The ‘B’ word (Breathability)

When discussing moisture risk and hygrothermal performance of buildings, the word ‘breathable’ is often used – and abused. What does it really mean?

It’s a strange word in this context – inanimate materials clearly don’t respire like animals. And normally we’d think about breathing air, whereas the main gas we’re interested in for hygrothermal analysis is water vapour, one component of air. Ventilation, infiltration and airtightness are important subjects, but aren’t usually associated with the word breathable – perhaps this is a piece of spin yet to be tried “our new airtightness membrane is fully breathable thanks to the large holes in it!”

In my experience, many people use the word breathable most commonly to mean a material or build-up is vapour open. Perhaps it is therefore carried over from the class of products generally known as “Breather Membranes”. To be pedantic, BS5250 refers to roofing membranes as either “LR” (low resistance) or “HR” (high resistance), the more modern products known as Breather membranes being the LR variety – resistance in this context being vapour resistance.

Without getting too philosophical, it might be reasonable to say that as long as it’s clear what a particular speaker or writer means, then all is well and good. However, there is a specific technical definition. A breathable material is one that is:

A vapour open material is one that a low resistance to the passage of water vapour. A capillary active material is one that can transport liquid water by capillary action – the same mechanism a tree uses to draw water up from its roots through xylem. A hydroscopic material is one that can absorb (and adsorb) water vapour from the air and release it again, with no ill-effects above a certain level.

According to this definition, a material cannot be called breathable unless it fulfils all these criteria.


Material Vapour open Hydroscopic Capillary Active Breathable?
PIR foam No No No No
Mineral wool Yes No No No
Wood fibre Yes Yes Yes Yes

It is particularly important when we remember that in many constructions, the transport of moisture is often dominated by liquid phases, i.e. capillary action is sometimes more important than vapour diffusion. In practice, I find it less confusing to avoid the ‘B’ word as much as possible, and instead talk about the relevant property or properties mentioned above.

So please, only use the B word when you really meant it!

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