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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    Michael Doerneman

    As wonderful as these explanations are;  the question is actually quite simple.

    I'd like to argue that in ARE Land; there is no grey area; they're looking for a simple straightforward answer that tests your understanding of the concept

    Like said before; the warm in winter side is where the vapor barrier is provided;

    The question will clearly state; the LOCATION in which you will be considering example:  "... for a building in Miami" -

    For stucco buildings in Miami the Vapor Barrier is provided right after the finish material; for brick or masonry it can be direct applied like a paint;  Basically you're looking for the answer with the layer that's closed to the "warm in winter side" - that's the closest arrow or layer to the outside

    Now for the Opposite;

    For other locations like Denver;
    in Denver it's most likely that the interior of the building interior will be "warm in winter" - and therefore you're looking for a layer that's closest to the interior;

    So realistically you have 2 scenarios possible - you will determine the answer by WHERE THE BUILDING IS LOCATED;

    Last I checked; I think Miami and Florida are the only US cities that would be warm in winter;  I'd be a little amused and shocked if they were talking about Puerto Rico, Virgin Islands, Hawaii, or Guam; Because if you asked a person a question about Hagåtña people wouldn't know where that was! XD  But my understanding is that you won't get locations where the climate flirts with being north or south; like the Carolinas;  Either way;  Check your architects graphic standards and they have the zones diagramed in there;  

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    Carie Penabad

    Thank you all for the thoughtful responses.  

    As a follow up to the comments, in Miami it can often be warmer inside than outside in January and February.  In this case, the vapor barrier in Miami would be placed similarly to that in Denver (although the temperature differential between outside and inside is far less severe).  Should one ignore this scenario and focus on the days where the outside is hotter than the inside (i.e. 80 outside and 75 inside)? 

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    Michael Doerneman (Edited )

    No- think like a test writer; I know that reasonably yes maybe there’s one day a year that may be colder or a family that runs the heat in January. But realistically; there are only two scenarios.
    As my good friends have told me: in ARE Land there is no grey area. Consider simple reasoning;
    Warm in winter - Miami will always be the outside of the assembly; always

    I have also professionally done work in southern tropical areas in Florida and Texas; this is true in real life scenarios as well. Never provide waterproofing similar to Denver. Ever. It causes moisture to build up in the cavity and rot it out.

    Less thinking! More ARE passing!

    Make these type of questions easy gimme questions; there’s only two choices - and if you’re lucky you’ll get asked it twice, lol that’s definitely a possibly.  they may use the exact same wording but just change the location you are considering.  There’s only so many questions in the bank so it may happen;

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    Gang Chen

    “…the most important tip for quality control of a project is “Don’t leak and don’t fall.” For the “don’t fall” part, you need to pick a good structural engineer, and you need to do a good job to coordinate with him. For the “Don’t leak” part, you need to make sure ALL your exterior details need to work and keep the water and moisture out of your building.”  (Quoted from my book, Building Construction)

    Keep this in mind and you’ll know what to look for in the details. Knowledge on drip edges, proper flashing locations, location of moisture barrier within a wall section, etc. is very important.

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

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