The problem with the emergency bivvy sacks made of the heat-reflective space blanket material is that while they provide warmth, they don't let moisture escape. Thus, you sweat. But a new material in AMK's SOL Escape bivvy changes that.
According to GearJunkie, the SOL bivvy is rain-, snow- and wind-resistant, but its material has a breathable design so that you don't feel like you're in a sauna. The $50 bag only weighs 10 oz. and rolls up to be approximately the size of a Nalgene bottle. Expect to see it in stores in 2012. [GearJunkie]