Back your spam self on out of these forums,sleeves. It's against garden web policy.

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.

"can you wait a year or two? " Sorry, I didn't catch that part of your post. A year or two is a long time to sit without being fully dried in. If it's financial, I would wait until you have the funds to do it right the first time out not relying on how long underlayments might hold up.

Deterioration from exposure to UV light starts immediately but doesn't get to be a problem for a few months.

Here is a link that might be useful: Owens Corning discussion

I'm curious. Is there anyone living in this house? I be surprised if you could get an occupancy permit with no siding.