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In a region that’s half-cold half-warm, does it matter which side of insulation you put a vapor barrier?

[email protected] | Posted in General Questions on

The vapor barrier goes on the warm side of the insulation, with warm side meaning the exterior if you’re in a hot climate and the interior if you’re in a cold climate.  What if you’re in a climate that’s half-hot half-cold?  Where the heating degree days and cooling degree days are equal, does it matter which side of insulation you put a vapor barrier?

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  1. Expert Member


    In that situation, I would put it "In the middle" and design the wall to dry towards both sides of the VB. In zone 3/4, there are hot summers and cold winters, and high humidity, and low humidity. If I had to choose inside or outside, I'd put the vapor barrier on the outside, unless it's really cold during the cold part of the year. The humidity outside (in zone A at least) will nearly always be greater than the humidity inside, and you want to keep that out. The HVAC should be able to handle the conditions inside.

    1. [email protected] | | #2

      I like that: Where temperatures are [even], vapor barrier goes on the humid side.

  2. Expert Member
    Akos | | #3

    I think vapor barriers in that climate are a bad idea. If you must have something, a vapor retarder is a better idea. You are also in a warm enough climate that almost any amount of exterior rigid provides condensation control, at which point no vapor retarders are needed.

  3. Expert Member
    PETER G ENGLE PE | | #4

    In the middle is always safe. And yes, it should be a Class 2 or 3 vapor retarder, not a Class 1 VB. If your outdoor temperatures don't vary from indoor by more than about 20F, then you don't need any VR at all. As the difference gets larger, you need to be a bit more careful about where and whether to add a VR.

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