MyFloridaCode.com

[Florida Code Talk] Vapor Barrier
Joseph Lstiburek joe at buildingscience.com
Sat Feb 6 15:22:31 EST 2010


You are correct that the vapor retarder requirments are geared to 
cold climates.

The ICC has very specific requirements on vapor retarders with 
unvented roof assemblies.  In climate zones 5,6. 7 and 8 any 
air-impermeable insulation must be a Class II vapor retarder or have 
a Class II vapor retarder or covering in direct contact with the 
insulation.

These requirements can be translated as follows - open cell foam (low 
density foam - 0.5 lbs/ft3) - must be used in with a vapor retarder 
in cold climate unvented attic assemblies.  Closed cell (high density 
foam - 2.0 lbs/ft3) does not need additional vapor resistance.

In warm climates open cell foam/low density foam does not need any 
vapor control.  So, in Florida, no vapor retarder is required in 
unvented attic assemblies.

In wall assemblies, no interior vapor retarder is required 
irrespective of insulation system.  You can use any insulation 
without any form of vapor control.


>Well, I just googled "open cell foam and vapor barrier" and most all the
>sites mentioned needing a vapor barrier with open cell.  Based on other
>research I've done in the past, I figured most of these were mainly geared
>toward the colder climates but wanted to make sure. 
>
>
>-----Original Message-----
>From: Joseph Lstiburek [mailto:joe at buildingscience.com]
>Sent: Saturday, February 06, 2010 1:23 PM
>To: Ken Rodgers
>Cc: codetalk at myfloridacode.com
>Subject: Re: [Florida Code Talk] Vapor Barrier
>
>An interior vapor retarder is not required in ICC climate zones 4 or lower.
>
>I am curious as to what research you have done regarding a vapor
>barrier over open cell foam?
>
>
>>Content-Type: multipart/alternative;
>>	boundary="----=_NextPart_000_0052_01CAA72C.7C932C60"
>>Content-Language: en-us
>>
>>Can someone give me the code reference for interior vapor barrier
>>requirements over insulation.  I'm specifically trying to find the
>>requirement (if any) for a vapor barrier over open cell foam.  The
>>research I've done seems to indicate the need for a vapor barrier
>>over open cell but there seem to be a few other opinions out there
>>as well with regards to our part of the country.  Just wondering if
>>the code addresses this specific situation.  I'm sure this question
>>will also spark a good foam vs. anti-foam debate so I look forward
>>to learning from that as well.
>>
>>Thanks
>>
>>Ken
>>
>>
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