vapor barrier on “warm side” refresher?



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    David Kaplan

    Jonathan - I got these questions on PDD only (just my personal experience).  Didn't get any of them on PPD.  Others may have had different experiences, but I think you'll find this to be the case.  PDD really gets more into the details of a project, wall sections, roof details, etc.  That's why it's more applicable to that test.

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    Jonathan Chertok

    thanks david
    i’ll try to post a graphic and some notes on this for others. i looked at it last night and it seems that a visual may be helpful to others in testing on this type of question.
    awesome help and THANKS

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    June Wang (Edited )

    the rule of thumb is to always have the air/vapor barrier right against the face of the wall (CMU/concrete/studded wall...etc)...then the insulation, air gap and finished material/panel on the exterior. it's rain screen approach.

    ultimately your waterproof membrane on the roof/terrace needs to be able to overlapped by the a/v they are always always on the face that's closest to the structure. i don't believe you should install a/v barrier on the interior side at all.

    i can't speak for FL weather...but this detail would apply to the rest of the US...

    to get back to your questions, warm side usually is indicating the interior side....your wall detail applies to a whole year so basically we design the wall for the winter months. when it gets warm you have a reverse climate condition, the wall still breathes...that's why we use a/v barrier instead of wpm on the vertical surfaces.

    are you serious you need to remember all the zones?? the zones are defined by the ASHRAE which i don't believe we need to memorize it....

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    Jessica Godwin

    The way I understand it is that you place the vapor barrier on either the "inside" or "outside" of the insulation. So for a humid climate, the vapor barrier would be on the "outside" because warm exterior air is meeting cold interior surfaces (the A/C is running). For a cold climate, the vapor barrier would be on in "inside" because warm interior air (the heater is running) is meeting cold exterior air.



    Btw, I saw a vapor barrier question on my PPD exam, so it's safe to assume you might see for both PPD and PDD. 

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    Jonathan Chertok (Edited )

    OK. i assume no one will ask about vapor barrier perm ratings but here is what i have. also, there is a good article in building science called "all about climate zones". i assume it is associated with lsitiburek.

    COLD climate (think northern US): IMPERMEABLE vapor barrier on /INSIDE/ (where the warm/moist air is made by people and cooking etc). in cold climate you /also/ can (or probably must, not sure) put a PERMEABLE vapor barrier on the outside to keep AIR out. this "NON-WARM SIDE" permeable vapor barrier has to be permeable to let the vapor out of the wall section if it gets in - since the interior (warm side) vapor barrier is a true barrier since it is impermeable. technically i am not sure when you just start calling a vapor barrier an "air barrier" but anyway, to continue...

    MODERATE climate (think mid atlantic, central US and parts of the western US): vapor PERMEABLE barrier on the /OUTSIDE/. it is /permeable/ so warm/moist air from inside can get out and is not trapped inside the wall section.

    HOT-HUMID, WARM HUMID and HOT-ARID: vapor barrier on /OUTSIDE/ (this is the warm side and you are keeping warm and or warm-humid air out of the building envelope). this breaks down slightly since the climate zones are sometimes mixed and sometimes humid (lots of vapor in the air you really want to keep out) and sometimes arid (not much vapor in the air to keep out). but for general answers vapor "barrier" on the outside.

    note western US is "Dry" until you get to the "Marine" zone on the west coast. eastern US is "Moist". i guess this has to do with global weather patterns (or something).

    "permeable vapor barrier" means it is keeping air out and moisture out to some extent.

    "impermeable vapor barrier" means it is keeping air out and moisture to a large degree depending on the perm rating.

    "vapor retarder" is probably a better way of saying "vapor barrier" since it is not really a barrier and is dependent upon the perm rating (which tells you how much vapor it can keep out).

    "air barrier" pretty synonmous with "vapor retarder" AFAIK. keeps air out but lets some vapor in/out depending on perm rating.

    air hole in your building envelope = way, way, way more important in terms of letting vapor into your building than the vapor retarder. if your vapor retarder is not tightly sealed you almost may as well not put one in.

    <edit: actually jessica's images might be more appropriate for candidates taking the exam since it is so straightforward. THANKS jessica.>

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    Benjamin Norkin


    I agree with your descriptions above, I think you have more knowledge than is required for the ARE. I made a video with some things I concepts I thought were important for PPD/PDD, I talk about vapor barrier location at around 6:40:

    June, I remember seeing an example problem where you had to know the climate zone numbers (which I don't, except for my own) but on the actual exam I'm pretty sure you just need to be able to distinguish between obviously North and obviously South climates, like Florida vs Maine.

    Jessica, on your warm humid diagram, the vapor barrier would more commonly go on the outside of the sheathing. This is usually an all-in-one product with your house wrap/wrb, like Zip sheathing, or Henry Blueskin or Typar, etc. That's what we do in the mid-atlantic. Like Jonathan mentions, it's a really complex subject, on the ARE I don't believe you have to differentiate between levels of permeability and distinguish a vapor barrier from a vapor retarder. Know which side is the warm side and try to understand how moisture might move through your wall. What if you put the vapor barrier on the wrong side? What if you forget it?

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    Jonathan Chertok

    hi benjamin
    thanks a ton
    did you ever work a dew point example? i have one in ballast and took a stab at it but hat to take another run at it. i’d like to run one and guess i am going to start a post on it. seems sort of straightforward but i seem to be getting hung up...

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    Benjamin Norkin (Edited )

    No, that's something I wish I knew more about. I'll wait for your diagram. I think you're good on this stuff for the test. You need to schedule an exam and take one already!

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    Jonathan Chertok

    hi benjamin,
    i always find these helpful. also i know everyone has a boat load of material to track down.
    presumably the answers are as follows(?):
    vapor barrier on the wrong side means you will trap moisture that /condenses/ in the wall section because it can’t get out from the warm side to the cooler side. you want to keep it out but you don’t want to trap it.
    if you forget it you are (presumably) letting moisture into the wall section (either from the interior or from the exterior) and it can condense in the wall section. or it can migrate /into/ the building which you don’t want. OTOH, you /do/ want it to get out.
    this is why a low perm rating is very important in many climates. and also why taking about locating “vapor barriers” and not discussing vapor retarders (instead) or perm ratings is so reductive as to induce a kind of cognitive dissonance that is difficult to parse. i.e. where do you locate a “vapor barrier” in a hot- humid climate? well, you don’t because it will trap moisture since “barrier” implies a high perm rating which by definition implies trapping moisture. where do you put an “air barrier”? same issue AFAIK. an air barrier in a cold climate in the outside of a wall section has to have a very low perm rating to work - and an air barrier can be a vapor /barrier/ as well or it can let moisture though (again depending on the perm rating).
    i mean, not to put too fine s point on it but

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    Jonathan Chertok

    one does have to sort of deductively think about these questions to get them right.

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