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  1. #1
    craig pedersen's Avatar
    craig pedersen Guest

    Default VAPOR BARRIER LOCATION

    I bought a house built in the 90s with vapor barrier on inside of wall behind drywall, no exterior sheathing, only 'bearhide' and vinyl siding. I want to replace the vinyl siding with fiber cement siding but am concerned about moisture getting trapped in wall, if I install a second vapor barrier under new fiber-cement siding. Installing a second vapor barrier seems like a bad idea as that would trap summer moisture in wall cavity/insulation.

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    Last edited by craig pedersen; 02-07-2013 at 07:22 AM. Reason: clarity
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  2. #2
    Join Date
    Mar 2007
    Location
    Orlando, FL
    Posts
    1,340

    Default Re: VAPOR BARRIER LOCATION

    Start with reading the guidelines by James Hardie, one manufacturer of fiber cement siding, for a primer on what's required.

    http://www.jameshardie.com/pdf/insta...plank-hz10.pdf


  3. #3
    craig pedersen's Avatar
    craig pedersen Guest

    Default Re: VAPOR BARRIER LOCATION

    Thanks, I am an architect, familiar with installation guidelines. Hardie suggests a VB behind siding. I believe two vapor barries, interior and exterior are a problem


  4. #4
    Join Date
    Feb 2008
    Location
    Caledon, Ontario
    Posts
    5,005

    Default Re: VAPOR BARRIER LOCATION

    You require an air barrier such as Tyvec behind the siding not a vapour barrier.


  5. #5
    Join Date
    Mar 2007
    Location
    Plano, Texas
    Posts
    4,170

    Default Re: VAPOR BARRIER LOCATION

    You are correct to worry about two vapor barriers. The wall has to be able to dry to one side or the other. It depends on the climate as to the best location for a vapor barrier. There are however vapor "retarders" that are somewhat vapor permeable. You definitely need a WRB (water repellant barrier) to stop liquid water behind the siding but some products like old fashioned building paper installed shingle style will fulfill both the WRB and permeable barrier.
    For a deeper understanding of the physics behind vapor barriers, thermal barriers, WRB and the like; visit the Building Science Corp. website.

    Building Science Information

    Jim Luttrall
    www.MrInspector.net
    Plano, Texas

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