I'm asking this as a neighbor has built a house but hasn't sided it yet. It's been about 9 years and I'm really curious to know if he's getting any R value anymore or not.

Besides that,, it's a real eyesore on our street. lol

Sweetrain, Most materials exposed to the elements have problems due to breakdowns from u.v. rays, temperature changes, heavy rains, icings, etc. Your best bet is to contact the manufacturer and get their take on it, though I also believe geographical location has a lot to do with how materials hold up. Some will do better in areas where it might not do as well somewhere else. We put a "newer" type underlayment on our build on the roof as we had to leave it for the winter and it wasn't ready for the roofing. It was claimed to hang in there for 6 months but failed miserably, leaving us a wet house when we returned in the spring. Looking back I simply would have used good old fashioned 30# felt, removed that,(due to wrinkling), when the time came to put on the roof with new underlayment. It's something to consider for you to simply use felt (with cap nails), for now and when it comes time to side, install the foam panels then.