When removing our 12 year old shingle roof due to hail damage the roofer discovered rot on the OSB that clads the outside of the SIPs. The rot was confined to the 1st 3′ of the panels from the soffit edge, under the ice and water shield. Rot was spotty, not continuous.The seams were still sealed tightly with no damage along seam lines. The rest of OSB was fine. Panels constructed with open cell foam rather than the newer, closed cell foam. Any thoughts about what’s causing the rot?
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Without knowing where you are I would guess a combo of snow accumulation on the eave and water vapor infiltration from below.
Probably wicking up from the unprotected lower edge of the OSB. The Ice and Water should have come to the bottom edge of the panels. I'd make sure it's done now while you have the chance.