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Community and Q&A

Questioning the Need for a Vapor Barrier

bonai | Posted in General Questions on

hi there,

I can not wrap my head about why a house in these 2 zones need a vapor barrier membrane on the interior walls.

Any one of you as a good way to explain it?


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  1. Expert Member


    Can you flesh out your question a bit? Are you asking why it's a code requirement? Why just those zones? Or why from building science perspective?

    Vapour-barriers stop interior moisture moving through the wall by diffusion and condensing on cold surfaces.

  2. bonai | | #2

    My question is from a building perspective.

    Why having a vapor barrier was an issue in Florida years ago? I live in a zone 7 and we get cold winters but are the temperatures low enough in Florida to have water condensate during winter time? Where was the water condensing? On the membrane and then making the gypsum in direct contact damp? Now that they don't install vapor barrier anymore, the moist air goes in the insulation and next to the sheathing isn't it? we are just displacing the issue, no?

    I think my problem is every time I read about cold surface in an article, I imagine winter cold crispy surface but the reality is for vapor to condensate, we need the temperature to drop close enough to the dew point of the surrounding air mass - so we can have condensation at 65F for example, correct?

    1. Expert Member
      MALCOLM TAYLOR | | #3


      As Jon says below: In warm climates like Florida, or in any climate where air conditioning use in the summer means the interior of the house is much cooler than the surrounding air, the vapour-barrier ends up on the wrong (cold) side of the wall, meaning that moisture in that air will condense on the interior poly.

      In cold climate like 7/8, warm-side air and vapour barriers are used to keep moist interior air from making it's way into the walls and finding a surface cold enough to cause it to drop its moisture (usually on the sheathing).

  3. Jon_R | | #5

    In Florida, moisture drive reverses and an interior side vapor barrier causes warm/humid outside air to condense/accumulate at the cold interior side (vs continuing on to the interior).

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