Insulation Challenge: Highest R-Value in 2x6 rafters

Hi,
I,m finishing a house in Michigan with cathedral ceilings (short spans) made with 2x6s. The local inspector says that I need to have R-30. How can I get that much in that little space.
One thought I had was to use Dow Thermax (3" thick boards with R-19) or Super Tuff-R (2" boards with R-13), but I need to know if I can "stack" these, since they are faced, I don't think so, but would like to hear from someone that has used these products before.
Also, what would be the best sealant to use with the foam boards (Great Stuff, other spray in foam insulation, or something else)?
Thanks,
Pat
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