higher than the energy of the non-salty brick.
We can also observe that the surface charges are not a fixed but variable and they can be altered - increased or decreased - by multiple mechanisms, by physical, chemical or electromagnetic means. Various environmental conditions - such as storm clouds moving in, carrying a lot of electrical charges; or the position of the Moon - affects the surface energy of masonries on an ongoing basis - in this case by electrostatic or electromagnetic means.
Masonries with high surface charges also attract and trap more moisture.
When similar salty and non-salty bricks have been subject to high air humidity for an extended period of time, the non-salty brick has accumulated 31 g of moisture while the salty brick has accumulated 141 g of moisture, under the exact same conditions. Just on the account of salts the salty brick has accumulated 4.5 times more moisture than the non-salty one.
Moreover, once the bricks have been removed from the humid environment, highly-charged salty surfaces (green) let go of moisture much slower than non-salty (blue) brick surfaces, drying out about 7 times slower - a significant difference.
This explains why a salt-free masonry is so much more breathable, why rain water (fresh water) can evaporate out easily from it - the electro-chemical surface charges being low. It also explains why the wall fabric near ground level - in the presence of rising damp and salts - tends to be much damper, the electrochemical surface charges here being much higher.
The high salt content of the masonry acts as a moisture collector, binding water electrochemically (hygroscopic effect), leading to reduced breathability and higher residual moisture content of the walls near ground level.
At higher elevations, outside of the salt band, the masonry - primarily being affected by rainwater - does not contain salts, the walls having high evaporation and a high breathability.
Thus, in porous masonries we can identify two types of moisture barriers:
Physical moisture barriers are externally added coatings or materials which interfere with the free movement of moisture. Most common such materials are cement, plastic membranes and foams. Physical barriers hinder evaporation and thus make a wall fabric less breathable.
While the moisture-related damages from moisture barriers are well known, the fact that various forms of energy can act as moisture barriers and hinder the evaporation of moisture, has been known to a less extent. As mentioned earlier, the surface of all solid materials contains small amounts of static electricity - known as surface energy or surface charges or adhesion forces - which are strong enough to attract and bond the tiny water molecules to capillary surfaces, resulting in some moisture accumulation.
Thus, from evaporation blocking perspective, salts behave similarly to cement. But, unlike cement which forms a visible and obvious moisture barrier, salts are spread invisibly throughout the pores, creating an invisible yet nevertheless significant moisture-blocking effect.
These findings explain why a wall fabric can dry out relatively easily after rain (intermittent wetting with little/no salts involved), and not dry out or just partially dry out when a significant amount of salts is present in the masonry – such as in case of farm or barn conversions, old churches and chapels, rising damp situation, around old chimney breasts, buildings close to the sea etc.
The presence of any physical moisture barriers (non-breathable materials) is bad for an old masonry designed to breathe because:
Thus, the removal of non-breathable materials is an important step, as this improves the evaporation rate allowing the wall fabric to become drier. This, in many instances, brings about a significant improvement especially in not too old buildings with low-to-moderate salinity cases. In high salinity cases, however, one might only experience partial results: the surface might dry out, but under the surface the fabric remains mostly damp.
How much moisture can accumulate inside the fabric under breathable and non-breathable finishes? The video below demonstrates the concept.
It is also important to understand that physical moisture barriers are not the primary cause of dampness problems. Moisture barriers just make some pre-existing dampness problem – often a hidden one – worse, by forcing moisture to accumulate.
Without an underlying moisture source dampness can’t accumulate. The reason why no one is concerned about the breathability of newer buildings is because the fabric is dry, so there's no need to evaporate out anything.
A common hidden moisture source in old buildings is rising damp caused by the evaporation of soil moisture from under the building - an invisible, slow yet persistent moisture source. Non-breathable plasters trap rising water vapours making it accumulate and liquefy inside the wall fabric. The increased moisture content dissolves and spreads the salts further, leading to ongoing plaster damages. Replastering with lime temporarily improves the problem, but if the underlying moisture source is not solved, the plaster damages will appear again after some time.
These and many other aspects related to rising damp are discussed in detail in the rising damp section.
To further reduce the moisture content of the masonry, especially when salts are also present, decreasing the surface energy of the fabric offers another alternative, complementing the removal of moisture barriers. The magnetic DPC technology does exactly that.
Here are some other related pages that you might want to read to broaden your knowledge in this field.
Here are the some recommended materials / products that can help solving or dealing with some of the problems discussed on this page.
