Used on our recent Ultralight Challenge, this is more of an emergency bivvy than a full on mountain bivvy, but can still be useful. Made of shiny shiny Thermolite fabric it reflects a significant proportion of warmth back, 80% according to the specifications, and could conceivably be used as a very warm weather sleeping bag, or above 9°C. It would be useful to take in winter if you’re after a bit of extra warmth, or perhaps while snowholing? The bag is roomy – significantly more so than the Rab Survival Zone, and i could turn around comfortably in this without taking the bag with me.
The side opens half way and uses velcro to close, leaving some gaps for ventilation. There’s also a mesh panel at the foot that should help ventilate further. Considering that the fabric isn’t breathable, you need all the help you can get, and you’ll find that condensation is unavoidable. I’ve found that the condensation hasn’t been excessive, and while it makes your sleeping bag damp, you’d still be good for another night. But that is certainly something that needs considering if you intend to use this over manynights. It’s waterproof, but with all the ventilation you wouldn’t really want to risk it out in the open, maybe under a tarp.
All that said, this is first and foremost an emergency product that also happens to do a decent job for occasional planned overnight use. So while it does have a few limitations mentioned above, it’s hardly surprising as it’s been designed for an unplanned night, sitting mostly in your pack and never being used. If you wanted a breathable bivvy at this weight, you’d need to shell out a bit more, but even then you’r probably not going to eliminate condensation altogether.
Summary: An emergency survival system that makes a passable, ultralight, fine weather bivvy for tarping.