2X8 rafter high R-value

newmanab

Need a little direction here:

2x8 ceiling joist in a bonus room above a garage, located in Michigan.
want the highest R-value without building ceiling down.

was looking at closed cell spray foam.
--will 2 pound density break away over time (to rigid?)
--should i get 1/2 pound density?

Should i do splash and dash?
-- or will that create 2 vapor barriers?

seems like everyone has their own idea on rafter baffles with spray foam!
-- what does the roof shingle manufacturers have to say?

any other ideas???

Thanks for your help in advanced.