Brainy Membrane

July/August 2006
This article originally appeared in the July/August 2006 issue of Home Energy Magazine.
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July 01, 2006
A vapor retarder that changes permeability with changing relative humidity provides added protection against moisture problems in exterior walls.
        In cold climates such as Wisconsin’s, the common practice of installing vapor retarders on the room side of wall insulation reflects the reality that indoor air humidity is higher than outdoor air humidity for much of the year. In this case, kraft paper or polyethylene vapor retarders restrict the diffusion of water vapor from household moisture sources into wall cavities. However, even if cold-climate building assemblies tend to dry to the outside, there are often times when there is a potential for drying to both the outside and the inside. In the case of a 2 x 4 bottom plate that is saturated due to a water leak, an interior vapor retarder eliminates the possibility of significant drying to the inside. All else being equal, that bottom plate will stay wetter longer if it can dry only to the outside than it will if it can dry to both the outside and the inside. In addition, there can be instances where there is a potential for building assemblies to dry to the inside but no potential for drying to the outside. Even cold climates have sunny and humid afternoons when the outside environment acts ...

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