Framingham, MA, US
2015iecc section 503.1.1 under exceptions
If you remove the drywall you only have to fill that cavities with insulation, not bring the wall up to code.
Isn't your roof assembly a bad sandwich? If water gets between the 2 layers of rigid, into the fiberglass batt area will it be able to get back out? The water will sit in the fiberglass and rot your rafters won't it? At least thats my understanding of roof assemblies.
Doesn't the paint on the zip sheathing change the air and water barrier properties of the zip product?
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