Cumulative effects of perm ratings in a wall assembly

[Michael Hurd]

A product manufacturer could not provide to me the perm rating of their insulation product. What they could provide me was the Perm rating of their product attached to 1" of concrete that is combined with .375" of granite. I have no idea why they tested their product this way. The concrete/granite assembly has a perm rating of .19, when the 2.5" of insulation is added to the concrete/granite the resulting perm rating, for the three products combined, is .10 perms. From what has been provided to me, Is there a calculation that can be performed to ascertain the perm rating of the 2.5" of insulation only?

[Manfred Kehrer]

Of course there is.
The resistances add up to the total resistance. Perm is the reciprocal of the resistance. So in this case:
Resistance of insulation = 1/0.1 - 1/0.19 which results in 0.21 perm for your insulation.

(P.S. Next time I will send you an invoice for that )

[Michael Hurd]

Well you can send an invoice, but whether or not I pay it is another matter! As always, thanks!