I am consulting with Borah right now and am planning on ordering one of their lightweight side zip bivies with argon 90. They recommended argon 90 over 67 as it was more breathable I guess. As far as claustrophobia...I can't see a bivy as any more confining than being inside my mummy bag zipped up. Sleeping inside the bivy in warm weather may be a concern as far as getting hot but I will have to wait till next year to find out. In that kind of weather I will not use my sleeping bag anyways but most likely just put my sleeping pad only inside and use the bivy as a bed sheet so to speak.