So, would something like this (Pentax Endurance) be an option?
http://www.integraldesigns.com/product_detail.cfm?id=894
They also make the same design out of "Lightweight 50D Pertex® Shield 2.5-layer fabric".

This is the kind of design that I hadn't found commercially up until just now. This is pretty much what I had envisioned making myself, it's not fully weatherproof clearly, but ideal as a sleeping bag cover without trapping exhaled air inside. Does seem fairly pricey, but on the flip side making one would be fairly time consuming.

This is another contender:
http://www.mountainlaureldesigns.com/shop/product_info.php?cPath=22&products_id=30

Comes back to the fabric question, water resistant highly breathable (Pertex Endurance), versus waterproof and breathable (eVent or Pertex Shield).

What I'm particualarly unsure of is how colder conditions impact these materials, are some more prone to losing breathability than others?

Edited by Simple_One on 02/09/2013 20:53:29 MST.

Get an eVent bivy. Breathability of eVent is unsurpassed.

Edited by randalmartin on 02/09/2013 20:48:11 MST.

Thanks all for the discussion so far. I'll be reading up on the fabrics over the next week or so, if I find anything of great interest I'll add a link here.

"What I'm particualarly unsure of is how colder conditions impact these materials, are some more prone to losing breathability than others?"

Once the bivy fabric is cold enough, water vapor condenses before it can pass through. Some fabrics are better at moving the condensed water thru them (kind of like a paper towel). The lighter nylon fabrics typically don't excel in this fashion.

I wonder how well a tyvek type bivy would perform in the above regard.

Edited by jnklein21 on 02/09/2013 21:09:00 MST.

the following web address is for an article about bivy condensation. The author experience is that air and vapor permeable fabric (Event or similar)perform the best in regards to condensation. He also suggest that a bivy with WPB top and bottom would be better than a bivy with a waterproof bathtube floor.

http://40yearsofwalking.wordpress.com/2011/07/04/the-bivy-condensation-conundrum/

However if you are only interested in protecting your sleeping bag from spray while sleeping under a tarp or from the wet walls of your tent a homemade Tyvek bivy might be a good choice:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=46661.

Tyvek doesn't have enough hydrostatic head to qualify a waterproof (about 40 to 50 inches),it's water resistant yet breathable. The type 14 tyvek used in the home made bivy is available on Amazon.com

I have an old Gortex hooped bivy and I don't believe the venting around by head reduces condensation significantly. The venting around my head doesn't seen to affect moisture around my feet. I have a preference for a side zipper because it makes getting in and out of a bivy very easy.

Edited by Surf on 02/10/2013 00:30:15 MST.

I have used a lot of different bivvies and in those conditions, my all eVent Integral Designs Overbag has worked the best. Sadly, it is also no longer made. EVent is true a miracle fabric In a bivy.

Hi,
have a look at Locus Gear from Japan for Tyvek Bivy and I think
Mr. Jotaro (the owner) is designing and testing new bivy made from
super breathable eVent fabric he uses to make his shelters.
Cheers!

I have a Black Diamond [Bibler] Hooped Bivy. It's pretty minimalist and very breathable. It comes in a variety of options. The fabric has a funny, fuzzy interior coating which helps transfer moisture. I've spent a lot of time in pouring rain with this bivy too - doesn't leak at all. It's pretty roomy, which is a good thing for a claustrophobe like me. I tend to use bivys only when a route doesn't have room for a tent or I need to go really light. Anyway, the BD bivys are worth a look. Coincidentally, you mentioned using your bivy in snow caves, I'll be using mine tonight in a snow cave!

Edited by redpoint on 02/10/2013 16:19:43 MST.