bivy sack? WTB need info, suggestions

i use a Montbell sleeping bag cover (bivy) regularly in the winter...though I use it along with either a megamid or a single wall tent.

www.backcountry.com/montbell-breeze-dry-...l-sleeping-bag-cover

It doubles as a waterproof backpack liner when I'm travelling during the day to keep my sleeping bag and gear dry, and at 6 ounces, it doesn't weight much more than a heavy duty contractor bag. 

The reason I use it is that it keeps falling condensation off my bag and also boosts the warm of my bag.   With the sleeping bag cover and a down jacket, I can comfortably get away with using a Feathered Friends Vireo bag (1lb no zippered bag) in the middle of winter. Without the bag cover, the system was noticeably colder.

I might not trust this on its own if the weather was going to be iffy, but its a good option when used with a tarp shelter.

i use a Montbell sleeping bag cover (bivy) regularly in the winter...though I use it along with either a megamid or a single wall tent.

Thanks Pete. That looks really interesting. That would have been just the ticket when I was getting snowed on INSIDE my rain-fly a couple weeks ago, due to condensation and strong winds. I thought my sleeping bag would get soaked.

The URL you linked says the bag cover is very breathable. That's critical because if it's not, the sleeping bag can get wet from your own perspiration.

yeah, i've had zero issues with my bag getting wet from my own perspiration...and it does a nice job shedding all the condensation snowflakes that form inside my Firstlight.

I love using a bivi in the winter and everyone who goes with me ends up using a bivi also.  It is the lightest and most compact way to travel (that I have found).
I dig a snow tube straight into a steep hillside and then bell out the head so I can sit up inside.  I can usually dig the tube in the same amount of time others prepare their tent platform and set up the tent. 
I use a BD winter bivi which only weighs 9 oz and fits into my sleeping bag stuff sack.  The bivi isn't gore-tex and breaths excellently.  I have never had condemsation on the inside or the bottom.  I use a Neo Air pad and a 30 degreee bag.  I sleep comfortably down to the single digits which seems to be the coldest I have found around here.
When the weather starts getting warmer where the pack settles then I use a heavier OR sack and sleep on top of the snow.
 

i use a Montbell sleeping bag cover (bivy) regularly in the winter...though I use it along with either a megamid or a single wall tent.

www.backcountry.com/montbell-breeze-dry-...l-sleeping-bag-cover

Turns out I already have one of these. (I checked my gear box in the basement.) I got it six years ago. Green on the outside, smooth white material on the inside. I don't know if they've changed the design since then.

I've used it only once, during the summer. I went on a short overnight hike in fair weather with dew overnight. The bag performed horribly in those conditions. I wrote in my journal, "It was like putting a vapor barrier on the outside of my sleeping bag." A huge amount of condensation on the inside of the sleeping bag cover. I took off the sleeping bag cover in the middle of the night, it was so bad. My sleeping bag is one of those super-light Western Mountaineering down bags that Jim Nelson sells.

I checked some reviews for lightweight sleeping bag covers and it seems like people have very different experiences with them. My theory is that they are breathable under lower humidity conditions, but they become garbage bags when the outside humidity is high enough. In my experience, Goretext bivi sacks (which are generally heavier) don't have the same problem.

I should give the MontBell bag another try, but I'm pretty leery of it.

well, i have only used mine in the winter...so it could very well behave like a trash bag in the summer/higher humidity situations...as you mentioned, most laminate fabrics don't do particularly well in those conditions.
still, for wintertime use, i've been pretty happy with it.