I’m considering insulating our garage: it won’t be conditioned unless I’m working in it (which is only occasionally).

My thought, though, is any insulation would benefit our home’s energy usage? (Attached garage)  

I’m trying to avoid pests, so it would seem to me closed cell foam would be best?  

Cost is an issue, but Rigid XPS foam may be in budget. 

My questions are: 

1: Would R10 XPS have significant gains over R7.5?

2: In 2×4 walls: would I still want a 6mil vapor barrier before my wallboard?  (Plan is to use canned foam to take up the gaps around the XPS)