There are two areas in which breathable conditions are vital. These are fabric health and human health.
In certain circumstances, building fabric can be damaged due to a lack of breathability, and human health can be affected due to a build-up of mould. Mould spores are incredibly small and can even penetrate solid rock. Damp structures (caused by trapped moisture) can lead to mould formation and the contamination of indoor air. The negative effect of mould on human health has been well documented.
The Movement of Water
As we explain in a previous blog post, breathability does not refer to the movement of air within a building, rather the movement of water vapour. There are other elements taken into account with breathability too. Capillarity measures the way a material absorbs water as a liquid and hygroscopicity refers to the way a material absorbs and desorbs water as the relative humidity changes.
There are very different qualities in different insulation materials. Some may be very vapour permeable but have very little hygroscopic or capillary qualities. Natural insulations are all vapour open, highly hygroscopic and have varying capillary qualities. These qualities must be taken into account in any understanding or modelling of the way that moisture interacts with buildings. Breathability is of utmost importance, and the many studies of non-breathable materials in construction (and particularly in retrofits) support this.
The Meaning of Breathability
To confine “breathability” to external walls and the issue of trapped moisture due to poor construction or ongoing building faults, however, is to miss out whole aspects of breathability. It has been known for a long while that hygroscopic materials can reduce levels of relative humidity in buildings and thereby create healthier indoor climates. This technique has been shown in museum and archive situations to be more effective in controlling RH than air conditioning or other mechanical means. Hygroscopicity as stated above is the quality of a material to absorb and desorb moisture as relative humidity changes. Most materials do this to some degree. As humidity changes so does the Equilibrium Moisture Content (EMC) of a material. This is usually measured as the amount of water per weight of material. It is natural materials and in particular natural fibres and unfired clay which have the best hygroscopic qualities of all construction materials, but there is still much work to be done to maximise their value in common building situations. It is clear from material testing and from case study examination though that they could play a very significant role in the future both in refurbishment and new buildings, particularly as buildings become more airtight.
A Robust Solution
To say that breathable materials have no effect in regulating internal condensation moulds is not only incorrect, but irresponsible too. They have an absolutely vital role to play and are playing this already through the many natural fibre furnishings and fittings in buildings. In future, with better research and monitoring, they may become an essential part of a robust solution for good indoor air quality in both new and refurbished buildings.