Exterior Wood Fiberboard Insulation with WRB
I am planning to build a wall with exterior wood fiberboard (MSL Eco4, R4, 26 perms) over 2×6 filled with Rockwool. The fiberboard will be covered with a regular WRB membrane, double furring to create a rainscreen cavity, and finally covered with vertical metal siding. Drying will be exclusively toward the exterior since I will be using a foil-faced polyiso board as my vapor-barrier on the warm side. Construction zone 6, Prov. of Quebec
I recently saw the 2021 Sweet 16 wall design contest and saw comments from Christine Williamson on two walls with exterior wood fiberboard covered with WRB housewrap. Her comments were that she found those assembly risky and that “…it is not practical to detail an unsupported membrane that is continuous enough in this application”. She suggested in one case to cover the WRB with exterior gypsum sheathing and in the other case using a drainage mat or textured WRB to allow some drying between the furring and the wood fiberboard. Both walls have rainscreen cavities.
I’m wondering if I should be concerned with my wall as detailed above, despite having a generous rainscreen cavity. Are we talking about a purely theoretical risk or are there documented issues with fiberboard insulation covered with a standard WRB membrane (beside sloppy installation that is). If I’m confident my builder will do a good job installing the WRB, should I change something to my wall to add another layer of protection? Adding 2000sq ft of drainage mat would be very costly. A textured wrap would be more feasible.
I realize that wood fiberboard used on the exterior side might not be as resilient as plywood if exposed to water but exterior wood fiberboard is apparently common in Europe and likely to become more popular in North America as well. Should these walls be protected differently than walls with plywood or OSB?
Thank you for our guidance
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