Momentum is supposed to have a durable water resistant coating. So, it is partially water proof. Is it enough? That is a judgment call.
Back in the old days, maybe thirty years ago, I sewed my first two bivy sacks. They had a coated nylon bottom and a Goretex top. Goretex is a little bit "old school," but it works, especially in wet and cold weather. The problem is that it is a little bit heavy. (This is BPL, right?)
Go to the other extreme and use an uncoated taffeta, and it would be very light and breathable, but maybe not waterproof enough at all.
I determined that Momentum had enough waterproofness balanced with light weight and breathability for me. YMMV.