Insulation, Radiant and Vapor Barriers - Ridgid foam insulation boards

I am in the process of finishing my basement and need a solution for a hallway. Space is tight so rather than framing out with 2x4 I am going to use 1" or 1 1/2" Z-Channel on the exterior concrete wall to save 2.5" or 3".

So.. this brings me to insulating that wall. I did some quick looking around, and the obvious easy find is Owens Corning "Foamular" Ridgid insulation that I can get from the local Home Depot. 1" is listed as R5, 1.5" is listed as R7.5 (I am assuming these are measurements of initial R value as this is closed cell extruded polystyrene and I am not sure how well this holds up over long term).

Can anyone point me to other similar products that might have a better R value rating, and something that holds up over time? Can you point me to where I can buy it? I have read some on the net about other materials that perform better than R5 per 1" of material.. but have not had any luck finding an actual product anywhere.

I would of course prefer to use the 1" z channel and save that extra half inch, but also want to get the highest insulating performance I can.... if it means using 1.5" z channel then I would do it.

Code in my area requires minimum R5, and I hate sitting right at that line.

Thanks for any info/advice.

Irish