how water proof is it

I am looking to make a poncho that I can use while riding a bike. I know that the silnylon is water proof for the purpose of tarps where nothing should touch it but is there any light weight material that is waterproof even when being touched? if not would there be any trick like a second layer that could be added to help with this problem? Thoughts:

Sil Nylons is waterproof even if you tuch it. I have a rain jacket and its fine. It is hot though becouse it does not breath. Should be fine for a poncho. Sil nylons is not wterproof under presure. I have a water bag that I made and if I squize it, beads of water form on the surfice. But if there is no presure it fine.