Replies

1. Expert Member
   MALCOLM TAYLOR | Jun 10, 2021 11:50am | #1

   Bonai,

   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 | Jun 10, 2021 12:25pm | #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 | Jun 10, 2021 01:15pm | #3

   Bonai,

   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. 
   creativedestruction | Jun 10, 2021 02:03pm | #4

   This may also be of help:
   https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

4. 
   Jon_R | Jun 10, 2021 02:04pm | #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).