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Community and Q&A

“Smart” vapor retarders for areas with low summer humidity

StephenW81 | Posted in General Questions on

Literature for “smart” vapor retarders all appear to describe use based on low humidity winters vs high humidity summers.  However, the climate in the inland (east of the Cascades) Pacific Northwest, is moderate humidity winters and low humidity summers.  In this climate, is there any value to “smart” vapor retarders?  If there is value, do the differing vapor permeability curves of the available membranes (e.g. Membrain, Intello, or even plywood) make any difference?


  1. Expert Member
    Dana Dorsett | | #1

    Yes, smart vapor retarders still make sense in your climate. The outdoor humidity is mostly irrelevant- it's all about the indoor humidity and outdoor temperature.

    Typical wintertime indoor humidity in a reasonably tight house in most of Eastern WA will be 30-35% RH @ 70F. (If it goes very much lower than that people will often actively humidify the air to mitigate chapped lips & cracked fingernail discomfort.) That air has a dew point of about 37-41F. On a wood sheathed house with out exterior insulation the wintertime average temperatures will be well below that temperature.

    When the sheathing is 25F the dew point of the entrained air in the fiber insulation will be 25F, the moisture being taken into the wood sheathing. That makes for a vapor pressure difference between the 35F+ dew point indoor air and the 25F cavity air, causing water vapor diffusion through the wall. But at 30-35% RH and lower the vapor permeance of a smart vapor retarder is pretty low, limiting the rate at which moisture can diffuse from the indoors into the wall cavities.

    The vapor permeance of the sheathing is also pretty low, but as more moisture accumulates in the wood the more vapor permeable it becomes. The wood sheathing is also a sort of "smart" vapor retarder. But to ensure that the moisture levels stay low enough to not create a mold condition in the spring when temperatures rise to mold-growth temperatures some sort of vapor retarder tighter than standard interior latex paint is called for.

    With a smart vapor retarder when the sheathing is warming enough to start releasing it's moisture burden into the cavity air, the RH next to the vapor retarder rises the vapor retarder becomes more vapor-open, allowing the moisture to dry toward the interior faster than it allowed it to enter when the sheathing was cold.

  2. StephenW81 | | #2

    Dana, Thanks for your response. The thought behind my post was that very low summer humidity on the house interior side of the vapor retarder might reduce its ability to become more vapor open, and therefore, the vapor retarder would actually reduce potential drying to the interior during the summer. This assumes vapor impermeable insulation on the exterior limiting any drying to the outside. However, my understanding of your comments is that this is not an issue and reducing moisture passage into the wall/roof during the cold parts of the year is more important.

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