Big wall question: Is a bivy sack NECESSARY if I have a ledge + rainfly?

I just ordered a Mountain Hardware Ultralamina zero degree synthetic sleeping bag. I also have a Fish Double Whammy Portaledge with a rain fly. I've heard people advocate using a bivy sack in addition to a rainfly/ledge combo, but it seems like having a good synthetic bag would be enough to keep me dry in the rain if I had a ledge+fly. I'm asking because I have never had to wait out a storm on a wall and don't have any experience with just how bad/crazy it can actually get up there. [For reference, the walls I'd be doing would mostly be in Yosemite from early spring through early summer].

Also, for routes with natural ledges for sleeping - would I be totally screwed if it rained on me and all I had was my synthetic bag and a heavy duty trashbag or a tarp or something? I know synthetic bags are supposed to stay warm when wet, but again, I have honestly never slept outside in the rain with just a synethic bag...and I imagine that a SOAKED bag would not actually keep me warm.

Bivy sacks can be quite expensive and there is already sooooo much gear involved that I'd really like to save myself the money if it's something that isn't totally necessary. However, if it is insane to go up there without a bivy sack, I'd like to know as well. So far the only walls I've done have been in perfect weather where rain was not an issue.

Anyway, that is my question - THANK YOU in advance for any advice!

I think it really depends on how wet and how cold it will be. If you were on El Cap waiting out a snow/rain storm for two days, then yes...you should have a bivy sac as a backup to a ledge and rainfly. Mini condensation rainstorms inside the rainfly will make things damp inside the fly and the bivy sac is your backup to that. But if it's not that cold and you just have to wait out a couple of hours of rain, then I would say you don't need the bivy sac. A bivy sac will add about 10 degrees of warmth to your bag...so it may be overkill with your 0 degree bag (you'll just sweet and get things wet from the inside).

In my opinion, you should get a bivy sac if you're sleeping on natural ledges and there is a chance of a cold rainstorm. Wind will make your tarp or trash bag useless if its a big enough storm. I've used a tarp in a small storm before and it flapped around like crazy.

Sac up Dude.

Route & weather dependent, if the route is in known water courses or has slabby sections, a rain fly can only do so much to keep you dry. On steep overhanging routes you are much more protected. If going up on a big wall in questionable winter weather, I would take a bivy sack and some large sponges..

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