The best stuff I've found so far is Epic Malibu, used by Black Diamond, about 1.8 oz/sq yd. It is occasionally available at thru-hiker and some other sites. It does not meet CPAI specs. Looking at the Essence and Mica jackets, I suspect that lighter and more breathable materials will become available soon. And from this forum, there is much info about the Tyvek. For my next project, I am going to go with the Epic, well vented.
Sam Farrington, Chocourua NH