We poured out basement in December.  Amvic ICFs.  I have noticed that the foam is really starting to yellow and disintegrates when touched with the finger.  Maybe 1/8th inch wears off or something.

Is this something to worry about?  We have hit a few major major snags and may not have the roof on our house until next summer.  That would protect the interior foam but not the exterior. 

Should I wrap this stuff with tyvek or or paper???

I would worry too much. UV light will penetrate about +/- 1/16". The chalky dust that forms will protect the foam from further damage. If you want to stucco/parge you will need to rasp the surface off prior to doing so.

That's good news. I plan to do some stone veneer on the exterior which requires metal lath. I assume then I wouldn't have to worry about wrasping it off then right?

That's good news. I plan to do some stone veneer on the exterior which requires metal lath. I assume then I wouldn't have to worry about wrasping it off then right?

The metal lath will be mechanically fastened to the webs (likely with stainless steel screws). This should hold the stone in place. If the mortar is an acrylic-based product, you could still rasp the foam to allow for additional strength by having the acrylic bond to the EPS. Downside is you will not be able to see your markings for web locations after rasping - best bet is to mark these locations prior to rasping surface.

My foam degraded a little in the sun, but less than I would have though over a year's time. I did cover the upper part with Tyvec just to slow it down. The lower part I used stucco, and I cleaned it with a pressure washer (gently!) after attaching the lath.

Don't clean it off until you are ready to cover it. Then pressure wash the area that gets the stone. The UV will have opened up the beads, like rasping does.