What is wall breathability?

Anyone who has done even a little research on sustainable buildings, or looked into designs that diverge from the mainstream ones, is sure to have come across the term ‘wall breathability’ – only because it is an indispensable component of safe and healthy infrastructure, but the term is often confused with having something to do with ventilation and aeration, when in fact it does not. ‘Wall breathability’ is a term that actually pertains to that particular characteristic of a wall which allows it to transfer moisture through it. This moisture can be water in its liquid form or gaseous state, which we know is virtually everywhere – in the atmosphere, in the pipes and on all sorts of surfaces; thus, it is inevitable that the building will be constantly exposed to it over the years. The amount of water that comes into contact with the building and the receptibility and reaction of the building to it manifests itself in the eventual health and condition of the infrastructure. To add further clarity to the term ‘wall breathability’, it can be described as the movement of water through the structure.

Why does wall breathability matter?

The materials that are used in the construction of walls, including the building material, paint, sheen and varnishes, all essentially come to define the breathability of a wall. The underlying features of these materials that determine wall breathability are their hygroscopicity and capillarity. The hygroscopicity of a material is its ability to allow water absorption and release; whereas, capillarity describes the movement of water particles through that material. Both of these characteristics define the retention and movement of water through the substance, so the greater the hygroscopicity and capillarity of a material is, the more its breathability will be. Now, the question that presents itself here is: do walls need to be breathable, or not?

Roger Hunt, the co author of ‘Old House Eco Handbook’, explains what happens to a building with minimally breathable walls in very simple terms: “What’s going on is similar to what happens when we wear a plastic mac. The moisture from our body cannot escape, it builds up between our skin and the plastic and we become very uncomfortable.” So, yes. We need the walls of our buildings to be adequately breathable to prevent the retention and build up of water; therefore, we must first understand the level of breathability of the materials at our disposable and then ensure such a selection and design that allows for optimal breathability. In general, it is advised that the external components of a building wall should be about four to five times more breathable than the internal components, so to create a gradient for the transfer of moisture. In this way, the gradient established and maintained facilitates the movement of water from the indoor environment to the outside -preventing the dampening of walls and build up of moisture and humidity which would otherwise impair the indoor air quality.

Impact of wall breathability on buildings

Looking more closely at this, we can identify a range of negative consequences that an imbalanced water-wall relationship might entail. Water tends to have an impact on different parameters of the wall which in turn induce an effect on the building as a whole. Both in its vapour and liquid state, water has the capacity to undermine the thermal insulation of a building. It does this by decreasing the thermal resistance of the building, making way for hotter indoor conditions in warm environments and colder indoor conditions in cold environments. Not only does this incur energy and financial losses, but also presents itself as a hazard to human health: furthermore, surface condensation on the indoor faces of walls can lead to loose paint, damaged fabric and generates an area that facilitates the growth of harmful moulds. All of these consequences threaten human health and well being and lead to economic damage, so it is imperative that we create buildings that are equipped to tackle this.

Another aspect of building performance that is influenced by breathability of walls is the airtightness of buildings. Airtightness of walls is a feature that ensures that no unaccounted for air escapes or enters the buildings through the walls. This quality is essential in defining the degree of thermal performance a building possesses. If there is inadequate airtightness, there will be improper insulation that can incur energy losses; therefore, it is crucial that we have a certain degree of airtightness without compromising on our wall breathability. Wall breathability is often confused with air permeability, which can result in an inadequate design that may hamper building condition. Breathability deals with the rate of water particle transmission through the walls; whereas, air permeability deals with the passage, or leakage, of air through gaps and spaces. The solution to ensuring breathable walls without undermining the airtightness of the buildings is to choose airtight and breathable membranes which are becoming increasingly available in the market.

Enhancing building performance with breathable materials

The reason wall breathability was not a subject of concern in the past is because old buildings were constructed from lime-based and porous mortars, plasters and renders that were readily available in that time, and also incorporated clay and earth. These sufficiently hygroscopic materials allowed for good wall breathability. Modern day buildings are made of cement based mortars and renders, synthetic and the popular ‘water proof’ paints and water resistant sealants, all of which entrap the moisture within the closed spaces – leading to break down of surface paints, dampening of wall, rotting and all kinds of infestations. This is not only a hazard to the building, but also to its inhabitants. 

Taking a lesson from this, we can focus on incorporating more organic and natural products in our buildings that are eco-friendly and offer considerable breathability. Using such materials offer a range of benefits. Some of the materials that can be utilised in designing breathable walls are: wood, straws and cellulose insulation. These are safer and more energy efficient to produce on a commercial scale because they do not generate large amounts of toxic emission that are harmful to both humans and the environment, and create minimal waste if utilised effectively. In addition to this, these organic alternatives use up large amounts of carbon dioxide in their growing stages, thereby reducing the effects of global warming on the planet – as opposed to their man-made substitutes that release large quantities of carbon dioxide in their production process.

Furthermore, there are a range of construction materials, such as natural fibres and unfired clay that are known to control indoor humidity in a manner that enhances the Indoor Air Quality (IAQ). These materials naturally regulate humidity by retaining it when the levels increase too much and releasing it when the indoor atmospheric levels fall – acting as a buffer that creates a more natural and controlled environment that suits the human health and condition. The reason this is needed is that our conventional buildings are not designed to tolerate humid conditions, which is manifested to us in the form of water condensation problems and areas of moulds. To regulate this, we need to have devices that remove and add humidity as per our needs which is very costly and wasteful of energy. 

In light of all of this, the strategies we adopt to ensure a controlled water-wall relationship will undoubtedly determine the quality and durability of our buildings. We must be careful as to ensure a regulated absorption, transmission and evaporation of water in the buildings to prevent the adverse effects of water on the fabric and material of the buildings for them to perform efficiently. In addition to choosing organically manufactured material to constitute the buildings, we can enhance the breathability of the walls by our choice of paints and finishes as well. A lot of the synthetic paints and finishes available in the markets can be impermeable and harmful. Such chemically construed products contain alarming levels of volatile organic compounds (VOCs) which are toxic chemical substances that can cause a variety of respiratory illnesses, heart diseases and even cancer due to prolonged exposure. Synthetic finishes contain compounds, such as toluene and xylene which are toxic to humans and animals, and impair Indoor Air Quality; therefore, choosing naturally produced paints and finishes can not only elevate the breathability of walls, but also has no negative consequence on human health. 

Conclusion: Why we need to focus on breathability

To conclude, breathable walls serve to reduce indoor air pollution, the risk of surface condensation and prevent the formation of toxic moulds. In addition to this, organically produced breathable walls  using natural breathable paints act as natural buffers of humidity, allow for vapour permeability and increase the performance energy efficiency of the building; moreover, buildings constructed from naturally occurring substances not only reduce the harm we might otherwise be exposed to as a result of their synthetic counterparts, but also offer create an environment of health and well being that is somewhat similar to that experienced outdoors. Above all, naturally built breathable walls contribute to the sustainability of our planet from the start of their production to decades after they become a part of our infrastructure. 

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