It's kept me and my hammock dry every time I've used it. I can't speak to the long term durability though as it gets set up, taken down, folded and handled. After 1 season, mine doesn't have any rips or tears, but the surface in some places looks a little frayed. Still hasn't leaked though. From what I understand, water vapour can penetrate tyvek but not water droplets (kind of like gore-tex). That's why it's good keeping homes dry. Best of luck. You're doing a great thing.
Did you sew it together ore hov did you make it wide enuf?