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Double stud wall and rainscreen

Stephen Watts | Posted in Green Building Techniques on

A double stud wall appears to be one of the primary framing techniques for a high R-value wall without foam.  However, a ventilated rainscreen appears to be required to maintain wall integrity.  There is a conflict between wildfire resilience and wall ventilation; the smaller the rainscreen cavity, the better from a wildfire perspective (all other things being equal).

Assuming a 10-12″ thick double stud wall insulated with cellulose, mineral wool, and/or fiberglass (R-36 to R-50 center of cavity) with effective interior air barrier and vapor retarder (e.g. Intello) in climate zone 5, how much of a rainscreen cavity is required for effective ventilation necessary to maintain long-term wall integrity?

Oregon Code requires a 1/8″ rainscreen airspace or a WRB with an integral drainage plane with >75% drainage efficiency (typically ~1mm airspace).  Is the Oregon requirement sufficient? Is it necessary for the cavity to be open at both top and bottom?  If you use a mat (e.g. VaproMat) to create a rainscreen cavity, it shouldn’t inhibit drainage, but does it inhibit ventilation compared to open airspace?

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Stephen,

    From Martin's comprehensive article "All about rain-screens":

    "How big a gap do I need?

    A rainscreen gap doesn’t have to be very big. Researchers have learned that even a 1/16 inch gap provides a capillary break, allows drainage of liquid water, and permits “diffusion redistribution.” However, you’ll probably find that (unless you are using a wrinkled housewrap) job-site realities and variations in material thicknesses usually require a rainscreen gap to be at least 1/4 inch deep.

    Having read this information, many builders use 1/4-inch wooden lath or rips of 1/4 inch plywood for their furring strips. Other builders use 1/4-inch-thick plastic drainage mats. These 1/4-inch gaps work well.

    However, some builders prefer more leeway for installation errors. They may be worried, for example, about puckers in the WRB or bulges in the sheathing that may compromise the 1/4-inch gap. They’re more comfortable with a deeper gap — perhaps 3/8 inch, or 1/2 inch, or 3/4 inch — because it allows for a few minor on-site problems or installation glitches. A 3/8 inch gap is more than enough for drainage, and is also enough to provide useful ventilation drying.

    Logic dictates that homes in wet climates benefit from deeper rainscreen gaps — up to about 3/4 inch — because deeper gaps allow faster ventilation drying.

    When choosing the depth of your rainscreen gap, there are issues other than water management that come into play. Builders know that 1/4-inch or 3/8-inch rainscreen gaps simplify exterior trim details. It’s much easier to trim out (and flash) a wall with 1/4-inch furring strips than a wall with 3/4-inch furring strips.

    Finally, if your furring strips are being installed over thick rigid foam, you don’t really have a choice on furring strip thickness. You’ll need 1x3s or 1x4s that are 3/4-inch thick in order to have something to attach the siding to."

  2. Jon R | | #2

    > how much of a rain screen cavity is required for effective ventilation

    Good interior side air sealing, cellulose, a smart retarder and permeable exterior (eg, plywood or gypsum) mean you need less than without these things. You could attempt to use WUFI to quantify the relative differences.

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