there is no closure on the other side of the insulation, no need to unless someone else is living on the other side..

Concern for the pest is legit, you need to seal proof all perimeters of foundation, no cracks.

plywood for floor base is common in new constructions.

问给你施工的contractor 啊。每个州要求都不同吧，这里的大妈有几个能知晓？

加建需要申请PERMIT,这个city都有检查......这样就通过了吗》

可以咨询：专业的加建公司，