FAQ Water Vapour Resistance.
Water vapour resistance is a measure of the extent to which a material prevents the passage of water vapour.
How is water vapour resistance measured?
There are several related units of measurement for water vapour resistance. The thickness dependent measure of vapour resistance is typically in units of MNs/g.
Water vapour resistivity - a measure of resistance independent of material thickness - is measured in MNs/gm. Like thermal conductivity, the water vapour resistivity is independent of insulation thickness.
What is the μ-value?
Because the units of water vapour resistance and resistivity are so small it can be hard to relate to them or to properly evaluate the effectiveness of different materials.
The μ-value is a dimensionless measure of water vapour resistivity that makes comparing performance easy. This is done by multiplying the water vapour resistivity value by the vapour permeability of still air (0.2 gm/MNs).
Like water vapour resistivity, the μ-value is an inherent property of the material and is independent of thickness.
What is the μ-value of Kaiflex?
Kaiflex materials have the following μ-values:
Moisture Resistance Factor μ||
> 10 000||
> 10 000||
> 4 500||
> 2 000||
> 2 000|
What is the equivalent air layer thickness?
Although the μ-value is independent of thickness the total vapour resistance offered by a material is dependent on the material thickness. Because of this the μ-value is most effective when comparing the water vapour resistance of materials of the same thickness.
Equivalent air layer thickness, sd, is a way of normalising data so that the total water vapour resistance of materials of different thicknesses can be compared. To obtain the equivalent air layer thickness the μ-value is multiplied by the insulation thickness (in m).
What is the minimum μ-value that I need?
Insulation needs to resist the passage of water vapour because the water vapour pressure pushes moisture towards cold pipes. If insulation applied around the pipe has insufficient resistance to the passage of water vapour the moisture will pass through the material and condensate on the pipe.
The colder the pipe, the greater the difference in water vapour pressure becomes. As the force pushing water vapour towards the pipe becomes stronger and so higher levels of moisture resistance are needed.