Perm Rating of sprayed polyurethane foam

[C. Furtaw]

In the US DB the perm in. rating of the above foam is 1.448 but there are two thicknesses given for this rating. Which one should be used?

[Manfred Kehrer]

This depends on what thickness your membrane has.
The material properties are independent of thickness. But of course a double- layer membrane results in the half of the permeance.

[C. Furtaw]

Manfred,
You confuse me when you write that the material propertities are independant of thickness.
For instance in your database spun-bonded polyolefin membrane with a thickness of 0.00787 has a permability of 0.392 perm in. Diividing this by 0.00787 gives a perm rating of a specific membrane of 49.8 perm which is the approx. rating of Tyvec. So, the material property in the DB is dependant on the thickness.
In my example the perm inch rating would then change for different thicknesses ? Why have different thicknesses in the DB?

[Manfred Kehrer]

A thicker membrane does change the permeance but not the permeability. Therefore the permeability is a thickness independent Material Property. The permeance is a property of that membrane with that thickness but not a material property of spun bonded polyolefin.

Because of numerical problems we normally use all membranes with an thickness of 1mm (and we adapt all material properties in the right way, please see the WUFI online help regarding this).
Only the membrane from the North America Database are still not converted. But we have normally no problems with them.

[C. Furtaw]

OK, if I understand you correctly I leave the permability factor alone even if I chenge the layer thickness. Yes?
What may confuse me is that some material suppliers provide a permeance on the supplied material thickness but some materials permeance are based on a different thickness such as Tyvec that, in its materials DB, lists it's perm rating as 30 or 40.
CF