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  1. #1
    Join Date
    May 2008
    Posts
    2

    Smile attic insulation

    Hi Question my house is 40yrs old has about 6 or 9 inches of insulation between rafter. Question is the vapor barrier is facing down is this correct?. Looking at the video Tom say if you live in a climate that is hot the vapor barrier should be up if this correct i would need to rip up the old insulation and replace it correct. We live in south.

  2. #2
    Join Date
    Jul 2009
    Posts
    5

    Default Re: attic insulation

    The place for a vapor barrier could be toward the inside, toward the outside, or both. Warm air can hold more moisture than cold air, and especially when warm air has high relative humidity, it is holding much more water vapor than cooler air can hold. When the moisture from warm air gets through the insulation (and some always does for insulation does not stop transfer, only slows it down) and comes into contact with the cold surfaces on the far side of the insulation and the cooler air next to them, that cooler air cannot hold as much vapor. So, some of the water condenses onto the cold surfaces, creating wet surfaces subject to rot, mold, and rust. In circumstances where the warm air is on the inside and the cold on the outside, the vapor barrier should be on the inside. Where the warm air is on the outside and the cooler air on the inside, the vapor barrier should be on the outside. In climates where both circumstances are true, depending on the season, a vapor barrier should be on both sides.

  3. #3
    Join Date
    May 2008
    Posts
    2

    Smile Re: attic insulation

    Gentlmen thank you both for your response will check with the building code.

  4. #4
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: attic insulation

    Quote Originally Posted by Rand View Post
    The place for a vapor barrier could be toward the inside, toward the outside, or both. Warm air can hold more moisture than cold air, and especially when warm air has high relative humidity, it is holding much more water vapor than cooler air can hold. When the moisture from warm air gets through the insulation (and some always does for insulation does not stop transfer, only slows it down) and comes into contact with the cold surfaces on the far side of the insulation and the cooler air next to them, that cooler air cannot hold as much vapor. So, some of the water condenses onto the cold surfaces, creating wet surfaces subject to rot, mold, and rust. In circumstances where the warm air is on the inside and the cold on the outside, the vapor barrier should be on the inside. Where the warm air is on the outside and the cooler air on the inside, the vapor barrier should be on the outside. In climates where both circumstances are true, depending on the season, a vapor barrier should be on both sides.
    I agree with jkirk.

    Rand .... your post was pretty good till the last comment which is incorrect .... you should never apply a vapor barrier to both sides of insulation.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  5. #5
    Join Date
    Jul 2009
    Posts
    5

    Default Re: attic insulation

    I stand corrected. The logic made sense, but it wasn't carried through sufficiently. Thinking about it more, if there is a barrier on both sides, any moisture that gets into the wall would be somewhat trapped.

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