µ Factor
The water vapour (or moisture) diffusion resistance factor is defined in DIN 52615 (Thermal Insulation - Mass Transfer - Physical quantities and definitions). It is obtained by dividing the water vapour diffusion coefficient in air, by the water vapour permeability of a given porous material.
The moisture diffusion resistance factor, commonly called µ factor, is therefore a dimensionless number describing how many times better a material or product is at resisting the passage of water vapour, compared with an equivalent thickness of air.
To obtain a µ factor it is assumed that the air layer is stationary and that the water vapour diffusion coefficient and moisture permeability are expressed in the same units:
 | |
|
| | |
| µ = | Water vapour permeability of air
Water vapour permeability of material | |
| | |
|
For the Armaflex range of thermal insulation materials the following values may be quoted:
| |
|
| Class O Armaflex | 5,000 |
| NH/Armaflex | 2,000 |
| HT/Armaflex | 3,000 |
|
The importance of high µ factor, when this is built into the material, is that it demonstrates the superior long term performance of the material in its ability to resist the diffusion of water vapour.
high µ factor = high resistance to water vapour transmission
|
|
|