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Condensation issue created if polyiso installed on exterior of kneewall containing kraft-faced fiberglass batts?

Chris Albrecht | Posted in Energy Efficiency and Durability on

I am working on a home in the warm humid region of the IECC designated zone 3. The finished room over the garage has kneewalls that currently have R-13 kraft-faced fiberglass batt insulation in the stud cavities.

I would like to add at least 1″ to 2″ of rigid foam board, preferably polyiso, in order to reduce air infiltration and increase the walls thermal resistance. I am concerned that I may initiate condensation issues within the wall cavities by combining the kraft-faced vapor retarder with the vapor barrier properties of the polyiso. The kraft paper retarder faces the conditioned space and is in contact with the back side of the 1/2″ drywall air barrier.

I intend on fastening the foam board directly over the studs on the unconditioned side of the kneewall and taping all of the seams. Will I instigate moisture and trap it in the stud cavities if this is done? Can I use less expensive extruded rigid foam board without the foil backing and achieve decent results?


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  1. GBA Editor
    Martin Holladay | | #1

    1. First of all, the kraft paper is not a vapor barrier -- it's only a vapor retarder. It is much safer than a polyethylene vapor barrier, because it allows some inward drying.

    2. Secondly, people install rigid foam on the exterior of walls all the time. As long as the foam is thick enough, it won't trap moisture. In fact, it makes the wall safer by keeping the stud cavities warmer (and therefore dryer, because they are less subject to condensation). To read more about the theory and the details of exterior rigid foam on walls, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

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