Double stud wall and rainscreen
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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