Vapor Barriers

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    David Kaplan

    Carie,

    I'll let others speak to the locations for both hot and cold climates.  I think that's a bit of a grey area and I don't think that you'll be asked such a question on the ARE.  It will be specific - one or the other - not both.

    However, you for sure do NOT want to have two vapor barriers in one wall section.  This creates a "sandwich" and if vapor should get in between the two layers, you've just now created a space where it can't get out.  We've had several product presentations in our office about vapor barriers and this has been addressed many times.  One only.

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    Michael Ermann

    This is way too complicated a topic to properly address in this forum (it is 1.25 hours of video in the Amber Book) . . . so I'll try to summarize what you need to know:

    1. The vapor control layer goes on the warm side of the insulation, against the insulation. Because the answer to your question is prohibitively complicated in the 80% of the US that has warm summers and cold winters, I don't think that it is worth knowing this for the test.

    2. That said, it is VERY worth knowing for your career, as this is by far the most common question i get from practicing architects. Know that the idea of a vapor "barrier" at all is based on 60 year old science, that wasn't good science even back then, and applied only to cold weather climates with old-style not-very-airtight building construction methods. Unless you are building in Minnesota with tar paper or tyvek over OSB, it really isn't that useful. The job of the vapor control layer, as we now know it (the A.R.E. doesn't know this yet) is not to prevent vapor from moving through an assembly, but rather to throttle it (WHILE ALLOWING VAPOR TO PASS THROUGH SO THAT BUILDING ASSEMBLIES CAN DRY OUT).

    3. As David wrote, the most important thing to know about a vapor "barrier" in your practice is to not specify a second vapor-impermeable layer (plastic or vinyl sheet product) in an assembly. Only one of those per assembly, please, so if you have a fluid-applied rain barrier, a peel and stick product, closed cell spray foam insulation, or taped rigid insulation (each of which doesn't allow water or vapor to pass through), you definitely don't want a second low-perm barrier to trap moisture.

    4. The building scientists tell us the best assembly for almost any climate is, from outside to inside: (1) exterior cladding (for instance, brick), (2) airspace, (3) exterior-grade insulation (for instance, XPS), (4) a single layer (often fluid-applied) that provides rain/air/vapor control, (5) sheathing (OSB or exterior-rated gyp), (6) an uninsulated cavity with pipes and conduit and ducts as necessary, (7) interior finish (for instance, interior rated gyp). See the writings of building science corporation or High Performance Enclosures by Straube  here, and follow Building Science Fight Club on Instagram.

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    Gang Chen (Edited )

    For ARE exams, a site is only in hot or cold climate, not both.

    It is also not a good idea placing a vapor barrier in 2 locations within the same wall section because it will trap moisture between the 2 barriers.

    Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)

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    Gang Chen (Edited )

    A real story can tell you why it is a terrible idea to use 2 vapor or moisture barriers:

    A friend of mine bought a house several years ago. The house looked very nice and had beautiful wallpaper on all walls in all rooms. One year after the sale, they wanted to do a remodel of house, but once they opened up the walls, they discovered the all the walls and some major structural members were molded. This was caused by the wallpaper acting as a second vapor barrier and trapped the moisture within the walls. This was in Southern California with a pretty dry climate.  You can imagine the problem will be even bigger in a humid climate.

    They had to spent over one hundred thousand dollars extra to fix the mold problem.

    Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)

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