What are breathable building materials and how do they work ?

What does breathable actually mean in the context of a building? Does the building breathe like I do with air coming into my lungs and back out again as carbon dioxide?

No, not exactly.

It means that the building materials can transmit moisture. Breathable building materials are permeable enough so that any moisture from humid air and moisture from surface wetting can be absorbed and can then easily evaporate during drier warm periods.

A lot of natural, traditional building materials have inherent properties that allow moisture vapour to pass through them; they are vapour permeable. These materials can also absorb and retain moisture in vapour form [like moist damp air in the background or from a bathroom], which means they are also moisture buffering – the scientific word for this is ‘hygroscopic’.

Another way to describe these breathable building materials is that they are ‘vapour open’ or ‘vapour permeable’.

This is very different from some modern, synthetic building materials that are made from fossil fuels and plastic and are heavily processed. These are the opposite and therefore classified ‘vapour closed’ or ‘non-vapour permeable’. They don’t ‘breathe’.

Here are a couple of cross-section sketches that show how this works in practice.

How breathable building materials function in a vapour open construction

Sketch 1 illustrates a breathable vapour OPEN construction.

Breathable vapour open buildings mean that background water vapour is intended to enter the constructional build-up – it’s not intended to stop it entering. Over the colder, damper winter months, with higher internal humidity, a certain amount of water vapour moisture can enter the build-up and is temporarily stored in the insulation. Then, over the warmer summer months, when the internal humidity drops, the stored water vapour moisture can evaporate harmlessly into the room.

Image 2 shows different insulation materials – the first is natural bio-based wood fibre insulation made from timber waste and the second is synthetic rigid PIR insulation made from fossil fuels.

Understanding a non-breathable vapour closed construction

Sketch 2 illustrates a non-breathable vapour CLOSED construction.

Sealed vapour closed buildings can work, but as they rely on internal vapour control membranes that are not vapour permeable, they need extremely good site workmanship [should that be ‘workpersonship’?] to ensure the insulation is fitted tightly without edge gaps and the internal membranes are installed fully sealed without any gaps or holes. In reality, it’s likely there will be gaps, as I understand it is impossible to ever build a 100% fully sealed construction.

Nature knows best – who’d have thought it?