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Community and Q&A

Two (air/vapor) barriers ?

tundracycle | Posted in General Questions on

When building a sauna you always have a foil barrier on the inside of the studs. Typically either FF-PIR or paper backed foil. Then an air gap and then interior cladding like T&G spruce. See ‘Notes on Construction’ here: 

Something like JM FF-PIR has a vapor perm of 0.05 and air barrier of <0.02 L/(s•m2)

What should the exterior be?  Tyvek is, supposedly, an air barrier but not a vapor barrier. If any moisture/vapor were driven in to the wall structure would it realistically be able to get out? Or best for the exterior to have no Tyvek or other air/vapor barrier?


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  1. Expert Member
    BILL WICHERS | | #1

    Tyvek is an air barrier, but is vapor permeable, so it's NOT a vapor barrier. The point of tyvek is to provide some protection against bulk (liquid) water, but to still allow drying of the materials behind it. Tyvek is usually considered to be a "weather resistant barrier", a "WRB", which will block rain, especially wind-driven rain, from getting into the structure, but without acting as a vapor barrier that would prevent drying of the structure and promote rot.

    You should be fine using Tyvek on the exterior, and it should make your structure more robust. With high interior side moisture drive, an exterior rainscreen -- which really helps to promote drying -- would be a good idea here too.


  2. tundracycle | | #2

    Thanks Bill. With what is effectively an air barrier on the inside (plus Tyvek on the outside) will the effective lack of airflow prevent moisture within from drying or should it still dry OK?

    Totally agree about the rain screen.

  3. Expert Member
    BILL WICHERS | | #3

    Airflow and drying aren't necassarily the same thing. Vapor open materials (like Tyvek) allow for drying while blocking airflow. You should be absolutely fine with Tyvek on the exterior in this assembly.


  4. tundracycle | | #4

    Thanks Bill.

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