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Sheathing moisture in a temperate climate

MALCOLM TAYLOR | Posted in General Questions on

I don’t want to sidetrack the discussion in the Blog on sheathing moisture, so I’ll ask a related question here:
In a temperate but wet climate like the PNW, are seasonal elevated moisture levels more serious as the outside temperatures never get cold enough to inhibit rot or mold?

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  1. GBA Editor
    Martin Holladay | | #1

    I don't think that there is a yes-or-no answer to your question.

    In any climate, you want to choose a forgiving wall assembly that promotes rapid drying of any component that may get damp. Local experience should govern; in the Pacific Northwest, the lessons from the so-called Leaky Condo Crisis in Vancouver apply -- namely, make sure that flashing details are perfect; be wary of stucco; and make sure to include a ventilated rainscreen gap between your siding and your sheathing.

  2. Expert Member
    Dana Dorsett | | #2

    +1 on the rainscreen gap for the foggy-dew side of the passes! See the test results of walls 9 through 12 (p.23-26), of this document, at a test facility in Puyallup, WA (Puget Sound region, near Tacoma, WA.)

    All four had interior side poly vapor barriers and fiber-cement siding. Walls #9 &12 were unvented but plywood & OSB respectively. Wall 10 had a 3/4" rainscreen gap, but only open at the bottom, wall 11 was open top & bottom, allowed to convect.

    The unvented walls both had excursions north of 15% moisture content in the sheathing (sensor MC3, in red), whereas neither the bottom vented or convecting vented versions of the wall broke 12% at any of the moisture sensors.

    One would have to speculate how well they would do without the poly sheeting, particularly in a high interior RH condition like the one tested (55% RH continously). I'm pretty sure they would all be running higher with only latex paint as the interior vapor retarder, and that the sheathing on the unvented siding assemblies would run MUCH higher. Under a high interior RH latex-only vapor retarder condition more pronounced differences between the bottom-vented and fully convecting rainscreened assemblies may show up as well.

  3. Expert Member

    Dana, that's encouraging. Our code mandates the use of rain screens and it is still common practice to use poly too. Using those strategies, how risky do you think double walls are in this climate?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "How risky do you think double walls are in this climate?"

    A. Like you, I'm interested in Dana's answer. However, this is a much-debated topic among building scientists right now, subject to a lot of ongoing research -- so Dana's valuable and educated opinion will simply be one among many. All of us who read the literature are reading the same data, so the answer to your question depends mostly on your appetite for risk.

    My advice (since I wrote my 2013 article) is: Use plywood, fiberboard, board sheathing, or DensGlass Gold for sheathing, not OSB, and include a ventilated rainscreen gap between the sheathing and the siding. If you do that, you shouldn't need to worry.

    If you want your sheathing to be as dry as possible all year 'round, including in February, don't build a double-stud wall. Instead, build a 2x6 wall with insulation (rigid foam or mineral wool) on the exterior side of the sheathing.

  5. Expert Member

    Sound advice.

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