Waterproof bivvy and down sleeper?!

I use a down sleeping bag and a UL water resistant/breathable bivy, but I always bring my tarp. The idea is that the tarp will shake off the tough stuff and my bivy keeps spray, dew, etc off the bag. Since it is water resistant instead of water proof, it is considerably more breathable and allows the moisture coming off my body to exit get away as it rises through my bag. When the weather is fairly nice, I just don't throw up the tarp.

Of course, YMMV, but this systems works really great for me.

sure, this approach seems well documented, and one I plan to use once I get my ass in gear to make a breathable bivvy! But I'm talking about a gore-tex or eVent bivvy and no tarp, with a down bag, that's what seems mad to me!

J.

I use an event bivy and a down bag, without tarp, I have experienced light rain, not severe downpours but to date no problems. Of course getting in and out of a bivy in the rain is the bigger issue in my view. If I am expecting rain then my poncho tarp is used to provide a drier access to the bivy.

I use my WM Summerlite with a MontBell Breeze Dry-Tec S Zip bivy and I think it works great. There have been some mornings where the foot of my bag was slightly damp ( I haven't had this happen consistently in any sort of weather I think it was more of an overly high amount of sweating due to dressing warmer than the conditions required so mostly error on my part) but camping in the rockies gives me ample sun to quickly dry the down in the bag. I have slept in this set up in light rain and though getting in is cumbersome I didn't really get the bag wet so everything was fine.

IME, Ryan's explanation is what most do. The only time I would use just my bivy, is when there is no rain in the forecast – and even then I would have my poncho/tarp with me just in case. You may be able to get away with a night here and there, but for continuous rain, I think even the WP/B stuff would wet through – or the condensation would soak you.
However, I am interested in hearing from those that use just a bivy…maybe the alpine guys?

…exactly- I think the key issue is not so much the efficacy of the WP/B material- which is likely to be good enough when combined with a somewhat water resistant shell on the sleeper, but the question of what to do if you get properly soaked before getting in the bag- which is why this approach may be more popular with alpine climbers because snow brushes off, whereas rain does not…

back to my big bertha of a synthetic as I trudge through the rain…