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Why is Vapour Permeability and Breathability Important?

When it comes to insulating your home or commercial building, factors such as thermal performance, durability, and cost-effectiveness are often top considerations. However, one crucial aspect that should not be overlooked is the vapour permeability and breathability of the insulation materials, especially in the case of External Wall Insulation (EWI). In this blog post, we delve into why vapour permeability and breathability are critical in EWI and how they contribute to a healthier and more energy-efficient building.

Understanding vapour permeability

Vapour permeability, also known as vapour diffusion, refers to the capacity of a material to allow water vapour to pass through it. This characteristic is often quantified as a “permeance” value. This measures how much water vapour can flow through a certain area of a material in a given amount of time.

Materials with high vapour permeability, or high permeance values, allow more water vapour to pass through them. Materials with low vapour permeability restrict vapour movement.

In the context of insulation, vapour permeability is critical for moisture management within the building structure. If moisture from inside the building (generated by activities such as cooking, bathing, or even breathing) can’t escape and accumulates within the insulation or walls, it can condense into liquid water. This potentially leads to problems like dampness, mould growth, and structural degradation.

This is particularly important in colder climates or during winter months when the indoor air is typically warmer and more humid than the outdoor air. The warm, humid indoor air can condense on the cooler surfaces within the wall or insulation, leading to moisture accumulation.

Understanding breathability

Breathability in building materials is closely related to vapour permeability but extends the concept a bit further. Breathable materials not only allow water vapour to pass through but can also absorb and release moisture without losing their structural or thermal properties.

This moisture buffering capacity can help to moderate indoor humidity levels, contributing to a more comfortable and healthier living environment. If the indoor air becomes too humid, breathable materials can absorb some of the excess moisture. Conversely, if the indoor air becomes too dry, they can release stored moisture back into the air.

Breathable materials are particularly valuable in buildings that are difficult to seal perfectly or that experience significant fluctuations in indoor humidity levels. They can also be an excellent choice for older or heritage buildings. The use of modern, non-breathable materials could upset the balance of the traditional building system and lead to problems with dampness and material degradation.

The importance of vapour permeability in EWI

In an insulated building, daily activities such as cooking, bathing, and even breathing release moisture into the air. Without a way for this moisture to escape, it can condense on cold surfaces, leading to damp and mould growth.

This is where the role of vapour permeability becomes significant in EWI systems. Here’s why:

  1. Preventing Moisture Build-up: A vapour-permeable insulation material allows the water vapour to pass through the walls and escape to the outside, thereby preventing the build-up of moisture within the wall structure.
  2. Preserving Structural Integrity: By allowing moisture to escape, vapour permeable insulation helps to maintain the structural integrity of the building. Trapped water can deteriorate building materials over time, potentially leading to costly repairs or replacements.
  3. Creating a Healthier Indoor Environment: High levels of humidity within a building can lead to mould growth. This can have serious health implications for the occupants, including allergies and respiratory problems. By managing moisture levels, vapour permeable insulation contributes to a healthier indoor environment.


The term hygroscopicity refers to the ability of a material to absorb moisture from the air. This concept is directly linked to the notions of vapour permeability and breathability, which we have previously discussed.

Hygroscopic materials, such as wood fibre, cellulose, and some types of mineral wool, can absorb water vapour from the air when the humidity is high, store it, and then release it back into the environment when the humidity drops. This moisture buffering action can help to regulate indoor humidity levels, improving indoor air quality and comfort.

Here’s how hygroscopicity comes into play in our discussion of vapour permeability and breathability:

  1. Enhanced Breathability: Hygroscopic materials enhance the breathability of a building structure. By absorbing and releasing moisture, they facilitate the transport of water vapour through the building fabric. Therefore, helps to prevent the accumulation of moisture and the associated problems of dampness and mould growth.
  2. Moisture Buffering: The ability of hygroscopic materials to absorb and release moisture buffers changes in indoor humidity levels. This can be particularly beneficial in buildings with variable occupancy patterns or in climates with significant diurnal or seasonal variations in humidity.
  3. Improved Thermal Comfort: By helping to maintain stable indoor humidity levels, hygroscopic materials can contribute to improved thermal comfort. High humidity levels can make the indoor environment feel hotter in summer and colder in winter. Therefore, by regulating the humidity, these materials can make the indoor environment feel more comfortable.
  4. Energy Efficiency: Finally, by improving thermal comfort, hygroscopic materials can potentially contribute to energy efficiency. If the indoor environment feels more comfortable, there may be less need for mechanical heating or cooling, thereby saving energy.

Breathability in external wall insulation

Breathability in EWI is closely linked to vapour permeability. Breathable insulation materials not only allow water vapour to pass through, but they can also absorb and release moisture without compromising their insulating performance.

In buildings, especially older ones, with solid walls that can’t be sealed perfectly, breathable EWI can help manage moisture levels, acting as a buffer against changes in humidity.

Moreover, breathable insulation can work in harmony with natural building materials, which also tend to be breathable. This can be particularly important in heritage buildings where using non-breathable insulation could lead to increased dampness and damage to traditional materials.

Choosing the right external wall insulation system

When selecting an EWI system, it’s important to consider both its thermal performance and its breathability. While synthetic insulation materials, like polystyrene and polyurethane, offer high levels of thermal resistance, they tend to be less breathable compared to natural insulation materials like wood fibre, cork, or sheep’s wool.

Hence, it’s crucial to balance these factors and consider the specific requirements of your building. In many cases, a combination of materials or breathable render finishes achieves the right mix of insulation, vapour permeability, and breathability.


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