Insulating and vapor barrier around chimney – GreenBuildingAdvisor

I am building a pretty good house with a cathedral ceiling and hot roof. I have a wood stove, and the chimney is framed in. It’s a metal Class A chimney with a square support box. The manufacturer (ICC) told me I cannot put spray foam within 1 inch of the support box, and that I should use Roxul to insulate right up next to the support box. 

The rest of my roof is insulated with closed-cell spray foam to prevent condensation on the under side of the roof deck. How can I prevent condensation around the chimney if I use roxul? Do they make any vapor-impermeable products I don’t know about?