silnylon or tyvek bivy tub ??

Going to build a bivy. I know neither is 100% waterproof, so the question.

It will be a 1.25oz tyvek bivy.

I have woken up more than once in water with the bottom of my sleeping bag soaked and its not a good experience.

or would I be better off with urethane coated nylon and more weight.

I am leaning towards just building the entire thing out of tyvek.

Thanks