Do My External Insulated Walls Even Need Sheathing?
I am building a heated garage to be built in Minnesota – climate zone 6. I have designed the walls as follows (in to out): gypsum wallboard -> 2×6 stud wall 16″ o.c. w/ cellulose insulated wall cavities -> 7/16″ OSB sheathing -> WRB -> 1 1/2″ XPS foam panels (R7.5) -> 3/8″ strapping to create rain screen (w/ venting above & below) -> cement fiberboard cladding above & stone veneer cladding below.
Having read a great deal about the risks of “injury” to the sheathing if temp at sheathing below dewpoint (e.g. https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing), I have a simple (stupid?) question….
…does one even need to put wood sheathing on?
While I could consider increasing my XPS layer to 2″ thick as advised here on GBA and leave the OSB sheathing, that extra 1/2 inch of wall thickness would entail multiple additional design changes that I would prefer not to deal with. So, maybe I could ditch the sheathing and increase the XPS to 2″ and never have to worry about the sheathing getting cold?
…or should I reverse the XPS and OSB? Thereby bringing the OSB into a space where it can dry – adjacent to the rain screen.?
Of course, James Hardie does not approve installation of their product over greater than 1.5″ of XPS (see http://www.jameshardie.com/pdf/USTB-Applying-James-Hardie-Siding-over-Advanced-Framing-or-Continuous-Insulation.pdf), so really, I have issues no matter what, it would seem.
Thanks for the insights! Great resource, for the DIY builder/nerd, by the way.
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