I am getting my rough in inspection for my new outlets Thursday. After that, I would like to insulate and sheath my walls. 

The space is currently vented with one gable vent and a ridge vent. I know this isn’t right but since it has been so open there have been no issues. I added the ridge vent when I did the roof with plans of venting the soffit and closing the gable like I did on the main house roof.  

I know it seems silly but right now I am just focusing on closing in the walls. The space is not normally heated. I need to plan for how I am going to take care of the ceiling at some point too. It is 24′ in depth and has rafter ties there now. 

I am thinking rockwool batts in the walls. I do not want to use plastic vapor barrier. I am thinking maybe the CertainTeed membrain although I do not believe the permeability rating meets the requirement for Maines climate. I don’t hate the idea of using plywood sheathing but drywall would work too. 

What would you do for insulation and finishing in this space?