Ok, so I've been using my MLD Soul bivy and thinking about how to tackle moisture, which is going to become more important as winter approaches down under (which here means lows of perhaps 5 F but usually warmer). And much as I enjoy buying new gear, I'm hoping to avoid buying a double wall tent.
Assuming I've already got the most breathable shells on the bag and bivy I can get, things I can think of:
1. Go for a wide bivy.
When pegged out, the Soul bivy does a really good job of a bathtub-like floor by having the waterproof bottom come up at the sides. Great for keeping out side on rain-splash etc, but the sides of my bag tend to come up against the non-breathable sides and condensation results. The problem will be worse with a thicker bag. So I figure if I have a wider bivy than strictly necessary there should no contact with the waterproof side and my bag. Same thing applies for the length, except I've already achieved this because a standard length MLD bivy is generously long for my 5'7" height.
2. Try clipping up the foot of the bivy, like the head is
The foot box is where condensation is worst. I really like how the head of the bivvy can be clipped up to the roof of the tarp, giving you lots of space over your head. I'm wondering if it would help if you tried a similar thing at the foot of the bivy, to create a buffer of air between the bag and the bivy top ? This one is fairly easy to experiment with.
3. Add a small mesh window to the footbox
Air circulation down the bottom of the bivy must be pretty much non existent. I wonder if adding a small mesh 'window' on top, combined with clipping the bottom end up away from my bag, would help out here. Ideally I guess it would be zippable to fine tune it for windy vs calm, but a fixed window would be light and cheaper. So something like the MLD superlight bivy's head, but at the foot: http://www.mountainlaureldesigns.com/shop/images/snewtieoutuperlight.jpg.
4. Don't rely on zipping right up to stay warm
It's attractive to add some warmth by zipping myself right in. Works in the short term, but I think this might be counter-productive over many nights though, because it makes it harder for moisture to escape resulting in a wetter bag. By the fifth night, say, with a damp collapsing bag, arguably you might have been warmer overall allowing more circulation from the outset (although colder on the first night!). So I'm going to aim for a bag warm enough that I don't need to seal myself right in, with the hope that it will stay drier longer.
Thoughts? Any other ideas?