What is Vapour Permeability, and Why is it Important in Breathable Roof Membrane?

Find Out why Vapour Permeability is a Key Property of Breathable Roof Membrane

Vapour permeability is a key property of breathable roof membrane. The name vapour permeability is often interchanged with ‘breathability’, but when referring to roof membrane, they both mean the same thing.

What is vapour permeability?

Breathable roof membranes must let moisture pass through their surface, otherwise they couldn’t be referred to as ‘breathable’. The phrase ‘vapour permeability’ simply refers to a materials ability to allow vapour to pass through it.

You may have heard builders or roofers refer to a membrane as having a ‘high’ or ‘low’ permeability. high permeability means more vapour can pass through the membrane in a 24 hour period. Low permeability means less vapour will be able to pass through, in the same amount of time.

Vapour Permeability is measured in perms. Perms determine how much vapour is able to through one square metre of membrane in a 24 hour period. The faster vapour can pass through the membrane, the greater permeability the membrane has.

Why is vapour permeability important?

No-one wants to bring their family photo album down from the attic to find it has been damaged by mould. However, without proper ventilation, excess moisture can become trapped in the air, and in turn cause property damage, rot and unhealthy mould to grow. This is where breathable membrane plays an important role. By allowing excess moisture to escape in the form of vapour, breathable membrane allows our homes to breathe. Breathable membrane also helps to keep your roof's structure healthy. For example, if any water manages to enter the structure, instead of staying trapped inside and causing further problems, the breathable membrane allows the water to evaporate and escape.