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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