Foam sandwich concerns
I suspect I know the answer to this already, but it bears exploring as the “risk” will impact my wall design for the future house.
First, the pertinent data: I live in zone 6 Minnesota, and the structure in question is a garage/shop. In floor radiant heat, no humidifier, design temp of 60F. 2×6 at 16″ o.c. construction with 7/16″ OSB sheathing. One and one-half inch XPS was placed outide the sheathing (before I educated myself and learned that 2″ XPS would have been a better choice for 2×6 in zone 6), followed by WRB wrap. 3/4″ 1×3 stapping to create rain screen, then fiber cement lap siding
Now for the question: how stupid would I be if I proceeded to apply 2″ of spray foam to interior stud bays?
I know that creating a “foam sandwich” would not provide an avenue for drying the OSB if it got wet, and thus is rightfully discouraged. But, with the rain screen and WRB, it seems that water will have difficulty getting to the OSB; and if it did, it must have accessed it through a crack that perhaps then would provide an equal opportunity for drying? In fact, the XPS is full of “cracks” anyhow as the seams between the XPS sheets didn’t get taped before some overzealous hired hands applied the WRB.
I am a fan of closed cell spray foam – for air sealing, adding structural strangth, and good R-value per inch of thickness. However, while the above described wall is nearly complete, the stub bays sit vacant so filling with unfaced fibergalss batts is an available option. I want to spray foam, but I do not want to create a wall destined to fail if that’s the opinion of people far smarter that I!
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