Just a few comments:
i have a friend who made a Tyvek bivvy and he said he had way too much condensation while snow camping. Exact material unknown, probably house wrap. I have a three layer gortex bivvy and it works. It has kept me dry in a few rain storms and I have never felt calmy or had condensation. However.....it weighs a pound and a half. I have a tent that is much more roomy and comfortable that weighs 2 pounds. And...most of all, I have decided that if it rains you need a tarp in addition to your bivvy because you need a place to change clothing, cook, sort through your pack and such. If it is really raining you can not do those things with just a bivvy. So the weight of an UL tent is equal and you have a better environment to wait out a storm. also, what do you do in a bivvy if it is really raining and you need to pee at 2 AM?
Jim M