Breathability (Vapour Permeability)
The ability of a material to let water vapour pass through it while still resisting liquid water.
On an actual building
It is the central issue in heritage repair. Lime-based fabric works by allowing moisture to evaporate outward; sealing it with an impermeable coating traps water inside the wall and accelerates the decay the coating was meant to prevent.
What we most often have to correct
That waterproof and breathable are contradictory. Penetrating repellents line pores without blocking them, achieving both.
Terms that come up alongside this one
- Lime Mortar
A traditional mortar using lime rather than cement as the binder, setting slowly by carbonation and remaining softer and more vapour-permeable than cement mortar..
- Silane-Siloxane Treatment
A penetrating water repellent that lines the pores of masonry and concrete without filling them, leaving the surface visually unchanged..
- Capillary Action
The movement of liquid through narrow spaces against gravity, driven by surface tension.
Breathability (Vapour Permeability) — questions we get asked
What does breathability (vapour permeability) mean in waterproofing?
The ability of a material to let water vapour pass through it while still resisting liquid water.
Why does breathability (vapour permeability) matter on a real project?
It is the central issue in heritage repair. Lime-based fabric works by allowing moisture to evaporate outward; sealing it with an impermeable coating traps water inside the wall and accelerates the decay the coating was meant to prevent.
What do people commonly get wrong about breathability (vapour permeability)?
That waterproof and breathable are contradictory. Penetrating repellents line pores without blocking them, achieving both.
Have a building showing this?
If a term on this page describes something you are looking at, a site inspection will tell you how far it has gone and what it needs. It costs nothing.
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