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I live in Iowa and I want to know if it is necessary to have a vapor burrier on my ceiling if I have roof vents

cwalker1965 | Posted in General Questions on

I’m also considering installing an attic exhaust fan, not sure how to determine the size of fan I need.

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Replies

  1. Dana1 | | #1

    In climate zones 5 & 6 you don't need a ceiling vapor barrier. The ceiling needs to be air tight however. Soffit to ridge venting is preferred, with the cross section of the soffit vent area 30-50% larger than the ridge vent area. Larger soffit than ridge vent area keeps the pressure difference between the conditioned & attic spaces at the ceiling plane limited, so as not to drive infiltration through the ceiling leaks with the stack effect of the attic height.

    Attic exhaust fans are a universally BAD idea in reasonably insulated houses, especially in cold climates. Depressurizing the attic draws conditioned space air into the attic through any & all leaks in the ceiling plane, which increases the heating & cooling loads. In hot climates small self-powered attic ventilators can sometimes reduce total energy use in houses with sub-code insulation levels, provided there is sufficient cross section to the soffit or gable vents to limit the amount of depressurization.

  2. GBA Editor
    Martin Holladay | | #2

    Chris,
    Dana Dorsett gave you good advice.

    For more information on these issues, see:

    Do I Need a Vapor Retarder?

    All About Attic Venting

    Fans in the Attic: Do They Help or Do They Hurt?

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