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Vented Attic Vapor Retarder

user-6651407 | Posted in Building Code Questions on

Getting confused in my research today, looking for feedback.

Climate Zone 5, Vented Attic with Blown-In Fiberglass.

Is it correct, vapor retarders are not required in climate zones other than climate zones 6 or higher for vented attic assemblies?

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  1. GBA Editor
    Martin Holladay | | #1

    Bridget,
    Code reference numbers change from cycle to cycle. You should always check with your local code authority, to see which version of the model codes applies in your area. In U.S. jurisdictions subject to 2006 codes, the language on vapor barriers should be as shown below:

    "N1102.5 Moisture control.
    The building design shall not create conditions of accelerated deterioration from moisture condensation. Above-grade frame walls, floors and ceilings not ventilated to allow moisture to escape shall be provided with an approved vapor retarder. The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation.
    Exceptions:
    1. In construction where moisture or its freezing will not damage the materials.
    2. Frame walls, floors and ceilings in jurisdictions in Zones 1, 2, 3, 4A, and 4B. (Crawl space floor vapor retarders are not exempted.)
    3. Where other approved means to avoid condensation are provided."

    In the 2009 codes and later, well ventilated attics are not required to have interior vapor retarders. However, note these provisions in the 2012 IRC -- provisions which apply to Zones 6, 7, and 8, but not Zone 5:

    "R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. ...

    R806.2 Minimum vent area. The minimum net free ventilating area shall be 1/150 of the area of the vented space.
    Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more of the following conditions are met:
    1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling. ..."

    -- Martin Holladay

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