I agree with your use of "arbitrary". I think EPDM roofing has about 7 perms
and that is not really permeable. 

Eric Hammarberg, Assoc. AIA
Vice President
Thornton Tomasetti
51 Madison Avenue
New York, NY  10010
T 917.661.7800  F 917.661.7801  
D 917.661.8160  
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-----Original Message-----
From: edison [mailto:[log in to unmask]] 
Sent: Friday, May 05, 2006 8:09 AM
To: [log in to unmask]
Subject: Re: [BP] Eco-Armour Cote/Rhino Shield? WTF?

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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