Breathability is about vapour movement. It is about the mechanisms and factors that control (facilitate or hinder) the movement of vapours through porous materials. Understanding the key factors that affect the movement of vapours is key to understanding breathability.
Regarding breathability, it is generally believed that:
These statements assume that a breathable wall fabric does not interfere with the passage of vapours - those just "fly through" the fabric without affecting it, meaning if 100% moisture goes into the fabric all 100% comes out on the other end, no moisture accumulation occurring in the fabric.Â
This is not the case.
Practical observations did not seem to substantiate this hypothesis, so we decided to explore this topic in depth which led to some important findings.
In the lab we have built a small test wall from of "old-style" soft, porous, new bricks placed on drained damp soil. At the beginning of the experiment the bricks were completely dry. Then we removed the plastic foil from under the bricks allowing water vapours to move into the bricks. We have monitored the movement of moisture from the soil, through the bricks, to the surface with a large array of sensors, taking consecutive readings from all masonry areas and the environment.
Once the masonry has made contact with the damp soil - we noticed that the moisture build-up occurred in two distinct phases:Â
This behaviour is caused by electrical surface charges (also known as surface energies or adhesion forces). These are small electric charges present on the surface of all solid materials which can attract light particles - e.g. dust to solid surfaces; or water vapour molecules to capillary surfaces. These charges attract and initially trap water vapours (Phase 1). Once the vapours have covered the capillaries with a thin water film, to some extent neutralizing the surface charges, subsequent water vapours can now fly through the capillaries, resurfacing on the other end, resulting in an evaporating flow (Phase 2).
These electric surface charges are the primary mechanism behind the accumulation of moisture and wetting of the wall fabric.
This experiment shows that moisture accumulation occurs in a a breathable material. Moisture initially fills it up to 100% RH, then the excess moisture starts "blowing over", resulting in evaporation through the surface.
The amount of moisture that accumulates from water vapours in a new brick in the lab or in newer buildings, is not significant. Expressed in percentages, the volumetric moisture content of new bricks subject to vapour movement only is typically between 0.1 - 0.5%.
However, in older buildings - even in well-maintained, well-heated and well-ventilated internal walls, not subject to water ingress or condensation - especially close to ground level, moisture levels can be much higher. This indicates there must be additional factors that affect the movement and bonding of vapours in a porous fabric.
Many technical experiments later we found a very important element that interferes with vapour movement: salts.
In dry state, salts are hard, solid minerals organized in a fixed 3D-lattice, held together by intermolecular electrical attraction forces.
In the presence of water, however things change. Salts get dissolved and broken down by water into their constituent parts - positive and negative salt ions. These travel independently with the liquid water from the soil into the masonry. Being much larger in size and having much larger electrical charges than water molecules, by being drawn into the wall fabric, salt ions will:
Salts reduce the breathability of the masonry.
This explains why the masonry tends to be damper near ground level. Amongst other factors, 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 -Â does not contain salts, the walls having high evaporation and a high breathability.
Putting it all together: here are the main factors that affect the breathability of porous masonries:
The size of a water molecules are extremely tiny (0.1 nm in size), these being about 10 billion times smaller than the human body. Water vapours can pass through most building materials even if they have no visible holes. A completely invisible 1 µm typical brick pore is about 10,000 times wider than the diameter of a water molecule. To put these relative sizes into perspective: if in our example we imagine the water molecule being 1 mm in diameter, the invisible brick crack width is 10 metres! Â
The movement of water vapours through the (mostly invisible) pores of porous materials is called vapour diffusion.
Breathability - or the ability of water vapours to penetrate other materials - is measured by the water vapour diffusion resistance or mju (μ) value. This is a material property, independent of thickness, the lower the μ-value the more breathable a material is.Â
The lightest material is air. The reference mju-value of 1 (μ = 1) is the resistance of a 1 metre thick air layer. Obviously, solid materials have higher resistance than air, their μ-value being higher than 1.
Here are some indicative μ-values of the most common building materials. Please bear in mind that these values can vary based on the porosity and composition of each type of material.
Using only highly breathable materials with the lowest possible μ-value is not the best choice for every situation. The more breathable a material is, the more moisture it can absorb.1Nienhuys, Sjoerd. (2012). Tables for Thermal Insulation in High Altitude Areas of The Himalayas, Technical Working Paper Number 3 with Adjusted Values for Reflective Foils, Researchgate
Very porous, very breathable materials absorb more moisture than denser materials. The absorbed moisture affects negatively the thermal performance of insulation materials as breathability is about a two-way airflow: from the material (drying) and into the material (wetting).
Based on their moisture absorption capacity, porous materials can also be classified as:
Here are some other related pages that you might want to read to broaden your knowledge in this field.Â
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