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 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 moisture that the wall fabric is constantly subject to is the soil under the building.
Soil evaporation is driven by the large-scale circulation of water in nature. known as the Water Cycle. Liquid water falls as rainwater, goes into the ground, then evaporates and rises up as vapors. As part of this cycle the soil constantly evaporates moisture into the air and into the porous wall fabric built onto it.
Recent research has confirmed that soil evaporation is a standalone moisture source leading to moisture accumulation in the wall fabric, independently of any other moisture sources such as rainwater ingress and condensation; the 3 main moisture sources, which often overlap, being:
- Rainwater penetration (Liquid moisture)
- Soil evaporation (Vapors)
- Condensation (Vapors to liquid moisture)
The Effect of Moisture Barriers and Non-Breathable Materials
Moisture barriers are those materials that block the movement of moisture. Most common moisture barriers include modern cement-based plasters, plastic membranes, bitumen, tanking slurries and various waterproofing materials designed to block the penetration of moisture.
Most of these materials not only block liquid moisture but also water vapors, making the old building fabric "non-breathable" - unable to discharge and evaporate out moisture.
It is commonly believed that moisture barriers are the main reason behind the decay of the old historic building fabric. This is not entirely true. In itself, moisture barriers are not a problem. Moisture barriers only become a problem if an underlying (often hidden) moisture source is present.
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. There must be some moisture source(s) present for moisture to start accumulating. On external walls this can be rainwater penetration, soil evaporation (near ground) or condensation (through thermal bridging). On internal walls the most common hidden moisture source is soil evaporation, causing rising damp.
Without a moisture source the wall fabric stays dry, regardless if a moisture barrier is present or not. 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.
A Breathable Wall Fabric - Can Moisture Accumulate?
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 reason behind the moisture accumulation are surface charges. 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 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 walls.
- In itself, moisture barriers are not a problem. They only become a problem in the presence of an underlying (often hidden) moisture source.
- 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.
- A fully breathable wall fabric can accumulate significant amounts of moisture, only evaporating out the excess humidity after reaching equilibrium with the environment.