I made myself a bivy using 1443R for the top part and a type of HomeWrap for the sides and bottom.
Only used it at home just to see if it works, for breathability, and it does but it is a bit fragile and I would not use it as a stand alone bivy but under a fly so that all it would have would be sprays not direct rain.
As a tent fly 1443R works a bit better at holding rain if there is no contact with it (you pressing against the walls) still it isn't waterproof just water resistant.
BTW one of the reasons I made that bivy was because Tyvek can be glued with PVA (Elmers glue) so no stitching required.