What is the permeability requirement for an effective vapour barrier?

The permeability requirement for an effective vapour barrier is low permeability. This is essential because vapour barriers are designed to restrict the passage of moisture in the form of water vapour. A low-permeability barrier significantly reduces moisture transmission, thereby protecting building materials from potential damage caused by humidity and condensation.

In building construction, a vapour barrier with high or variable permeability would allow water vapour to pass through, which could lead to moisture accumulation within walls or ceilings. This moisture can foster mold growth, decay, and deterioration of structural elements. Additionally, a material with no permeability would not allow for any moisture movement, which may not always be practical, as buildings require some level of breathability to avoid trapping moisture inside. Thus, a low permeability vapour barrier strikes the right balance by preventing excessive moisture flow while allowing for necessary moisture management in a building.