30 perms represents 10x more water vapor transmission through a film of 
specified thickness per unit of time under specified temperature and humidity 
differential. Or the short version: Yeah, it's ten times more breathable than 
3 perms.

1 perm is a somewhat arbitrary value that is generally considered to be the 
dividing line between breathable and non-breathable. Materials like epoxy 
coatings and terra cotta glaze (unless cracked or spalled) are close to zero.

Mike E.

  So are 30-50 perms  10x 
> more or 10x  less breathable than 3-5? What numbers do we want to 
> permit vapor  transmission (isn't that what we're measuring with 
> perms?), and what  numbers do we want to block vapor transmission 
> (which I assume we  theoretically want to do  sometimes? 

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