Understanding Breathability
Breathability is a property that prevents the accumulation of harmful water—liquid water or persistently high humidity—within a building’s structure. While it does not replace primary systems like ventilation or weatherproofing, it is an important factor in maintaining stable, harmless moisture levels within the building fabric.
The 4 Essential Components
For a structure to be effectively “breathable,” four elements must work together:
- A Moisture Pathway: Materials must be vapour permeable, meaning they have microscopic pores that allow water vapour molecules to migrate through them.
- A Driving Force: Moisture naturally moves from high-humidity areas to low-humidity areas; this “humidity gradient” drives the movement of moisture through the fabric.
- A Sorptive Material: Using materials that can bind and release moisture helps regulate humidity by buffering and ensures water stays in its least harmful form as it passes through the building.
- Vapour Control: Components must be organized in the correct order of resistance to regulate how much moisture enters and leaves the fabric, preventing “bottlenecks”.
Note: Because water vapour is a gas with incredibly small molecules, materials can appear solid to the naked eye while still allowing large quantities of moisture to pass through safely.
Breathability in Practice
A successful strategy relies on combining traditional moisture management with breathable materials:
- Primary Moisture Control: Most of the moisture should still be managed by good ventilation (internal) and effective weatherproofing/drainage (external).
- Material Selection: The most effective structures incorporate sorptive materials — such as sheeps wool — which suppress the harmful effects of moisture by temporarily binding it.
- Design Planning: Proper air-tightness and vapour control plans, alongside moisture modelling, can determine the best combination of products to prevent damaging water build-up.
