The concept of breathability is paramount in building conservation. Breathability means the fabric of old building needs to be open-pore or permeable to moisture. This way water vapors can penetrate the fabric and moisture can evaporate out from the fabric into the surrounding environment. The free evaporation of moisture from the depth of a porous wall fabric is called casually “breathing”.
The Full Cycle of Breathability
The cycle of breathability in professional circles is generally regarded as the evaporation of moisture from the wall fabric to the environment.
Recent research has shown that the “wall fabric > environment” path is only a partial cycle of breathability. The full breathability cycle does not start from the wall fabric but from the soil, the full breathability cycle being: “soil > wall fabric > environment”.
An important source of vapors that the wall fabric is constantly subject to is the soil under the building.
Because the primary moisture sources affecting old buildings, mistakenly, have been thought to be rainwater penetration and condensation, the effect of soil evaporation has remained hidden and as such disregarded for a very long time. As a result actions solving dampness problems in old buildings focused solely on resolving liquid water penetration and condensation problems.
Recent research has confirmed that the main moisture sources in old buildings are in fact:
- Rainwater penetration (Liquid moisture)
- Soil evaporation (Vapors)
- Condensation (Vapors to liquid moisture)
Solving the problem of liquid moisture and condensation alone often did not result in a dry wall long-term, unless the problem of soil evaporation has also been dealt with.
Rainwater vs Soil Evaporation – Key Differences
The two most important moisture sources in old buildings are:
- Rainwater penetration: which affects the outer envelope of all buildings, including roofs, chimneys and all external walls. This can take many forms, including: water ingress, driving rain, splashback at the base of the walls, sideways water penetration for cellars etc.
- Soil evaporation: which affects the base of all walls, both internal and external, with no damp proof course. The role of a damp proof course is, according to latest research, to be a vapor barrier, blocking the evaporation (and accumulation) of moisture inside the wall fabric.
There are significant differences between these two moisture sources, the most important ones being highlighted in the table below:
Soil Evaporation (Vapors)
Ongoing wetting (24/7), no breaks
Limited amount of moisture
Unlimited (linked to the soil and water table)
Generally a non-saturated environment
A saturated environment
Relatively easy to spot, visible
Invisible, can be difficult to spot
Abundant, easy to see, fast damages
Slow, invisible in the background but ongoing
No salts present
Salts present (ions, electrical charges)
Causes water ingress
Causes rising damp
In the past soil evaporation has been often confused with condensation. Research, however, has confirmed that condensation and soil evaporation are different phenomena, although some similarities exist between them.
The concept of breathability works well for walls as rainwater is intermittent wetting: there are periods of rain, followed by periods of sunshine. When there is no rain, the wall fabric can dry by evaporation.
Applying the concept of breathability to dampness problems caused by soil evaporation at the base of old walls (rising damp) does not work, for the simple reason that soil evaporation is not intermittent but a 24/7 ongoing wetting. The wall base has no chance to “catch a break” to breathe out the moisture as incoming moisture is “always on”. Recent research has confirmed that under these circumstances moisture accumulates even in a fully breathable wall fabric.
The concept of breathability does not work for the base of the walls, where soil evaporation, the dominating moisture source, is causing ongoing wetting.
Can Moisture Accumulate in a Breathable Wall Fabric?
Here is an interesting question: if we build a fully breathable brick wall onto a well-drained damp soil, only subject to soil evaporation but no capillary action (no liquid moisture is present), will the brick accumulate moisture from the soil, or moisture will evaporate freely through the bricks leaving them dry?
The general assumption is, that vapors from the soil evaporate through the bricks, leaving them dry.
Research data, however, has now conclusively proven that this isn't always the case. Bricks in the vicinity of a well-drained but damp soil do accumulate significant moisture. We can distinguish two different phases:
- Phase 1 - Moisture accumulation: dry bricks placed on damp soil instantly start accumulating moisture. Bricks act as holding tanks, behaving like empty vessels or vacuum, and in the presence of humidity they start collecting moisture. This goes on until they reach some sort of equilibrium with the environment.
- Phase 2 - Evaporation: evaporation only starts after the equilibrium phase has been reached, when the bricks start evaporating out the excess moisture in an attempt to preserve their equilibrium state.
In other words breathable bricks first collect a significant amount of moisture from the ground, reach an equilibrium, and only then start evaporating (breathing out) the excess moisture.
The explanation behind the phenomenon are most likely electrical surface charges. Charged wall surfaces attract the charged water vapors, leading to their adhesion to capillary surfaces, resulting in the accumulation of moisture inside the fabric.
This phenomenon has been observed while doing some research experiments about the movement of moisture. Here are some more details for those interested. Additional experiments are under way.
Experimental Setup and Methodology
We have built a small wall consisting of 4 bricks. Instead of mortar we used dry brick dust from the same bricks, a common practice for lab simulations.
The bricks were placed on a 100 mm thick well-drained but humid soil-bed placed in a deep perforated tray, positioned in a second, shallower tray used to control the amount of water vapors present under the bricks, with precise wetting of the soil from the bottom.
Detailed measurements have been taken during the experiment using an array of embedded micro-sensors, collecting humidity and temperature readings from the soil, mortar courses, depth and surface of the bricks as well as of the ambient environment. In addition, various electrical parameters of the fabric such as the spontaneous voltages and currents present in them have also been recorded.
The readings have been taken by an 80-channel Keithley Tektronix DAQ-6510 professional data logging system operating at 0.0025% accuracy.
Once bricks are placed on a well-drained but moist soil, the moisture content of the bricks in depth (dark blue) starts rising immediately, peaking at 87% RH in a 40% RH ambient environment (orange line).
Additional wetting of the soil after 11 days pushes the depth humidity of the bricks to 100% RH. (Graph 1)
Although the depth humidity (dark blue) of the brick is high (about 85%), the surface humidity (light blue) stays around 40% at the level of ambient humidity (orange). This indicates that while the brick keeps accumulating moisture no additional evaporation is taking place through the surface. (Graph 2)
The Effect of Moisture Barriers and Non-Breathable Materials
Moisture barriers are those materials that block the evaporation of moisture. Most common moisture barriers are:
- Cement-based plasters
- Plastic membranes
- Bituminous felt of membranes
- Tanking slurries or various waterproofing materials designed to block the penetration of moisture
Once a breathable wall fabric is covered by a moisture barrier, in the presence of a moisture source moisture starts accumulating fast behind the surface.
The problem is not the cement or moisture barrier but the underlying moisture source. Without an existing (often hidden) moisture source, moisture barriers would not make too much damage to the fabric. Proof to this concept are newer buildings with working damp proof courses (DPC), which is a moisture barrier blocking the evaporation of moisture from the soil into the building fabric. As a result the wall fabric stays dry even in the presence of modern, non-breathable materials.
Without a moisture source there is no moisture to breathe out and the question of breathability becomes irrelevant.
How much moisture can accumulate inside old walls with breathable and non-breathable finishes? A research video below demonstrates the concept.
- The full cycle of breathability starts from the ground, the moisture evaporation path being: soil > wall fabric > environment.
- One of the most common, often hidden, moisture sources is soil evaporation from under the building.
- Just because a wall fabric is breathable it does not mean it's dry.
- A fully breathable wall fabric can accumulate significant amounts of moisture, evaporating out the excess humidity after reaching equilibrium with the environment.
- Moisture barriers make things worse because they "amplify" an underlying moisture problem. Without that moisture can't accumulate.