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Why choose a breathable external wall insulation system?

What does ‘breathable’ mean?

When referring to the breathability of building materials, one is generally talking about the interaction of moisture with the building fabric, and the ease with which moisture travels through the structure. ‘Moisture’ can either take the form of vapour or water.

Breathable materials are those that allow for this to happen, letting moisture pass through the walls without it becoming trapped. This relationship between moisture and buildings is essential for structural health and performance. A lack of breathability in building materials can lead to interstitial condensation, also known as damp, and a whole host of issues.

Breathability, thermal performance, and airtightness have become an essential consideration for modern building technologies, so in today’s blog we’re going to take a look at the advantages of choosing a breathable solid wall insulation system.

The dangers of water vapour and interstitial condensation

Interstitial condensation occurs when warm air diffuses into a vapour permeable material, whether it’s your solid brick wall or a fibrous insulating material. The problem is caused when water vapour reaches a point which is 2-3 degrees cooler and condenses into water. This is known as the dew point. When this occurs within the structure of the building it is known as interstitial condensation. Any materials that slow the escape of moisture, or are non-breathable which block the exterior of the property (i.e. sand and cement render) will inhibit the ability of the moisture to escape from the building fabric; and the resulting freeze-thaw cycles can cause serious structural damage. Thus, damp, mould, rot and structural issues will inevitably occur. The worst thing about interstitial condensation is that because it occurs inside the walls, it’s very difficult to catch and control.

Preventing interstitial condensation

Controlling condensation comes down to four key things: heating, ventilation, insulation and controlling the vapour permeability and ‘breathability’ of the system – moisture must be able to get out quicker than it can get in.

To prevent interstitial condensation with insulation, the temperature across the entirety of the the insulation system must remain higher than the dew point temperature of the water vapour. In practical terms, this means keeping the walls warm enough to prevent condensation. Vapour control layers can also be installed on the warm side of the insulation to slow the rate of vapour entering the wall, but thermal bridging needs to be completely eliminated to ensure that there are no cold spots in the walls where condensation can gather.

A ventilation strategy should also be established by your installer at the beginning of the project to ensure adequate exchange of air. Regular heating during colder weather is also important for ensuring these ‘warmer walls’.

Choosing breathable materials to reduce interstitial condensation?

To be classed as ‘breathable’ a material must possess the following characteristics: the material must be vapour permeable, and capillary active. The best insulation material to choose for preventing interstitial condensation is one that is also hygroscopic. This means water vapour can pass through, condensed water vapour can be absorbed but also, importantly, the water vapour can be routinely released.

Wood Fibre insulation is an excellent choice when it comes to choosing an insulation system that will actively prevent interstitial condensation. This is because Wood Fibre insulation can hold a significant amount of water vapour (20-30%) without reducing its thermal capabilities or structural integrity.

The advantage of Wood Fibre is that the water vapour stored within the material is gradually released from the fabric as the external temperatures improve. This creates a healthy internal environment and a dry building structure – the optimal building conditions to aim for.

Choosing a breathable render to pair with the Wood Fibre insulation is the next step to ensuring an entirely breathable system. Because Wood Fibre is a natural material, we recommend using lime based renders, as lime is renowned for being natural, eco-friendly and also highly breathable. Alternatively, a thin coat silicone coloured render would also be suitable.

It is essential that a breathable basecoat needs to be used to ensure consistency across the structure, whatever render is chosen.

Thermal bridges and surface condensation

Oftentimes with solid wall insulation systems, the quality of installation can affect the performance and create issues relating to condensation. This typically occurs when gaps are left between insulation boards, creating cold spots within the system which are a magnet for condensation. To combat this, the Wood Fibre insulation boards specified by EWI Pro are tongue and grooved to lock together to protect against thermal bridging caused by gaps.

The main benefit of solid wall insulation is that it creates a complete thermal envelope around the exterior of the property, thereby dramatically reducing the risk of thermal bridging. Therefore, correct installation is essential for ensuring structural integrity; build-up of condensation, both interstitial and on the surface of the internal walls can lead to system failure and in particularly severe cases can result in structural deterioration.

Installers recommended through EWI Pro have been fully trained and instructed on the proper installation of our insulation systems so that thermal bridging is avoided. For a local installer recommendation, you can call our sales team or visit our ‘find an installer’ page to fill out our installer request form.

Our technical team are available to offer detailed design and specification advice, as well as on-site support and training to ensure every solid wall insulation installation across the UK is a success.

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