Vapour Barrier Location

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    Daniel Spilman

    Amy, 

    As you stated above, the general rule of thumb is that the vapor barrier should go on the warm side of the insulation. This is because warmer typically has a greater moisture content. However, as I'm sure you have discovered, there are exceptions to this rule. 

    Some of the best explanations I have found were in Graphic Standardsin the Exterior Vertical Enclosures section, page 432 in the 12th edition. 

    You could also check out this website which gives good insight, however its a little heavy on the technical information: 

    https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

    Lastly, here is a white paper by DuPont Building Innovations which gives a good explanation as well:

    http://cdnassets.hw.net/ea/14/96f9adb541d9aa368182e7420e56/vapor-permeable-or-impermeable-k-26352-tcm131-2135974.pdf

    I affirm your instinct to study this topic. I came across a couple questions in PPD and several questions in PDD that required an understanding of this topic. Hope this helps! 

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    AMY NICHOLSON

    Thank you Daniel.

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    Stephen Parra

    Just to add to Amy's concern regarding how candidates should handle the industry-wide ambiguity regarding vapor barrier standards absent more information from NCARB durging the exam... in terms of terminology does anyone know how NCARB actually defines a vapor retarder? i.e. Does NCARB adopt the terminology suggested in the building science article referenced above and differentiate between a "vapor barrier" as an impermeable class I vapor retarder and "vapor retarder" as semipermeable or permeable vapor retarder? 

    The warm-in-winter-side is great as a rule of thumb but every authoritative source in NCARBs handbook such as AGS understandably either qualifies and equivocates on standards to avoid liability or admittedly uses the term vapor retarder and vapor barrier interchangeably which is confusing. If the exam says "vapor retarder" in a "mixed climate" the warm in winter side rule becomes less useful without knowing the permeability rating of the proposed retarder. On top of it all I live in Miami!  Warm in winter side here still means OUTSIDE!

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    Daniel Spilman

    I don't believe anyone is just going to give you an answer here. Anyone that is studying for these exams needs to study this topic. There is are correct and incorrect applications of vapor barriers / vapor retarder. I would recommend reading the section in Graphic Standards to clarify correct application. It provides a clear explanation of how to evaluate the permeance (or permeability) of each material in a wall assembly to be able to identify where moisture issues could occur and therefore where to locate the vapor barrier. Graphic Standards is one of the sources that NCARB lists as a reference, so I believe this to be a trustworthy source to consult for the correct understanding. 

    As architects, we need to have a clear understanding of this because it is a very integral part of correctly detailing a wall assembly. Its a huge problem if we allow moisture to get into parts of the wall where it cannot drain out. This is something that anyone passing the ARE should clearly understand and be able to analyze any wall section in any climate. I will tell you that the questions that I came across concerning this topic had a clear correct answer if you have an understanding of the concepts. I do not believe you will be required to differentiate between a vapor barrier and a vapor retarder. 

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    Lale Ceylan (Edited )

    https://en.wikipedia.org/wiki/Vapor_barrier There is a very helpful map in this Wikipedia article.

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