I am about to begin construction on a house in central North Carolina. It is a basic rectangular gable roofed story and a half with dormers on the front. Due to the design of the house it will have the HVAC for the second floor in the attic. This obviously means I will need a sealed/conditioned attic. I do not want to use spray foam in the attic because I feel there are too many unknowns with it. I believe cellulose is the best way to go but do want to maintain an airspace in case of a roof leak that always seems to occur at some point. I also like the idea of using the exterior sheathing as an air barrier as described in a recent blog post at https://www.greenbuildingadvisor.com/blogs/dept/musings/airtight-wall-and-roof-sheathing.

My thinking was to follow the approached shown in the mentioned blog. On top of the sealed roof sheathing I would apply furring strips followed by another layer of osb/plywood. Any water that leaks past the top layer of sheathing would be caught by the second layer. In addition I think it would make it easier to vent the valley areas where the dormers and main roof meet.

My question is what potential problems have I overlooked in this design? Will I have any problems with not having an airspace between the cellulose and the first layer of the roof sheathing? Do I need to have an air barrier like drywall or thermaply over the exposed insulation in the attic rafters? Any other issues I should be aware of?

Thanks for your input.