Last edited 24 May 2019

Vapour Permeability

Vapour permeability, or "Breathability" in a building refers to the ease with which water vapour passes through building elements. Building elements where vapour permeability is poorly designed can result in condensation, leading to unhealthy living environments and degradation of fabric.

Vapour permeability can be expressed in several ways:

  • vapour resistivity of a material, measured in GNs/kgm (giga Newton seconds per kilogram metre) or MNs/gm (mega Newton seconds per gram metre)
  • vapour resistance for a given thickness of material, in GNs/kg or MNs/g
  • vapour resistance factor, also known as th µ-value, which is the resistivity of a material relative to that of still air. It is a factor, and has no units.
  • "perms", short for permeability, used commonly in North America, measured in grains/(ft2 ·h·inHg)

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