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Discussion Forum

Vapor Barriers

mman | Posted in Energy, Heating & Insulation on September 8, 2007 11:02am

I’d like to comment and solicit advice on vapor barriers.  It is my understanding that the purpose of a vapor barrier is to keep moisture from condensing in the wall/insulation.  So, since hot air carries more moisture, it makes sense that the vapor barrier should go on the hot side of the wall. In hot humid climates, where the houses are mainly cooled, then the barrier should go on toward the outer surface of the exterior wall.  In cold climates where houses are mainly heated the vapor barrier should go on toward the inner surface of the exterior wall. 

<!—-><!—->  <!—->

Now for my questions, first, I am building a house in <!—-><!—->Delaware <!—-> (<!—-><!—->Kent <!—-> <!—->County <!—-><!—->).  It seems that this area has a blended climate with three equal periods 4 months of heating, 4 months of cooling and who knows what in between the two seasons (some heating, some cooling and a lot of time with open windows).  I am very uncertain, where do I put the vapor barrier, or should I just use un-faced insulation in the Mid-Atlantic States like <!—-><!—->Delaware<!—-> <!—->? 

<!—-> <!—->

Second, I used to live and work in <!—-><!—->Michigan <!—-><!—-> and Upstate New York and throughout the 1980’s it was common for builders to use plastic for the vapor barrier.  When doing remodeling work in Upstate New York, I’ve seen about 3 out of 5 walls with plastic vapor barriers having some signs of rot.  Most of the time, the walls would be dry to the touch, but the wood and sheathing would have black areas or be streaked black.  Yes-the vapor barrier was applied to the interior face of the wall, but some how moisture seems to find a way into the wall cavity and once there it is trapped.  For that reason, I will only use craft-face for vapor barriers.  Maybe my experience is just a fluke, so I want to know have other people been seeing a signs of rot and mildew when they open up the exterior wall of homes with plastic vapor barriers? 

<!—-> <!—->

Help, I would really like to see some responses to both of these questions.

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  1. User avater
    BillHartmann | Sep 08, 2007 11:28pm | #1

    There are vapor BARRIERS And vapor retarders.

    Often when the term vapor barrier is used that means poly plastic. The problem is that is also a WATER BARRIER. And an condensation or leakage from the outside can dry out through the interior walls.

    You are right, you are in a Mixed Humid climate.

    In such a climate if you air seal the walls the then amount of humidity that goes through the building materials is low enough to prevent any excesive condensation.

    You can do that will with air tight drywall systems. Also sprayed foam is great. Along with desnse pack cels.

    But you really need to look at the whole wall construction from the interior paint to the exterior finish.

    http://www.buildingscienceconsulting.com/designsthatwork/hygro-thermal.htm

    http://www.buildingscienceconsulting.com/designsthatwork/default.htm

    Then go the technical research.

    http://www.buildingscienceconsulting.com/

    .
    .
    A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
    1. mman | Sep 09, 2007 12:57am | #2

      Thanks--that was quite a lot to asorb, but basically the paper suggest not to use a vapor barrier. 

  2. DanH | Sep 09, 2007 03:28am | #3

    One detail to understand. You only need the warm-side vapor barrier/retarder when the temperature on the cold side is likely to be below the dew point on the warm side for an extended period of time. Basically, so far as cooling season is concerned, this would mean that there are times during the year when condensation forms on the OUTSIDE of single-pane windows due to the air conditioning inside. Unless you keep you house as cold as an igloo, I doubt that this occurs very often in Delaware.

    Also, you can get mildew/rot in any wall if rain can get inside. Will happen regardless of the presence/absence of vapor barriers/retarders.

    So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
  3. hvtrimguy | Sep 09, 2007 05:32am | #4

    I think you have good insight. I too have been leary of the plastic vapor barier because I fear the moisture once in the wall system may get trapped. I suppose detailing it well would esure this wouldn't happen but in all reality, how many jobsites are carefully detailed. I like to think idiot proof nyself.

    I also agree with some other posts that keeping rain water out of the siding / roofing detail is also key. I would venture to guess that some of that mold or black streaking might be more of a product of that. A housewrap is meant to shed water as a secondary defense but also let the house dry to the outside so unless the conditions for condensation were constant, I don't think there would be a mold problem from the outside.

    On a third note, I'm begining to think that bathrooms need their own vapor bariers beyond the kraft. plastic might make sense there, but again detailing would be important. I know in a steam shower, all the joints and seams of the cement board need to be caulked and the cement board itself is best sealed before the tile install.
    so all that said, I think a lot comes down to how much attention is paid to lapping, taping, caulking, flashing, painting, etc. the vapor barrier is just one small component to the wall system

    "it aint the work I mind,
    It's the feeling of falling further behind."

    Bozini Latini

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