Breathable insulation helps to avoid the problem of damp and mould

Breathability refers to the building envelope’s ability to deal with water vapour arising from activities within a building or from water entering from outside. The capacity for a building envelope to breathe is a crucial part of maintaining the health of buildings and their inhabitants.

Buildings that do not perform well in this area may develop sick building syndrome and chronically suffer from problems such as mould and damp, which create very unhealthy living and working environments.

Breathability is not historically an issue that has been given much consideration by the construction industry, which is largely due to the poor breathing performance of some synthetic building products and materials that are widely used in mainstream construction.

However, attention to the issue of trapped moisture in buildings has grown in recent years. This is largely as a result of increasingly stringent Building Regulations for energy efficiency, which have led to the combination of greater levels of insulation and airtightness.

If the building shell is not designed and constructed correctly, these factors significantly increase the risk of moisture being trapped.

Rainwater penetration, internal or interstitial moisture condensation or interior surface condensation can have a very negative effect on both building performance and human health.

For example, damp in external walls will considerably lower their thermal performance, while interior surface condensation causes moulds, which are harmful to a building’s inhabitants.

Assessing breathability There are three key elements that need to be considered when it comes to assessing the breathability of a building product or material. These include hygroscopicity, which refers to a building material’s ability to absorb water vapour from the surrounding air, capillarity, which refers to a building material’s ability to absorb water as a liquid, and vapour permeability.

Vapour permeability, which is perhaps the most important of the three, refers to the ability of a building material to allow water vapour to pass through it. Because more moisture is typically produced inside than outside a building, the vapour pressure internally is usually higher than it is externally. To balance this difference in pressure, water molecules, in the form of vapour, naturally move towards the area of low pressure. This means that in vapour-open or breathable buildings, water vapour will move through the building envelope from the inside to the outside.

Vapour permeable structures are more preferable than vapour-closed structures because they help to preserve the health and performance of the building and its inhabitants.

Effective design When designing a breathable building envelope, it is crucial to consider the breathability of all building materials in the structure to ensure optimum vapour management and control is achieved.

There are many different types of insulation available, which offer very different levels of performance in terms of thermal and acoustic efficiency, fire performance, sustainability and, of course, breathabililty. For example, mineral wool is breathable, whereas synthetic plastic foam insulation materials do not exhibit breathable characteristics.

If a material such as plastic foam, which is not breathable, is used as an external wall insulation, moisture may become trapped between the interior face of the insulation layer and the building structure, particularly in old buildings where there is residual damp. This could result in a chronically damp wall, which, because it is wet, may have considerably reduced thermal performance and develop mould.

However, if a breathable insulation is used, such as mineral wool, the moisture in the building structure is allowed to pass through the insulation layer into the external environment. This helps to avoid the problems of damp and mould, while ensuring the thermal performance of the building is maintained.

In addition, if water enters the building fabric as part of the construction process it is better able to escape once the building is complete if breathable insulation materials, such as mineral wool are used. If this process is prevented, trapped water can cause mould and decay, for example, in timbers. Furthermore, any incompatibility between breathable timber and, for example, vapour closed insulation, may cause interstitial condensation during the life of the structure, resulting in rot and damage.

Buildings and health Getting breathability right is an essential part of creating buildings with healthy interior environments. The consequences of trapped moisture in airtight buildings, both in the fabric of the building and in fittings and furnishings, such as mould, is now linked directly to allergic reactions, such as asthma.

The UK has a particularly poor record in this area, with up to 15% of the population suffering with asthma, which is a higher percentage than any other country in the world.

There is substantial evidence that links this health issue with trapped moisture in buildings, particularly housing. As such, it is absolutely essential that more focus is put on the issue of breathability and proper ventilation when it comes to building design.

This needs to begin with a greater understanding of the behaviour of water in buildings and how it affects their performance and the health of their inhabitants. By giving more consideration to the breathability of the building materials used in modern construction, it is possible to achieve very high levels of build quality, thermal efficiency, acoustic performance and sustainability, as well as excellent breathability.

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